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Deep Cycle Battery 101

Read the following information and you will have a good basic understanding about batteries and what your system needs are.



What is a deep cycle battery?

The essential difference is in the lead plates. In normal car batteries, these are sponge type structure or consists of many thin plates, giving a greater surface area to generate big amps for short periods; whereas in deep cycle batteries, the lead plates are thicker and solid.

A deep cycle battery usually has multiple times the RC (reserve capacity) of a car battery, but a substantially lesser CCA (Cold Cranking Amps) rating - the focus is on consistent lower level supply rather than intermittent short bursts of high supply.

Why not car batteries?

Car batteries are meant to give large amounts of juice for a very short period. Once they've done their work turning over the engine, they are immediately recharged by the car's alternator.

In a solar power setup, while the draw mightn't be as brutal in short bursts, it can continue for a very long time - for example, overnight. Car batteries just aren't made for this type of application and will wear out quickly. I found this out the hard way.


Different types of deep cycle batteries


These are very much like car batteries only with thicker lead plates and have many of the issues that car batteries have when used in a stand alone power system - serviceable life, electrolyte dissipation, stratification, transporting problems and the dangers of explosive gas.


Gel deep cycle batteries contain acid with the addition of silica, which turns the acid into a jelly. Even when broken, the acid won't spill. These batteries are ideal for daily use and deep discharge and will work well in high temperatures. They can be partially recharged without causing serious battery damage and readily accept charging due to low internal resistance; however they shouldn't be recharged using high voltages such as what a car alternator cranks out.


AGM stands for Absorbed Glass Mat. A Boron-Silicate glass mat is placed between the lead plates, which immobilizes the electrolyte making it unspillable. This type offers the same sorts of benefits as gel batteries and AGMs are also tolerant to high voltage charging.

Another important safety feature of both AGM and Gel is that either do not off-gas under normal usage. They are hermetically sealed and recombine the oxygen and hydrogen produced within the battery.

Lifespan of Batteries

The lifespan of a deep cycle battery will vary considerably with how it is used, how it is maintained and charged, temperature, and other factors. In extreme cases, it can vary to extremes - we have seen L-16's killed in less than a year by severe overcharging, and we have a large set of surplus telephone batteries that sees only occasional (5-10 times per year) heavy service that are now over 25 years old. We have seen gelled cells destroyed in one day when overcharged with a large automotive charger. We have seen golf cart batteries destroyed without ever being used in less than a year because they were left sitting in a hot garage without being charged. Even the so-called "dry charged" (where you add acid when you need them) have a shelf life of 18 months at most. They are not totally dry - they are actually filled with acid, the plates formed and charged, then the acid is dumped out.

These are some typical (minimum - maximum) typical expectations for batteries if used in deep cycle service. There are so many variables, such as depth of discharge, maintenance, temperature, how often and how deep cycled, etc. that it is almost impossible to give a fixed number.

  • Starting: 3-12 months
  • Marine: 1-6 years
  • Golf cart: 2-7 years
  • AGM deep cycle: 4-7 years
  • Gelled deep cycle: 2-5 years
  • Deep cycle (L-16 type etc): 4-8 years
  • Rolls-Surrette premium deep cycle: 7-15 years
  • Industrial deep cycle (Crown and Rolls 4KS series): 10-20+ years
  • Telephone (float): 2-20 years. These are usually special purpose "float service", but often appear on the surplus market as "deep cycle". They can vary considerably, depending on age, usage, care, and type.
  • NiFe (alkaline): 5-35 years
  • NiCad: 1-20 years

Starting, Marine, and Deep-Cycle Batteries


  • Starting (sometimes called SLI, for starting, lighting, ignition) batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a Lead "sponge", similar in appearance to a very fine foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).
  • Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates - not sponge. This gives less surface area, thus less "instant" power like starting batteries need.
  • Unfortunately, it is often impossible to tell what you are really buying in some of the discount stores or places that specialize in automotive batteries. The golf car battery is quite popular for small systems and RV's. The problem is that "golf car" refers to a size of battery (commonly called GC-2, or T-105), not the type or construction - so the quality and construction of a golf car battery can vary considerably - ranging from the cheap off brand with thin plates up the true deep cycle brands, such as Crown, Deka, Trojan, etc. In general, you get what you pay for.
  • Marine batteries are usually a "hybrid", and fall between the starting and deep-cycle batteries, though a few (Rolls-Surrette and Concorde, for example) are true deep cycle. In the hybrid, the plates may be composed of Lead sponge, but it is coarser and heavier than that used in starting batteries. It is often hard to tell what you are getting in a "marine" battery, but most are a hybrid. Starting batteries are usually rated at "CCA", or cold cranking amps, or "MCA", Marine cranking amps - the same as "CA". Any battery with the capacity shown in CA or MCA may not be a true deep-cycle battery. It is sometimes hard to tell, as the term deep cycle is often overused. CA and MCA ratings are at 32 degrees F, while CCA is at zero degree F. Unfortunately, the only positive way to tell with some batteries is to buy one and cut it open - not much of an option.


Inadequate battery reserve power has long been the Achilles' heel of
 RVers who like to get away from the usual trappings of civilization,
 including hookups.  While an AC generator can be used to supply
 auxiliary power, it can't be operated continuously, and RVers who lack
 both a generator and campground electrical hookups are very battery-
 Beyond conventional 12-volt appliances, owners who have discovered the
 benefits of power inverters (see "Inverters" - April 1994) to operate
 120-volt AC appliances often find their previously adequate auxiliary
 batteries lacking.  To power all these newly added luxuries, batteries
 must provide adequate output and must be kept in excellent condition.
 The lead-acid battery types that are most common in successful RV
 auxiliary-power applications are all of deep-cycle design.  This is
 important because a deep-cycle design stands up to repeated heavy
 discharge-recharge usage much better than an ordinary automotive
 battery.  An automotive battery is designed to deliver very large
 bursts of current for short periods (when starting an engine) and then
 is immediately recharged (by the vehicles' alternator).
 Most RV 12-volt DC and inverter power applications require the battery
 to provide current for extended lengths of time before receiving any
 recharge.  An automotive battery will lose a significant percentage of
 its full storage capacity after being heavily discharged just one time.
 It will typically lose half of its capacity after 50 discharge-recharge
 cycles. (A heavy discharge is one that removes all but 20 percent of the
 battery's original full charge.)
 By contrast, even the lightest-duty deep-cycle battery will typically
 tolerate 200 to 300 such discharge-recharge cycles before reaching a
 similar state;  some of the heavier deep-cycle designs can exceed
 10,000 such cycles.  In short, no matter how "heavy duty" a battery is
 claimed to be, if it isn't a deep-cycle design it won't last very long
 in most inverter applications.  The only battery in an RV that needn't
 be of deep-cycle design is the one that starts the vehicle's engine.
 When a battery becomes too old and weak to sustain a usable charge,
 sulphation is most frequently the culprit.  Every time a battery is
 discharged, its sulfuric-acid solution is gradually broken down, leaving
 deposits on the battery's lead plates.  If the battery is promptly
 recharged, most of this sulphation is driven back into solution, leaving
 the plates in an essentially unchanged state.  Leaving the battery in a
 discharged state for extended periods, however, allows the sulphation to
 harden into a form that permanently embeds itself within the plates.
 Suplhation deposits permanently reduce the battery's storage capacity.
 Chronic undercharging or excessive discharge also lead to plate
 shedding, in which some of the active solid-plate material flakes off
 and accumulates in the bottom of the battery.  This accumulation
 eventually sorts out the plates, resulting in a dead cell. Consequently,
 if full storage capacity over a long service life is to be realized, it
 is important to fully recharge a battery promptly and to avoid over-

Depth of discharge - extending battery life

Deep cycle batteries are designed to be discharged much lower than standard car batteries and be recharged many more times. The life of a deep cycle battery under normal conditions is anywhere from 3 - 10 years.

Like any other piece of equipment, the more you hammer it, the less it will last you and one of the key strategies for getting the longest life out of your deep cycle battery relates to depth of discharge; i.e, how much juice you suck out of it. If a battery is discharged to 50% (around 12.06v under load or 12.24v with no load - this can be measured with a cheap multimeter), it will last about twice as long as if it is taken down to 80% discharge.

Cycles vs Life


A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. You have to be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged. For example, one of the widely advertised telephone type (float service) batteries have been advertised as having a 20-year life. If you look at the fine print, it has that rating only at 5% DOD - it is much less when used in an application where they are cycled deeper on a regular basis. Those same batteries are rated at less than 5 years if cycled to 50%. For example, most golf cart batteries are rated for about 550 cycles to 50% discharge - which equates to about 2 years.

Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film. The graph above shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary.

How many/big a battery to buy?

Really, the bigger/more the better. For example, I have a 100 AH (amp hour) battery and under normal weather conditions, even under partial cloud, that does me fine. One day of total overcast isn't a problem either, but a couple of gray days back to back and I start getting around the 50% discharge mark at which point I grudgingly have to fire up the generator.

As mentioned, given that my 130 watt panel recharges the battery by midday under sunny/mostly sunny conditions; that means I have spare output that is being wasted - so I would have been wiser to buy the 150 AH battery recommended to me which would help better carry me through overcast days.

What's with the AH?

AH, or amp hours, is how deep cycle batteries are rated. If you have an appliance that draws two amps; then it could run for 25 hours on a 100 AH battery before you'd hit the 50% DOD (depth of discharge).

Converting watts into amps

Most appliances are rated in watts rather than amps, so how do you make the conversion?

Use this simple formula:

Amps = Watts / Volts

Tip: Sometimes amps are noted on appliances, check the manufacturer label on the back of the item.

Calculating your deep cycle battery needs

After determining the amp rating of your appliances, then multiple each by the number of hours of use per day. Then add up all those figures and you'll get some idea of the sized battery you'll need - but bear in mind the 50% depth of discharge recommendation; i.e. a 100 AH battery really only gives you 50 AH if you wish for the battery to last well.

For example, if you had a notebook computer running at 3 amps and a light drawing 1 amp, a fully charged 100 amp hour battery could run both for 12.5 hours without any input from your solar panel before you'd hit the 50% discharge mark.

If determining the amperage of all your appliances doesn't appeal to you; try this solar system builder that will give you a full system recommendation based on your needs.

 Figure 1 - Battery State of Charge
 Charge   Voltage    Voltage    Specific
 Level    (12v)       (6v)      Gravity
 ------   -------    -------    --------
  100%     12.7        6.3      1.265
   75%     12.4        6.2      1.225
   50%     12.2        6.1      1.190
   25%     12.0        6.0      1.155
    0%     11.9        6.0      1.120
 The maximum storage capacity of a deep-cycle lead-acid battery is
 usually rated either in amp-hours, or in minutes of reserve capacity.
 The amp-hour value refers to the number of amps a battery will deliver
 over a specified period of time (generally implied to be 20 hours if not
 specifically stated), before the battery has discharged to a useless
 level (10.5 volts for a 12-volt battery).
 The reserve capacity value specifies the number of continuous minutes
 the battery can last while delivering 25 amps before dropping to this
 same 10.5 volts.  As a rule of thumb, for the smaller batteries you can
 multiply the number of reserve minutes directly by 0.6 to arrive at an
 approximate equivalent amp-hour rating for the battery.
 Therefore, a 50 amp-hour battery (or a battery with approximately 83
 minutes of reserve capacity) can be expected to deliver at least 2.5
 amps for 20 continuous hours, or at least 1 amp for 50 continuous hours.
 Note that at current drains much higher than those specified at the 20-
 hour rate, however, the capacity of the battery starts to decline due
 to internal losses and chemical inefficiencies at high currents.
 Consequently, this same battery might only be able to deliver 5 amps for
 nine hours (45 effective amp-hours), instead of the 10 hours (50 theo-
 retical amp-hours) implied by the battery's amp-hour rating. In general,
 bigger batteries can deliver higher currents without incurring this
 The life expectancy of a deep-cycle battery, like all lead-acid
 batteries, is directly dependent upon how heavily the battery is
 routinely discharged before being recharged.  Batteries that are
 regularly discharged until only 10 percent of their rated capacity
 remains have a much short life expectancy than identical batteries that
 are rarely discharged below 50 percent.  Therefore, you should not buy
 a 100 amp-hour battery if you plan on routinely using all 100 amp-hours
 between recharges.
 A good rule of thumb is that a deep-cycle battery should not be depleted
 beyond 80 percent of capacity, with 50 percent being even better. A 50
 percent discharge represents a good compromise between battery life and
 reasonable battery-bank size.  Therefore, you would do well to buy at
 least 200 amp-hours worth of batteries to meet an anticipated 100 amp-
 hour discharge "budget".
 Ambient temperature also has a strong effect on battery performance.
 Performance of most batteries is rated at around 80 degrees F.  At
 higher temperatures, they have greater capacity, but their life span is
 shortened, due to the acceleration of detrimental chemical reactions.
 At lower temperatures, they last longer than normal (provided the
 electrolyte is not allowed to freeze), but their capacity drops.
 At 32 degrees F, typical capacity is reduced by 35 percent; at zero
 degrees F, it is reduced by 60 percent; and at minus 20 degrees F, it
 is reduced by better than 80 percent.  A battery's ability to accept a
 charge also drops along with the thermometer.  In general, the best
 trade-off between efficiency and long life occurs when the battery is
 maintained at around room temperatures.  For RV owners, this means that
 batteries in a compartment that is insulated from extreme cold and heat
 will last longer and deliver more consistent power.
 As a battery is discharged, the sulfuric-acid solution inside each cell
 is gradually converted to water.  Consequently, the specific gravity of
 this solution also drops as the battery discharges.  This change can be
 easily measured with a hydrometer in order to determine the battery's
 state of charge.  A good battery hydrometer includes a temperature-
 correction scale (specific gravity versus battery charge varies somewhat
 with temperature) and will often yield readings that are more precise
 than those obtained with a voltmeter.  Using a voltmeter is usually more
 convenient, however, and is the only accurate method of checking sealed
 batteries.  Consult Figure 1 when determining the state of charge of a
 battery, using either a voltmeter or a hydrometer.
 Specific gravity readings should be taken by inserting the hydrometer
 suction pipe into the battery cell, squirting the electrolyte into and
 out of the hydrometer several times (electrolyte agitation improves
 accuracy), and then reading the hydrometer while the suction tube is
 still inserted into the cell.  Keeping the suction tube in the cell
 while taking readings minimizes the chance of spilling the electrolyte,
 which could cause burns or destroy clothing.  Read the hydrometer scale
 at the center of the fluid inside the tube, not at the edges.  Note that
 any heavy battery charge or discharge currents drawn just prior to
 taking specific gravity or voltage measurements will have an adverse
 effect on the accuracy of the readings.  The greatest accuracy is
 obtained after the battery sits idle for at least 24 hours prior to
 taking hydrometer or voltmeter readings.
 Specific gravity readings are also helpful in determining the overall
 health of a battery.  For example, differences in specific gravity of
 more than 0.050 between any two individual cells in a battery generally
 indicate that the battery is headed for problems.  By taking specific
 gravity readings every month or so, owners can catch battery problems
 before they cripple the entire system.
 Regardless of what type of battery is selected, all the house batteries
 in an RV should ideally be the same age, size, and brand.  This is
 because unsimilar batteries tend to charge and discharge at differing
 rates, leading to some of the batteries in the group being consistently
 undercharged during recharge and overstressed during discharge. Matching
 batteries will ensure maximum life for the entire battery bank.  If the
 bank is diligently maintained, all batteries will wear out at about the
 same time, allowing the entire bank to be changed out after a long
 service life.
 In buying batteries, look for similar date codes stamped on each one.
 If the batteries have sat on the dealer's shelf for more than a month,
 use a hydrometer or voltmeter to ensure that the state of charge has
 been maintained.  Don't buy old or partially discharged batteries.  If
 in doubt, ask the dealer about the date of manufacture and shelf
 storage procedure.
 Among the deep-cycle variants, the most common type is the RV/marine,
 typically sold by hardware and department stores and by RV-parts
 counters in automotive package (or group) sizes 24 and 27.  Typical
 ratings for this class of battery are approximately 80 amp-hours (110
 minutes) for size 24 and 105 amp-hours (170 minutes) for size 27.  These
 batteries represent a reasonable value in smaller systems that are
 equipped with inverters, or in installations where space is at a
 premium.  As deep-cycle designs go, however, they are lightweights, with
 relatively short life expectancy in heavy service (typically two to
 three years).  This deficiency is primarily due to the use of thin lead
 plates in their construction and the low antimony content of the plates
 The next most common deep-cycle version is probably the golf
 cart/electric vehicle, typically sold through battery-supply houses,
 some wholesale clubs, and occasionally department stores (frequently
 by catalog only).  These batteries are all of 6-volt design (connection
 of two in series produces 12-volt output) and typically cost a tad more
 per pair than a single size 27 RV/Marine battery.  They provide superior
 service in most RV applications (due to thicker plates and higher
 antimony content) and probably represent the best value for installations
 that can accommodate their large size (10-1/4 inch width, 7-inch depth,
 and 11-inch height).  Typical ratings are 220 amp-hours, or 400 minutes
 of reserve capacity.  Expected life is typically three to five years.
 Note that connecting two 6-volt batteries in series does not double the
 amp-hour or reserve capacity ratings, but connecting two of the resulting
 12-volt battery banks in parallel (a total of four golf-cart batteries)
 Gelled-electrolyte ("gel-cell") batteries are becoming cheaper and more
 popular among Rvers.  Available in group 24, 27, 4D, 8D, and 6-volt
 golf-cart sizes, they offer very good performance with virtually zero
 maintenance.  Where ordinary "wet-cell" batteries require monthly checks
 of electrolyte levels, the gel-cells are sealed, using an electrolyte
 that is jellied with nothing to replenish.  They also offer higher
 charging efficiency than ordinary batteries and provide slightly higher
 output voltage down to complete discharge.  Expected life is two to
 three years, although some models may better this estimate by a great
 Examples of this class of battery are the Interstate, Dryfit Prevailer,
 Sonnenschein, Deka, Johnson Dynasty, and Exide Nautilus Megacycle brands.
 Don't confuse these batteries with the "maintenance-free" wet-electrolyte
 RV/marine batteries being sold in some department stores under brand
 names such as Delco Voyager and GNB Stowaway.  Unlike the true gel-cells,
 these batteries are basically sealed RV/marine batteries with slightly
 altered plate chemistries that reduce battery gassing (and, consequently,
 water loss).
 To determine how much battery capacity your application requires, add up
 the total anticipated amp-hours of all the 12-volt DC appliances you
 will be operating between recharges, including the demands of an inverter
 if you have one.  Select batteries that meet or exceed this amp-hour
 value, plus a considerable safety margin.  As an example, assume you will
 be recharging the batteries every day adnd your appliance use habits
 are as shown in Figure 2.
   AC              Current        Daily          Total Daily
 Appliance       Consumption**      Use          Consumption
 ------------   --------------   ----------     --------------
 TV set             5 Amp-hr      6.0 hours     30.0 Amp-hr
 Microwave         85 Amp-hr      0.1 hours      8.5 Amp-hr
 Hair Dryer       125 Amp-hr      0.1 hours     12.5 Amp-hr
 VCR                3 Amp-hr      3.0 hours      9.0 Amp-hr
 120-v Light        1 Amp-hr      3.0 hours      3.0 Amp-hr
 120-v Light        1 Amp-hr      4.0 hours      4.0 Amp-hr
 Blender            3 Amp-hr      0.1 hours      0.3 Amp-hr
 Toaster           90 Amp-hr      0.1 hours      9.0 Amp-hr
   Total AC appliance usage:                    76.3 Amp-hr
 ** Measured at the 12-volt input to the inverter.
   DC              Current        Daily          Total Daily
 Appliance       Consumption**      Use          Consumption
 ------------    ------------    ----------     -------------
 Refrigerator    0.25 Amp-hr     18.0 hours      4.5 Amp-hr
 Propane Alarm   0.35 Amp-hr     24.0 hours      8.4 Amp-hr
 Water Pump      4.00 Amp-hr      0.2 hours      0.8 Amp-hr
 Cassette Player 2.00 Amp-hr      4.0 hours      8.0 Amp-hr
 Porch Light     1.80 Amp-hr      3.0 hours      5.4 Amp-hr
 Interior Light  1.80 Amp-hr      4.0 hours      7.2 Amp-hr
   Total DC appliance usage:                    34.3 Amp-hr
   Total Battery Usage:        76.3 + 34.3 =   110.6 Amp-hr
 In this case, figuring a 50 percent safety margin, you would need at
 least 221.2 amp-hours worth of batteries.  Consequently, installing a
 pair of golf-cart batteries would meet your needs, with no power to
 spare.  likewise, three group-27 batteries would suffice, with some
 reserve power.
 Although routinely overlooked in battery manufacturers' literature and
 in many reference, most deep-cycle batteries (with the exception of the
 gel-cell and other sealed varieties) are benefited by a periodic
 controlled overcharge, which is often referred to as an equalization
 charge mode.  To equalize a battery, the charging is allowed to continue
 well after the point at which the battery is normally considered to be
 "full", taking care to avoid excessive battery heating or electrolyte
 In a typical equalization cycle, the battery voltage is allowed to rise
 to approximately 16 volts, where it is maintained for up to eight hours
 by adjustment of the charging current.  This process helps to mix up the
 electrolyte, which otherwise tends to "stratify" (i.e., separate into
 overlapping layers of acid and water), and is also useful in removing
 some sulfate deposits.  When performed properly, equalization doesn't
 make the battery boil over, but does produce fairly vigorous bubbling.
 At the end of this cycle, you can expect to add some water.
 Most battery manufacturers consider one equalization charge per month
 to be appropriate for batteries that are in a continuous state of charge
 and discharge;  less often is adequate for batteries that see a lot of
 standby service.  Due to the generation of considerable gas that
 accompanies this process, equalization shoud NEVER be performed on a
 sealed or gel-cell battery.
 Also, most 12-volt DC appliances will not tolerate the 16-plus volts,
 so remember to disconnect everything or detach the battery cables before
 you equalize.  Refer to Figure 3 for the suggest maintenance charge and
 equalization voltages for various batteries.  Obviously, a charger with
 equalization capability is needed; there is no way to alter voltage
 output on most RV converters.
                      Charge Cutoff   Maintenance    Equalization
                        Voltage         Voltage        Voltage
 Wet-Cell Battery         14.4            13.5           16.3
   @ 80 degrees F
 Wet-Cell Battery         13.9            13.3           15.8
   @ 100 degrees F
 Gel-Cell Battery         14.4            13.8           NA
   @ 80 degrees F
 Gel-Cell Battery         14.1            13.8           NA
   @ 100 degrees F
 The "charge cutoff voltage" is the battery voltage at which heavy
 recharging should cease; the "maintenance voltage" is the voltage at
 which the battery can be safely maintained for long periods of time
 without excessive water loss.
 As a final thought, remember that lead-acid batteries generate highly
 explosive gases.  The larger the battery bank, the more gas is produced.
 Do not mount any battery in an unvented location, and avoid any sparks
 or open flame around the battery (particularly during and shortly after
 recharging).  Making or breaking electrical connections at the battery
 terminals is particularly dangerous.  Battery explosions often shower
 large areas with acid.  Wear eye, face, and skin protection, and give
 the bank plenty of time to "air out" before attempting any maintenance
 or inspection.

Learn About Wiring Solar Panels And Batteries


There are three types of wiring configurations that are relatively easy to learn. Once mastered, the job of wiring batteries or solar modules becomes easy as pie. The three configurations are: 


Series wiring


Parallel wiring


And a combination of the two known simply as series/parallel wiring.


In any DC generating device such as a battery or solar module, you will always have a negative (-) terminal and a positive (+). Electrons or (current) flows from the negative terminal through a load to the positive terminal. 


For ease of explanation we shall refer to a solar module or battery as a "Device"


Series Wiring


To wire any device in series you must connect the positive terminal of one device to the negative terminal of the next device




Important: When you wire devices in series the individual voltages of each device is additive. In other words if each device in the above example had the potential of producing 12 volts, then 12 + 12 + 12 + 12 = 48 volts. If these devices were batteries then the total voltage of the battery pack would be 48 volts. If they were solar modules that produced 17 Volts each then the total voltage of the solar array would be 68 volts.


The second important rule to remember about series circuits is that the current or amperage in a series circuit stays the same. So if these devices were batteries and each battery had a rating of 12 Volts @ 220 Amp hours then the total value of this series circuit would be 48 Volts @ 220 Amp hours. If they were solar modules and each solar module had a rating of 17 volts and were rated at 5 amps each then the total circuit value would be 68 volts @ 5 amps.


In the example below two 6 Volt 350 Amp hour batteries were wired in series which yields 6 Volts + 6 Volts = 12 Volts @ 350 Amp hours.



If the above devices were solar modules which were rated at 17 volts each @ 4.4 amps then this series circuit would yield 34 volts at 4.4 amps.


Remember the Voltage in a series circuit is additive and the Current stays the same.



Parallel Circuits


To wire any device in parallel you must connect the positive terminal of the first device to the positive terminal of the next device and negative terminal of the first device to the negative terminal of the next device.



Important: When you wire devices in parallel the resulting Voltage and Current is just the opposite of a series circuit. Instead the Voltage in a parallel circuit stays the same and the Current is additive. If each device in the above example had the potential of producing 350 Amp hours then 350 + 350  = 700 Amp hours, the Voltage would stay the same. 


If these devices were batteries then this parallel circuit would yield total voltage of 12 volts @ 700 Amp hours. If these devices were solar modules that produced 17 Volts @ 4.4 amps each then the this parallel circuit would yield 17 Volts @ 8.8 amps.


In the example below four 17 Volt @ 4.4 Amp solar panels were wired in parallel which yields 4.4 Amps + 4.4 Amps + 4.4 Amps + 4.4 Amps = 17.6 amps total @ 17 volts



if the above devices were batteries which were rated at 12 volts each @ 220 Amps hours then this parallel circuit would yield 12 volts @ 880 Amp hours.


Remember the Voltage in a parallel circuit stays the same and the Current is additive.


Series/Parallel Circuits


Hold on to your hats because here's where it gets a little wild. Actually you've already learned all you need to know to under stand series/parallel circuits.


A Series/parallel circuit is simply two or more series circuits that are wired together in parallel.






In the above example two separate pairs of 6 Volt batteries have been wired in series and each of these series pairs have been wired together in parallel. 


You might be asking why in the world would someone want to put them self through this ? Well lets say that you want to increase the Amp hour rating of a battery pack so that you could run your appliances longer but you needed to wire the pack in such a way as to keep the battery pack at 12 volts, or you want to increase the charging capacity of your solar array but you needed to wire the solar modules in such a way as to keep the solar array at 34 volts, well, series/parallel is the only way to do that. 


Remember in parallel circuits the current is additive so thus you increase your run time or Amp hour capacity or in the case of solar modules, you increase your charging current by wiring the batteries or solar modules in parallel. Since we need 12 volts and have 6 volt batteries or in the case of solar modules we need 34 Volts and have 17 Volt modules on hand on hand, wiring the batteries or solar modules in series allows us to get the 12 Volts or 34 Volts that we need. 


An easy way to visualize it would be to start by wiring the batteries in individual sets that will give you the voltage that you need. Lets say that you need 24 volts but have six volt batteries on hand. First wire four of the batteries in series to get 24 volts. (Remember wire in series to increase the voltage) and continue to wire additional sets of four batteries until the batteries are used up. 


Next wire each series set of four batteries in parallel to each other (Positive to positive to positive and so on and then negative to negative to negative and so on) until each series set is wired together in parallel. If each series set of batteries equals 24 Volts at 350 Amp hours then five series sets wired to each other in parallel would give you a 24 Volt @ 1750 Amp hour battery pack.


Remember: In a series circuit the current stays the same but the voltage is additive. In a parallel circuit the voltage stays the same but the current is additive


Here is a sample wiring diagram for a 12v inverter and eight 6v batteries.

below is a description of what he did to wire them.

(inverter is a 12V inverter, I had to wire the batteries together to make up a 12V battery bank. To start with, each battery is 6V. Just like with solar panels, connecting batteries in series (positive to negative) causes the voltage to add while the current (AHr) stays the same. So I connected the positive of one 6V, 225AHr battery to the negative of another 6V, 225AHr battery to end up with a 12V, 225AHr series string of batteries. That gave me my 12V. So I repeated this with the remaining 6 batteries giving me 4 separate 12V, 225AHr series strings. Since connecting them further in series would give me too high a voltage, all that was left was to connect these strings in parallel. Connecting in parallel leaves the voltage the same but the current (AHr) adds. So now I had 4 parallel connected strings for a total battery bank size of 12V, 900AHr (4 x 225). Unfortunately I was too busy installing the system to remember to take a photo but the following diagram illustrates what I did.)
- Battery wiring diagram. -


How to Size Wiring for Your System


Properly sized wire can make the difference between inadequate and full charging of a battery system, between dim and bright lights, and between feeble and full performance of tools and appliances. Designers of low voltage power circuits are often unaware of the implications of voltage drop and wire size.

In conventional home electrical systems (120/240 volts ac), wire is sized primarily for safe amperage carrying capacity (ampacity). The overriding concern is fire safety. In low voltage systems (12, 24, 48VDC) the overriding concern is power loss. Wire must not be sized merely for the ampacity, because there is less tolerance for voltage drop (except for very short runs). For example, at a constant wattage load, a 1V drop from 12V causes 10 times the power loss of a 1V drop from 120V.


Universal Wire Sizing Chart
A 2-Step Process

This chart works for any voltage or voltage drop, American (AWG) or metric (mm2) sizing. It applies to typical DC circuits and to some simple AC circuits (single-phase AC with resistive loads, not motor loads, power factor = 1.0, line reactance negligible).

STEP 1: Calculate the Following:

VDI = Voltage Drop Index (a reference number based on resistance of wire)
FEET = ONE-WAY wiring distance (1 meter = 3.28 feet)
%VOLT DROP = Your choice of acceptable voltage drop (example: use 3 for 3%)

STEP 2: Determine Appropriate Wire Size from Chart

Compare your calculated VDI with the VDI in the chart to determine the closest wire size. Amps must not exceed the AMPACITY indicated for the wire size.

Wire Size
Area mm2
Not Recommended
Metric Size
by cross-sectional area
(VDI x 1.1 = mm2)
(VDI x 1.7 = mm2)
Available Sizes: 1 1.5 2.5 4 6 10 16 25 35 50 70 95 120 mm2


20 Amp load at 24V over a distance of 100 feet with 3% max. voltage drop
VDI = (20x100)/(3x24) = 27.78
For copper wire, the nearest VDI=31.
This indicates #2 AWG wire or 35mm2

NOTES: AWG=Amercan Wire Gauge. Ampacity is based on the National Electrical Code (USA) for 30 degrees C (85 degrees F) ambient air temperature, for no more than three insulated conductors in raceway in freee air of cable types AC, NM, NMC and SE; and conductor insulation types TA, TBS, SA, AVB, SIS, RHH, THHN and XHHW. For other conditions, refer to National Electric Code or an engineering handbook.


Use the following chart as your primary tool in solving wire sizing problems. It replaces many pages of older sizing charts. You can apply it to any working voltage, at any percent voltage drop.

Determining tolerable voltage drop for various electrical loads

A general rule is to size the wire for approximately 2 or 3% drop at typical load. When that turns out to be very expensive, consider some of the following advice. Different electrical circuits have different tolerances for voltage drop.

LIGHTING CIRCUITS, INCANDESCENT AND QUARTZ HALOGEN (QH): Don't cheat on these! A 5% voltage drop causes an approximate 10% loss in light output. This is because the bulb not only receives less power, but the cooler filament drops from white-hot towards red-hot, emitting much less visible light.

LIGHTING CIRCUITS, FLUORESCENT: Voltage drop causes a nearly proportional drop in light output. Flourescents use 1/2 to 1/3 the current of incandescent or QH bulbs for the same light output, so they can use smaller wire. We advocate use of quality fluorescent lights. Buzz, flicker and poor color rendition are eliminated in most of today's compact fluorescents, electronic ballasts and warm or full spectrum tubes.

DC MOTORS may be used in renewable energy systems, especially for water pumps. They operate at 10-50% higher efficiencies than AC motors, and eliminate the costs and losses associated with inverters. DC motors do NOT have excessive power surge demands when starting, unlike AC induction motors. Voltage drop during the starting surge simply results in a "soft start".

AC INDUCTION MOTORS are commonly found in large power tools, appliances and well pumps. They exhibit very high surge demands when starting. Significant voltage drop in these circuits may cause failure to start and possible motor damage. Follow the National Electrical Code. In the case of a well pump, follow the manufacturer's instructions.

PV-DIRECT SOLAR WATER PUMP circuits should be sized not for the nominal voltage (ie. 24V) but for the actual working voltage (in that case approximately 34V). Without a battery to hold the voltage down, the working voltage will be around the peak power point voltage of the PV array.

PV BATTERY CHARGING CIRCUITS are critical because voltage drop can cause a disproportionate loss of charge current. To charge a battery, a generating device must apply a higher voltage than already exists within the battery. That's why most PV modules are made for 16-18V peak power point. A voltage drop greater than 5% will reduce this necessary voltage difference, and can reduce charge current to the battery by a much greater percentage. Our general recommendation here is to size for a 2-3% voltage drop. If you think that the PV array may be expanded in the future, size the wire for future expansion. Your customer will appreciate that when it comes time to add to the array.

WIND GENERATOR CIRCUITS: At most locations, a wind generator produces its full rated current only during occasional windstorms or gusts. If wire sized for low loss is large and very expensive, you may consider sizing for a voltage drop as high as 10% at the rated current. That loss will only occur occasionally, when energy is most abundant. Consult the wind system's instruction manual.

More techniques for cost reduction

ALUMINUM WIRE may be more economical than copper for some main lines. Power companies use it because it is cheaper than copper and lighter in weight, even though a larger size must be used. It is safe when installed to code with AL-rated terminals. You may wish to consider it for long, expensive runs of #2 or larger. The cost difference fluctuates with the metals market. It is stiff and hard to bend, and not rated for submersible pumps.

HIGH VOLTAGE PV MODULES: Consider using higher voltage modules and a MPPT solar charge controller to down convert to the system voltage (e.g. 12, 24 and 48V) to compensate for excessive voltage drop. In some cases of long distance, the increased module cost may be lower than the cost of larger wire.

SOLAR TRACKING: Use a solar tracker (e.g. Zomeworks or Unirac) so that a smaller array can be used, particularly in high summer-use situations (tracking gains the most energy in summer when the sun takes the longest arc through the sky). The smaller PV array will require smaller wire.

WATER WELL PUMPS: Consider a slow-pumping, low power system with a storage tank to accumulate water. This reduces both wire and pipe sizes where long lifts or runs are involved. A PV array-direct pumping system may eliminate a long wire run by using a separate PV array located close to the pump. Many of our solar water pumps are highly efficient DC pumps that are available up to 48V. We also make AC versions and converters to allow use of AC transmitted over great distances. These pumps draw less running current, and far less starting current than conventional AC pumps, thus greatly reducing wire size requirements.


Wire Gauge Tables 


Give it a try!


AWG Copper Wire Table
AWG Diam. (mils) Circular mils Ohms/1000ft Current Carrying Fusing Current Feet per Pound
0000 460 212000 0.050 - - 1.56
000 410 168000 0.063 - - 1.96
00 365 133000 0.077 - - 2.4826
0 324.85 105531 0.096 - - 3.1305
1 289.3 83694 0.1264 119.6 - 3.947
2 257.6 66358 0.1593 94.8 - 4.977
3 229.4 52624 0.2009 75.2 - 6.276
4 204.3 41738 0.2533 59.6 - 7.914
5 181.9 33088 0.3915 47.3 - 9.980
6 162 26244 0.4028 37.5 668 12.58
7 144.3 20822 0.5080 29.7 561 15.87
8 128.5 16512 0.6405 23.6 472 20.01
9 114.4 13087 0.8077 18.7 396 25.23
10 101.9 10384 1.018 14.8 333 31.82
11 90.7 8226 1.284 11.8 280 40.12
12 80.8 6529 1.619 9.33 235 50.59
13 72.0 5184 2.042 7.40 197 63.80
14 64.1 4109 2.575 5.87 166 80.44
15 57.1 3260 3.247 4.65 140 101.4
16 50.8 2581 4.094 3.69 117 127.9
17 45.3 2052 5.163 2.93 98.4 161.3
18 40.3 1624 6.510 2.32 82.9 203.4
19 35.9 1289 8.210 1.84 69.7 256.5
20 32.0 1024 10.35 1.46 58.4 323.4
21 28.5 812 13.05 1.16 - 407.8
22 25.3 640 16.46 .918 41.2 514.12
23 22.6 511 20.76 .728 - 648.4
24 20.1 404 26.17 .577 29.2 817.7
25 17.9 320 33.0 .458 - 1031
26 15.9 253 41.62 .363 20.5 1300
27 14.2 202 52.48 .288 - 1639
28 12.6 159 66.17 .228 14.4 2067
29 11.3 128 83.44 .181 - 2607
30 10.0 100 105.2 .144 10.2 3287
31 8.9 79 132.7 .114 - 4145
32 8.0 64 167.3 .090 - 5227
33 7.1 50.125 211.0 .072 - 6591
34 6.3 39.75 266.0 .057 5.12 8310
35 5.6 31.5 335 .045 4.28 10480
36 5.0 25.0 423 .036 3.62 13210
37 4.45 19.83 533 .028 - 16660
38 3.97 15.7 673 .022 2.5 21010
39 3.5 12.47 848 .018 - 26500
40 3.14 9.89 1070 .014 1.77 33410
41 2.8 7.842 - - 1.52 -
42 2.494 6.219 - - 1.28 -
43 2.221 4.932 - - 1.060 -
44 1.978 3.911 - - 0.916 -
45 1.761 3.102 - - - -
46 1.568 2.460 - - - -
47 1.397 1.951 - - - -
48 1.244 1.547 - - - -
49 1.107 1.227 - - - -
50 0.986 0.973 - - - -


Observe Proper Wire Size

The most important wiring practice is to observe proper wire size. Failure to use adequate size can result in fire. Even if fire doesn't result, wires that are too small will cause marginal performance of electrical equipment.


  Distance - Feet
  10 15 20 25 30 40 50
Amps Wire Gauge
5 18 16 14 12 12 10 10
10 14 12 10 10 10 8 6
15 12 10 10 8 8 6 6
20 10 10 8 6 6 6 4
25 10 8 6 6 6 4 4
30 10 8 6 6 4 4 2
40 8 6 6 4 4 2 2
50 6 6 4 4 2 2 1
60 6 4 4 2 2 1 0
70 6 4 2 2 1 0 2/0
80 6 4 2 2 1 0 3/0
90 4 2 2 1 0 2/0 3/0
100 4 2 2 1 0 2/0 3/0
120 4 2 1 0 2/0 3/0 4/0
140 2 2 0 2/0 2/0 4/0 4/0
160 2 1 0 2/0 3/0 4/0 4/0+4
180 2 1 2/0 3/0 3/0 4/0+10 4/0+2
200 2 0 2/0 3/0 4/0 4/0+4 4/0+0

Using the Table

The table shows the wire size required for a 3% voltage drop in 12 Volt circuits. To use the table, first calculate the total length of the wire from the source to the device and back again. Next, determine the amount of current in the wire. The wire gauge is found at the intersection of Amps and Feet. In most load circuits, a 3% drop is quite acceptable. In charging circuits it often pays to have less of a drop. Always use one size bigger if practical.

The National Electrical Code [NEC] requires their own cable sizing for premises wiring. Refer to the NEC rules to determine building wiring, as this page relates to electronic equipment wiring. For reference, the ampacity of copper wire at 300C for common wire sizes
14 AWG may carry a maximum of 20 Amps in free air, or 15 Amps as part of a 3 conductor cable.
12 AWG may carry a maximum of 25 Amps in free air, or 20 Amps as part of a 3 conductor cable.
10 AWG may carry a maximum of 40 Amps in free air, or 30 Amps as part of a 3 conductor cable.
8 AWG may carry a maximum of 70 Amps in free air, or 50 Amps as part of a 3 conductor cable.

How Much Voltage Should Your Solar Panel Be?

A good question when building solar panels is how much voltage do you want it to put out? It's basically a question of balancing size and power. First though, let's talk about the minimum voltage that you'd need for the 3 main renewable energy system voltages: 12V, 24V and 48V.

A fully charged 12 volt battery doesn't exactly hold 12 volts. Usually the fully charged voltage is anywhere from 12.6 volts and 14.8 volts (depending on the type of battery you have). That means at a minimum your solar panel will need to put out at least 15 volts. Since each solar cell puts out 0.55 volts you'll need at least 27 cells to charge your battery. Since each solar panel should have cells that are multiples (2x2, 3x3 etc) your panel should have at least 28 cells for aesthetic reasons. It would look a little weird to see a panel with only 27 cells.

The same applies to 24 and 48 volt panels. Fully charged 24 volt batteries (usually two 12 volts wired in series) hold between 25 and 30 volts. Your panel should generate at least 30 volts (55 cells minimum). A 48 volt panel fully charged will be between 50 and 60 volts so you'll need to generate that much.

Solar Energy, Power, Electricity

Multicrystalline Solar Cell

Basic about Solar Energy, Solar Power and Solar Electricity.
In basic about solar energy, solar power and solar eletricity we will talk about the basic things behind this power. Formulas that will be used to find out which Solar Panel you should use and which battery you should select. And how much Solar Panels do you need to power up lights and other applications. Here are the main things you need to know and that will be used to calculate your needs. AC-DC system, Volt, Current(Ampere), Power(Watt), Resistance, Series and Parallel connecting.

AC-DC system
Ac stands for Alternative Current. Alternative current is almost that we found in wall outlet or electric outlet. Clever say'd that we found in wall. It is 230 Volt. DC stands for Direct Current. In solar panels it is used 12 volt dc system. DC is that current we can found in cells, batteries, and using adapters or regulators. See the picture of a dell charger. Dell charger also converts AC Current to DC 5.4 Volt and 2410mA. Solar Panels also uses DC voltage and Current.

Dell Charger


Power Circle
Power Circle

Voltage is the electromotive force (pressure) applied to an electrical circuit measured in volts (E).
Example. P=200W, I=4.0A. If we have a value of watt and ampere and we want to find out how much volt does it use then we should use this from Power Circle. E=P/I. 200/4.0=50V. So we found voltage is 50V.

Current is the flow of electrons in an electrical circuit measured in amperes (I).
Example. P=100W, E=12V. We want to find out how much ampere does it use. We take a look at circle. I=P/E. 100/12=8.33A. Current usage is 8.33A.

Power is the product of the voltage times the current in an electrical circuit measured in watts (P).
Example. E=220V, I=0,4A. This example is taken from picture of Dell Charger. We have 400mA=0,4A. Take a look at circle P=E*I. 220V * 0,4A=88W. Answer is dell charger use 88Watt.

Resistance is the opposition to the flow of electrons in an electrical circuit measured in ohms (R). Increased resistance gives higher voltage and higher power(watt).
Example. E=12V, I=3.0A. We want to find out resistance. We use formula R=E/I. 12V/3.0A = 4 Ohm.

Parallel Connecting - 12V System
Parallel Connecting solar panels gives higher current. And voltage will remain the same. Parallel Connecting is best for us. Because we do not need high voltage. Normal battery is 12v. And by selecting high voltage require higher voltage charge controller. To connect solar panels in parallel we have to connect plus + to plus and minus - to minus. See the picture for details.

Parallel connected solar panels


Series Connecting - 24V System
By connecting Solar Panels in series connection. It will increase Voltage and current Amps. will remain the same. To connect solar panels in series we have to connect plus + to - minus on next panel. See the picture for details. In this example we have connected 2 solar panels in series which will give 24v output.

Series connected solar panels


Series and Parallel Connecting - 24v System
If we are creating a 24v system. So then we have to connect two solar panels in series and connect two series connected panels in parallel as showed in figure under. Which increases voltage to 24v and when we have connected two other 24v connected solar panels in parallel so voltage remains same and increases current Amps. If we want to increase output current on 24v so we connect more solar panels in parallel same way under as shown in figure under.

Series and Parallel connected solar panels




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