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

Flooded

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

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

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-
 dependent.
 
 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-
 discharge.

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
 effect.
 
 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.
 
 WHAT TO BUY
 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
 themselves.
 
 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)
 does.
 
 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
 margin.
 
 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.
 
 Figure 2 - TYPICAL POWER CONSUMPTIONS
   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.
 
 HOW TO KEEP THEM HAPPY
 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
 boil-off.
 
 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.
 
 Figure 3 - BATTERY VOLTAGES
                      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