To determine if the battery system is experiencing a problem, fully charge the batteries then shut off the charger and remove all electrical loads. Allow each battery in the system to stand on open-circuit for about one hour. Measure the voltage of each battery. If the battery voltage spread exceeds .30 volts for a 12 volt battery, a problem is indicated. Battery voltage alone does not confirm a problem. When the voltage spread indicates a problem, confirmation is accomplished by taking electrolyte specific gravity readings using a hydrometer. If the specific gravity readings show a spread greater than .030 (30 points), give the batteries equalization charge i.e slow constant current charge @ 3% of the battery capacity in amperes.

Lead-acid battery is a electrical storage device that uses a reversible chemical reaction to store energy. It uses a combination of lead plates or grids and an electrolyte consisting of a diluted sulphuric acid to convert electrical energy into potential chemical energy and back again. The electrolyte of lead-acid batteries is hazardous to your health and may produce burns and other permanent damage if you come into contact with it. Thus, when dealing with electrolyte protect yourself appropriately.

Batteries are typically built for specific purposes and they differ in construction accordingly. Broadly speaking, there are two applications that manufacturers build their batteries for: Starting and Deep Cycle. As the name implies, Starter Batteries are meant to get combustion engines going. They have many thin lead plates which allow them to discharge a lot of energy very quickly for a short amount of time. However, they do not tolerate being discharged deeply, as the thin lead plates needed for starter currents degrade quickly under deep discharge and re-charging cycles. Most starter batteries will only tolerate being completely discharged a few times before being irreversibly damaged.
More
Deep Cycle batteries have thicker lead plates that make them tolerate deep discharges better. They cannot dispense charge as quickly as a starter battery but can also be used to start combustion engines. You would simply need a bigger deep-cycle battery than if you had used a dedicated starter type battery instead. The thicker the lead plates, the longer the life span, all things being equal. Battery weight is a simple indicator for the thickness of the lead plates used in a battery. The heavier a battery for a given group size, the thicker the plates, and the better the battery will tolerate deep discharges. Some "Marine" batteries are sold as dual-purpose batteries for starter and deep cycle applications. However, the thin plates required for starting purposes inherently compromise deep-cycle performance. Thus, such batteries should not be cycled deeply and should be avoided for deep-cycle applications unless space/weight constraints dictate otherwise.

Battery Containers come in several different configurations. Flooded Batteries can be either the sealed or open variety. Sealed Flooded Cells are frequently found as starter batteries in cars. Their electrolyte cannot be replenished. When enough electrolyte has evaporated due to charging, age, or just ambient heat, the battery has to be replaced. Deep-Cycle Flooded cells usually have removable caps that allow you to replace any electrolyte that has evaporated over time. Take care not to contaminate the electrolyte - wipe the exterior container while rinsing the towel frequently. VRLA batteries remain under constant pressure of 1-4 psi. This pressure helps the recombination process under which 99+% of the Hydrogen and Oxygen generated during charging are turned back into water.
More
The two most common VRLA batteries used today are the Gel and Absorbed Glass Mat (AGM) variety.
Gel batteries feature an electrolyte that has been immobilized using a gelling agent like fumed silica.
AGM batteries feature a thin fiberglass felt that holds the electrolyte in place like a sponge.
Neither AGM or Gel cells will leak if inverted, pierced, etc. and will continue to operate even under water.

Battery Cells are the most basic individual component of a battery. They consist of a container in which the electrolyte and the lead plates can interact. Each lead-acid cell fluctuates in voltage from about 2.12 Volts when full to about 1.75 volts when empty. Note the small voltage difference between a full and an empty cell (another advantage of lead-acid batteries over rival chemistries).

The nominal voltage of a lead-acid battery depends on the number of cells that have been wired in series. As mentioned above, each battery cell contributes a nominal voltage of 2 Volts, so a 12 Volt battery usually consists of 6 cells wired in series.

The State of Charge describes how full a battery is. The exact voltage to battery charge correlation is dependent on the temperature of the battery. Cold batteries will show a lower voltage when full than hot batteries. This is one of the reasons why quality alternator regulators or high-powered charging systems use temperature probes on batteries.

The Depth of Discharge (DOD) is a measure of how deeply a battery is discharged. When a battery is 100% full, then the DOD is 0%. Conversely, when a battery is 100% empty, the DOD is 100%. The deeper batteries are discharged on average, the shorter their so-called cycle life. For example, starter batteries are not designed to be discharged deeply (no more than 20% DOD). Indeed, if used as designed, they hardly discharge at all: Engine starts are very energy-intensive but the duration is very short. Most battery manufacturers advocate not discharging their batteries more than 50% before re-charging them.

The Amp-hour (Ah) Capacity of a battery tries to quantify the amount of usable energy it can store at a nominal voltage. All things equal, the greater the physical volume of a battery, the larger its total storage capacity. Storage capacity is additive when batteries are wired in parallel but not if they are wired in series. Most marine, automotive, and RV applications use 12V DC. You have the choice to either buy a 12V battery or to create a 12V system by wiring several lower-voltage batteries/cells in Series.
More
When two 6V, 100Ah batteries are wired in Series, the voltage is doubled but the amp-hour capacity remains 100Ah (Total Power = 1200 Watt-hours).
You may decide to wire batteries in series because a single 12V battery with the right storage capacity is simply too heavy, unwieldy, or awkward to lift into place. Batteries consisting of fewer cells (and hence lower voltage) in series can provide the same storage capacity yet be portable. It is not unusual to see solar power installations where the battery bank consists of a sea of 2V batteries that have been wired in series.
Two 6V, 100Ah batteries wired in Parallel will have a total storage capacity of 200Ah at 6V (or 1200 Watt-hours).
Battery banks consisting of 12V batteries wired in parallel are often seen on OEM installations in boats and RVs alike. Such banks are simple to wire up and require a minimum of cabling. However, the wiring must have the capacity to deal with a full battery bank.

Since batteries depend on a chemical reaction to produce electricity, their Available Capacity depends in part on how quickly you attempt to charge or discharge them relative to their Total Capacity. The Total Capacity is frequently abbreviated to C and is a measure of how much energy the battery can store. Available Capacity is always less than Total Capacity. Typically, the amp-hour capacity of a battery is measured at a rate of discharge that will leave it empty in 20 hours (a.k.a. the C/20 rate). If you attempt to discharge a battery faster than the C/20 rate, you will have less available capacity and vice-versa. The more extreme the deviation from the C/20 rate, the greater the available (as opposed to total) capacity difference. However, as you will discover in the next section, this effect is non-linear. The available capacity at the C/100 rate (i.e. 100 hours to discharge) is typically only 10% more than at the C/20 rate. Conversely, a 10% reduction in available capacity is achieved just by going to a C/8 rate (on average). Thus, you are most likely to notice this effect with engine starts and other high-current applications like inverters, windlasses, desalination, or air conditioning systems. For example, the starter in an engine will typically quickly outstrip the capacity of the battery to keep cranking it for any length of time. Hence the tip from mechanics to wait some time between engine start attempts. Not only does it allow the engine starter to cool down, it also allows the chemistry in the battery to "catch-up". As the battery comes to a new equilibrium, its available capacity increases. A very elegant equation developed in 1897 by a scientist called Peukert describes the charging and discharging behavior of batteries.

As you can see below, the Peukert equation consists of several factors.
Peukerts Equation: Inx T = C
where
I is the current (usually measured in amperes)
T is time (usually measured in hours)
n is the Peukert number/exponent
C is the theoretical storage capacity of the battery (usually measured in amp-hours). Use the C/100 capacity or add 10% to the storage capacity at the C/20 rate
More
As you can see, the available current is dependent on the rate of discharge and the Peukert exponent for the battery. The closer the exponent is to 1 (one), the less the available capacity of a battery will be affected by fast discharges. Peukerts numbers are derived empirically and are usually available from manufacturers. They range from about 2 for some flooded batteries down to 1.05 for some AGM cells. The average peukerts exponent is 1.2 though the exact number depends on the battery construction and chemistry. The following image shows the dramatic impact of the Peukerts exponent on the available capacity of a 120Ah battery, depending on the ampere draw. As you can see, the lower the Peukerts Exponent, the lesser the effect on available capacity. Note the dramatic difference in Available Capacity between the average flooded cell (n = 1.20) and a deep cycle AGM (n = 1.08) with high-current applications. When the time comes to charge a battery, the Peukerts effect also comes into play. The capacity of a battery to absorb a charge during the bulk phase is also dependent on it's Peukerts number. This is one of the reasons why AGM cells can be bulk charged at much higher rates than either Gel or Flooded cells.

Reserve Minutes are a measure of how long your battery can sustain a load before it's available capacity has been completely used up. This measure is especially useful for folks who want to run inverters, fridges, and other large loads. The following chart has a logarithmic time scale (minutes) - hence, the non-linear nature of the Peukert effect is smoothed out quite a bit. Note how batteries that have a high Peukerts Exponent will quickly run out of capacity with high loads. Here, the low-exponent battery will last over 100 minutes with a 50 ampere load, while the high-exponent battery will last about 20 minutes. Thus, anytime you deal with large loads relative to the battery capacity available, chose a low-exponent battery. This is why many wheel-chairs and other electrically motorized vehicles use AGMs. This chart answers why starter batteries are built to have a low Peukerts exponent. Otherwise, they'd simply not be able to crank an engine for more than a few seconds. However, the thin plates that allow flooded cells to work as starter batteries also make them too fragile for deep-cycle use.

The conversion efficiency denotes how well a battery converts an electrical charge into chemical energy and back again. The higher this factor, the less energy is converted into heat and the faster a battery can be charged without overheating (all other things being equal). The lower the internal resistance of a battery, the better its conversion efficiency. One of the main reasons why lead-acid batteries dominate the energy storage markets is that the conversion efficiency of lead-acid cells at 85%-95% is much higher than Nickel-Cadmium (a.k.a. NiCad) at 65%, Alkaline (a.k.a. NiFe) at 60%, or other inexpensive battery technologies.

Battery manufacturers define the end-of-life of a battery when it can no longer hold a proper charge (for example, a cell has shorted) or when the available battery capacity is 80% or less than what the battery was rated for. The life of Lead Acid batteries is usually limited by several factors: Cycle Life is a measure of how many charge and discharge cycles a battery can take before its lead-plate grids/plates are expected to collapse and short out. The greater the average depth-of-discharge, the shorter the cycle life. Age also affects batteries as the chemistry inside them attacks the lead plates. The healthier the "living conditions" of the batteries, the longer they will serve you. Lead-Acid batteries like to be kept at a full charge in a cool place. Only buy recently manufactured batteries, so learn to decipher the date code stamped on every battery... (inquire w/manufacturer). The longer the battery has sat in a store, the less time it will serve you! Since lead-acid batteries will not freeze if fully charged, you can store them in the cold during winter to maximize their life. Construction has a big role in battery life too, some designs are better at preserving batteries than others and the suitability of a design for a given application plays a role also. For example, flooded lead-acid cells will typically fare worse than their VRLA bretheren in operations that involve a lot of jerky motion - the immobilized plates in VRLA cells will be stressed less than suspended plates in cheap flooded cells. Plate Thickness helps - the thicker the plates, the more abuse, charge and discharge cycles they can take. Thicker plates will also survive any equalization treatments for sulphation better. The heavier the battery for a given group size, the thicker the plates are, so you can use weight as one guide to buying lead-acid batteries. Sulphation is a constant threat to batteries that are not fully re-charged. A layer of lead sulphate can form in these cells and inhibit the electro-chemical reaction that allows you to charge/discharge batteries. Many batteries can be saved from the recycling heap if they are Equalized In closing, the design life of a battery depends in part on its construction, its type, the thickness of the plates, its charging profiles, etc. All these factors come together to determine just how long your battery may ultimately serve you.

Sulphation layers form barrier coats on the lead plates in batteries that inhibit their ability to store and dispense energy. The equalization step is a last resort to break up the Sulphate layers using a controlled overcharge. The process will cause the battery electrolyte to boil and gas, so it should be only done under strict supervision and with the proper precautions. It is much more tricky to equalize a VRLA battery than a flooded battery with removable caps.

Batteries start to gas when you attempt to charge them faster than they can absorb the energy. The excess energy is turned into heat, which then causes the electrolyte to boil and evaporate. The evaporated electrolyte can be replenished in batteries with removable caps such as most flooded deep-cycle batteries. Many car batteries are sealed and thus need to be replaced when their electrolyte evaporates over time. Since AGM and Gel cells are always sealed, it is very important to guarantee they are not overcharged. The only way to ensure this is to use a temperature-compensated charging system. Such chargers use a temperature probe on the battery to ensure that the battery does not get too hot. As the battery heats up, the charging current is reduced to prevent thermal runaway, a very dangerous condition.

This is a very dangerous condition that can occur if batteries are charged too fast. One of the byproducts of Gassing are Oxygen and Hydrogen. As the battery heats up, the gassing rate increases as well and it becomes increasingly likely that the Hydrogen around it will explode. The danger posed by high Hydrogen concentrations is one of the reasons that the American Boat and Yachting Council (ABYC) requires that batteries be installed in separate, well-ventilated areas.

The self-discharge rate is a measure of how much batteries discharge on their own. The Self-Discharge rate is governed by the construction of the battery and the metallurgy of the lead used inside. For instance, flooded cells typically use lead alloyed with Antimony to increase their mechanical strength. However, the Antimony also increases the self-discharge rate to 8-40% per month. This is why flooded lead-acid batteries should be in use often or left on a trickle-charger. The lead found in Gel and AGM batteries does not require a lot of mechanical strength since it is immobilized by the gel or fiberglass. Thus, it is typically alloyed with Calcium to reduce Gassing and Self-Discharge. The self-discharge of Gel and AGM batteries is only 2-10% per month and thus these batteries need less maintenance to keep them happy.

Battery Acid can cause burns: Use extreme care when handling acid. If electrolyte is spilled or splashed onto clothing or the body, wash with water and neutralise with a solution of baking soda and water. Electrolyte splashed into eyes is extremely dangerous. If this occurs, gently open eyes and wash with cool clean water for 5 minutes. Call a doctor. If electrolyte is swallowed: Drink large quantities of water or milk and follow with milk of magnesia, beaten eggs or vegetable oil. Call a doctor. If it is necessary to prepare electrolyte: Always add concentrated acid to water - never water to acid. Store electrolyte in plastic containers with sealed cover. Do not store in the sun.