A battery is an electrical storage device. Batteries do not make electricity, they store it, just as a water tank stores water for future use. As chemicals in the battery change, electrical energy is stored or released. In rechargeable batteries this process can be repeated many times. Batteries are not 100% efficient - some energy is lost as heat when charging and discharging. If you use 1000 watts from a battery, it might take 1200 watts or more to fully recharge it. Slower charging and discharging rates are more efficient. A battery rated at 180 amp-hours over 6 hours might be rated at 220 AH at the 20-hour rate, and 260 AH at the 48-hour rate. Typical efficiency in a lead-acid battery is 85%, in a NiCad battery it is about 65%.
Major Battery Types
Batteries are divided in two ways, by application (what they are used for) and construction (how they are built). The major applications are automotive, marine, and deep-cycle. The major construction types are flooded (wet), gelled, and AGM (absorbed glass mat).
Automotive, or SLI (starting, lighting & ignition)
SLI batteries are commonly used to start and run automobiles, where a very large starting current is needed for a short time. SLI batteries have many very thin plates with a large surface area, designed to be discharged no more than 1 to 5% from full charge. These can be damaged quickly if deep cycled.
Marine / Deep Cycle
Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates.
The so-called Marine deep-cycle batteries are actually a "hybrid", and fall between the SLI and deep-cycle batteries. These should not be discharged more than 50%. SLI batteries are usually rated at "CCA", or cold cranking amps, or "MCA, Marine cranking amps". Any battery with the capacity shown in "cranking amps" is NOT a deep-cycle battery, and some marine batteries with MCA ratings may be a hybrid (basically a battery with thicker plates than an automotive battery, but thinner than a true deep-cycle).
Industrial deep cycle batteries
These are often called traction batteries because of their widespread use in forklifts, golf carts, and floor sweepers.
These are used where power is needed over a longer period of time, and are designed to be "deep cycled", or discharged down as low as 20% of full charge (80% DOD, or Depth of Discharge).
Deep cycle batteries have much thicker plates than automotive batteries: Automotive batteries typically have plates about .04" (40/1000") thick, while forklift batteries may have plates as much as 1/6" (170/1000") thick - 4 times as thick as auto batteries. Most industrial deep-cycle batteries use Lead-Antimony plates rather than the Lead-Calcium used in most deep-cycle batteries. The Antimony reduces gassing when these batteries are subjected to their normal very high charge and discharge rates.
Deep Cycle Batteries
Deep cycle batteries are designed to put out 80% of their capacity time after time without damage. Automotive batteries can be severely damaged if heavily discharged a few times.
When discharged 80% and then recharged fully, deep cycle batteries can handle from a few hundred to 1000 complete cycles, where an automotive battery might be able to last for only 30 to 50 cycles. Deep cycle batteries may not work well in automobiles because they are not designed to put out the massive power needed for startup.
Many marine and RV batteries are "dual purpose" - these are deep cycle batteries that will also work as SLI batteries. Nearly all marine and many industrial batteries are sealed and may also be gelled, glass mat, and/or maintenance free. "Sealed" batteries are not totally sealed - the actual name for them is "Valve Regulated Sealed Lead-Acid Batteries", or VRSLB, sometimes called VRLA, or "Valve Regulated Lead-Acid".
These batteries may be standard or the so-called "maintenance free". All gelled batteries are sealed and a few are "valve regulated", which means that a tiny valve keeps a slight positive pressure. Nearly all AGM batteries are valve regulated..
Gelled batteries, or "Gel Cells" contain acid that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass that looks like hard Jell-O. The advantage of these batteries is that it is impossible to spill acid even if they are broken - the disadvantage is that they must be charged at a slower rate (C/20) to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive charger or they may be permanently damaged. This is not usually a problem with solar electric systems, but if an auxiliary generator or inverter bulk charger is used, current must be limited to the manufacturers specifications. Most better inverters commonly used in solar electric systems can be set to limit charging current to the batteries.
A "cycle" is a term used to describe the process of discharging a fully charged battery down to a particular state of discharge. The term "deep cycle" refers to batteries in which the cycle is from full charge to 80% discharge. A cycle for an automotive battery is about 5%.
How many times a battery can be cycled before it reaches the point where it can only be charged up to (usually) 80% of it's original capacity. This depends on how deep the battery is cycled. Cycle life ratings are not commonly published, and in many cases may not have been done, as it can be quite time consuming. A battery with a cycle life of 900 would take 3 years just to test.
Batteries that are designed to put out 80% of their capacity time after time without damage
Direct Current (DC)
The only type of electrical current which a PV module or battery can supply. Most home systems and some RV and boats use an inverter to convert the DC into 117 volts alternating current (AC), which is the standard household power.
Discharge and over discharge
Discharging is the process of using power from the battery. As power is drawn from the battery, chemical changes take place between the Lead, the acid, and the Lead Sulfate. Charging reverses this chemical change. Next to overcharging, the worst thing for a battery is to remain fully or partly discharged for long periods of time. This causes sulphation which is a hard Lead Sulfate coating on the plates. Sulfation can reduce battery capacity dramatically.
Depth of discharge. How much of the available charge has been used compared to 100%. SOC (state of charge is similar - it is how much is left). Most deep cycle batteries are considered to be at 0% SOC, or 100% DOD, when cell voltage is 1.75 volts, or 10.5 volts for a 12 volt battery.
Gassing occurs when more current is being fed to the battery than it can use. The excess current produces Hydrogen and Oxygen gasses. Some gassing is normal, but excessive gassing can indicate that the batteries are being overcharged. The gasses released are explosive if a spark or flame occurs, so adequate ventilation must be provided. Batteries normally start gassing at about 80-90% of full charge. A common fallacy is that you should stop charging as soon as the battery starts gassing. Most batteries start gassing at about 80% SOC, so if you quit charging at that point, you will never get a full charge. Most better chargers cut back on the current when the battery reaches this point to prevent excess gassing.
An instrument used to check the specific gravity (strength) of the electrolyte in the battery. Most Lead-Acid batteries will be in the range of 1.1 to 1.3 specific gravity, with most fully charged batteries being about 1.23 to 1.30 (some hydrometers multiply this number by 1000, so 1.23 would read as 1230.) Some batteries manufactured for use in very hot or very cold climates may have stronger or weaker acid. If so, it is usually marked on the battery.
Overcharging is one of the most destructive elements in battery life (the other is long term undercharging). Overcharging causes the plates to disintegrate and shed. These particles end up on the bottom of the cell. Eventually, the cells will short out, fall apart, break apart, or generally die. Overcharging also increases water loss tremendously, causing even more problems. Gelled cells can be damaged faster than flooded, and flooded can be damaged faster than AGM batteries by overcharging. Water loss is a particular problem with sealed gel cells, as the water cannot be replaced. In some cases, severe overcharging can also cause considable heat in cheaper batteries with high internal resistance, causing plates to buckle and cases to warp and break.
Reserve capacity is sometime used to rate deep cycle batteries. It is the number of minutes that a battery can maintain a useful voltage under a 25 amp discharge rate. Reserve capacity is probably a more useful measure than AH for batteries that run heavy loads, although most batteries also have tables that show the AH capacity at different discharge rates.
Batteries connected in series have the Positive (+) terminal of one battery tied to the Negative (-) terminal of the next battery. Power is taken from the two terminals at the end of the series string. Batteries wired in series supply the same current but the voltage is higher - for example four six-volt batteries in series will supply 24 volts. The amp-hour rating will be that of the smallest battery if different types are connected.. If old and new batteries are used together, the maximum current will be that of the weakest battery.
A condition in which a short electrical path is unintentionally created. An example would be setting a wrench on top of a battery touching the terminals. Batteries can supply thousands of amps if short circuited, melting the wrench, the terminals, and showering sparks and molten metal.
Specific Gravity (SG)
The measurement used to express electrolyte strength. SG compares the weight of the electrolyte to water, which has a SG of 1.000. SG changes somewhat with temperature, so most hydrometers come with a correction chart. A full charge should be about 1.265 at 77 degrees F (25 degrees C). This changes with temperature. This cannot be measured in sealed batteries. Pure acid has a SG of 1.835. A fully discharged battery will have a SG of about 1.12. SG should not be measured right after water is added as the reading will not be accurate until the electrolyte is fully mixed.
Even though Lead Sulfate is created in the materials of plates during normal discharging, this term is used to describe the generation of a different form of Lead Sulfate which will not readily convert back to normal material when the battery is charged. Sulfation occurs when a battery is stored too long in a discharged condition, if it is never fully charged, or if electrolyte has become abnormally low due to excessive water loss from overcharging and/or evaporation. Often sulfation can be corrected by charging very slowly (at low current) at a higher than normal voltage, usually at about 2.4 to 2.5 volts per cell at 2 to 8 amps.
The unit of measurement of electrical potential or "pressure". Most batteries come in 6, 12, & 24 volt. A single cell is 2 volts.
Volts per cell - a six volt battery has 3 cells, a 12 volt has 6. All Lead Acid batteries are 2 volts (nominal) per cell.
A term used to measure total power. It is amps multiplied by volts. 120 volts @ 1 amp is the same as 12 volts @ 10 amps. It is also amps x amps x resistance. One horsepower = about 750 watts. A battery that can supply 220 AH at 12 volts is equal to 2640 watts.