Batteries - An Overview:
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%.
Nearly all large rechargeable batteries in common use are Lead-Acid type. (There are some NiCads in use, but for most purposes the very high initial expense does cannot be justified by most consumers). The acid is typically 60% Sulphuric acid and 40% water. NiFe (Nickel-Iron) batteries are also available - these have a very long life, but rather poor efficiency (60-70%) and the voltages are different, making it more difficult to match up with standard 12v/24v/48v systems.
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 battery is actually a "hybrid", and falls 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).
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. If 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 start-up. 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".
Sealed Batteries
These batteries are made with vents that cannot be removed. Maintenance free batteries are also sealed. Sealed batteries are not totally sealed, as they must allow gas to vent during charging. If overcharged too many times, these batteries can lose enough water that they will die before their time. Most deep cycle batteries use Lead-Calcium plates for increased life, although most forklift batteries use Lead-Antimony. A few industrial batteries have special caps that convert the Hydrogen and Oxygen back into water, reducing water loss by up to 95%. Lead-Antimony batteries have a much higher self-discharge rate than Lead or Lead-Calcium, but the Antimony improves the mechanical strength of the plates, which can be an important factor in electric vehicles. They are generally used where they are under constant or very frequent charge, such as forklifts and floor sweepers. The Antimony increases plate life at the expense of higher self discharge. If left for long periods unused, these should be trickle charged to avoid damage from sulfation.
Temperature
Battery capacity is reduced as the temperature goes down. This is why your car battery dies on a cold winter morning. Capacity is increased at temperatures over 25° C (77° F), but battery life is shortened. Battery capacity is reduced by 50% at -22° F and for every 15° F over 75° F, battery life is cut in half, this is for ANY type of Lead-Acid battery, whether sealed, gelled, etc.
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%. 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 (Depth of Discharge). If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50% - but to do so, you would need a very large battery capacity in most cases.
Battery Voltages
All Lead-Acid batteries supply about 2.14 volts per cell when fully charged. Batteries that are stored for long periods will eventually lose all their charge, at about 1% to 10% per month. A "float" charge should be maintained on the batteries even if they are not used. Batteries self-discharge much faster at higher temperatures. The old myth about not storing batteries on concrete floors is just that - a myth. Lifespan can also be seriously reduced at higher temperatures - most manufacturers state this as a 50% loss in life for every 15°F over 77°F cell temperature. Lifespan is increased at the same rate if below 77°F, but capacity is reduced at lower temperatures at the rate of 10% per 15°F. Most deep cycle Lead-Acid batteries are actually an alloy of Lead and Calcium and/or Lead and Antimony for better plate life.
State of Charge
State of charge, or conversely, the depth of discharge (DOD) can be determined by measuring the voltage and/or the specific gravity of the acid with a hydrometer. This will NOT tell you how good the battery condition is - only a sustained load test can do that. Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC, and at 0% will be 1.75 VPC or less. Specific gravity will be 1.265 for a fully charged cell, and 1.13 or less for a totally discharged cell. Many batteries are sealed, and hydrometer reading cannot be taken, so you must rely on voltage.
"False" Capacity
A battery can meet all the tests for being at full charge, yet be much lower than its original capacity. If plates are damaged or partially gone from long use, the battery may give the appearance of being fully charged, but in reality acts like a battery of much smaller size. What is left of the plates may be fully functional, but with only 20% of the plates left... Batteries usually go bad for other reasons before reaching this point, but it is something to be aware of if your batteries seem to test OK but lack capacity and go dead very quickly under load.
Battery Charging
Battery charging takes place in 3 basic stages: Bulk, Absorption, and Float.
Bulk Charge
The first stage of 3-stage battery charging. Current is sent to batteries at the maximum rate they will accept while voltage rises to full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts.
Absorption Charge
The 2nd stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.4 to 15.5 volts.
Float Charge
The 3rd stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it's main purpose is to keep an already charged battery from discharging. Voltages at this stage are typically 13.2 to 13.6 volts.
Battery Chargers
Most garage and consumer type battery chargers are bulk charge only. Most of the cheaper charge controllers for PV systems are bulk only, or bulk plus partial absorption. When using larger battery banks or the more expensive gelled or AGM batteries it is important that the charge controller you choose has features to protect and prolong the life of your battery.
Charge Controllers
Regulators for solar systems are designed to keep the batteries charging at peak without overcharging. Meters for Amps and Volts are optional with most types. Some charge controllers have built in automatic equalization and taper charge or a maximum power point tracker, which optimizes power output from the PV panels.
Maximizers