The battery bank in a home power system serves two purposes. It acts as a voltage stabilizer for the system, moderating the high voltages that can occur during battery charging and minimizing the low voltages common in high demand situations. It also acts as a power reservoir, supplying the power needed when the load demand exceeds the capabilities of the power (charging) source. For instance, if you have a solar panel that produces 51 watts of power and want it to power a light bulb that requires 100 watts, the additional 49 watts of power required by the light bulb will be supplied by the battery. The power used by the battery is then replaced when the light bulb is not in use.
RV and marine batteries are available in a variety of sizes to 100 amp hour and are normally 12 volt. They may be of the standard, serviceable type or the sealed, "maintenance-free" style. They are common in small home power and portable power systems.
These batteries are available in 220 to 300 amp hour capacities and are normally 6 volts per battery. They are a good choice for small to medium home systems.
These batteries, which are normally manufactured as individual 2 volt units, are available in a broad range of capacities to 3000 amp hours. Six 2 volt units are connected in series for 12 volt systems. They are an excellent choice for medium to large capacity home power systems.
The batteries previously discussed are called "lead-acid" batteries in that they consist of lead plates in a sulfuric acid solution and are the most common batteries utilized in home power applications. Nickel cadmium and nickel iron batteries consist of nickel alloy plates in an alkaline solution which dramatically alters the operating characteristics of the battery. These batteries are also good choices for home power systems but involve special considerations.
The normal use of a battery is known as cycling. Cycling is the process of removing electricity from and replacing it to a battery system. When electricity has been consumed and then replaced, it can be said that the battery has been cycled. The extent of the cycling or the depth of discharge is usually expressed as a percentage of the total battery capacity. Thus, if 50 amp hours is consumed from a 100 amp hour battery , it is said to be 50% discharged. A cycle exceeding about 20% of a batteries capacity is said to be a deep cycle, while a discharge and replacement of less than 20% is referred to as a shallow cycle.
Not all the energy that is put into a battery can be taken back out. Some 10 to 20 percent will be lost ultimately to heat through the electrochemical charging process. As such, 110 - 120 amp hours must be imparted to a battery to provide 100 amp hours of usable energy.
In addition to the losses incurred during charging, another source of energy loss is self-discharge. The "typical" lead acid battery will lose 10 - 20% of its energy in a month, more at high temperatures, less at lower temperatures. Lead calcium batteries have lower self discharge rates than lead antimony types, but perform poorly as true deep cycle batteries.
As well as affecting self discharge rates, temperature affects battery performance in other ways. The optimum performance temperature range for batteries is 60 - 80 degrees Fahrenheit. At these temperatures, the battery will perform at 100% of its rated capacity. As temperatures drop, battery longevity increases, but performance drops. The battery goes into a state of partial "suspended animation" and only some of itís potential power is available. You may have experienced this while starting your car in cold weather. (unless you are fortunate enough to live where there is no such thing as cold weather.) For example, at freezing (32 degrees Fahrenheit) some 65% of battery capacity can be utilized, but at zero only 40 percent is available.
Freezing of batteries is a major concern of northern climate inhabitants. A fully charged battery typically will not freeze down to 70 to 90 degrees below zero, while a fully discharged battery is susceptible to freezing at +32 degrees. This is because of the chemical process which creates electricity in a battery. As a battery becomes discharged, the sulfuric acid in the electrolyte gradually bonds to the lead oxide in the battery plates. As this process continues, the electrolyte becomes less and less concentrated, until finally it is (theoretically but I wouldnít drink it) pure water. Since water freezes at +32 F, the dead battery will then freeze at this temperature. Damage caused by freezing is mostly mechanical, I.E. the bursting of cases, plate breakage, separator failure, mechanical shorting, plate material delamination and many other woes too hideous to mention. Although batteries can sometimes survive even a severe freeze-up, there is always damage done, and reduced life can be expected.
A properly maintained battery bank can last 10, 20, or even 30 years in rare instances. A poorly maintained bank of the same quality can be ruined in a matter of months (or even days at the hands of an expert). This is why battery maintenance is so important. Here are the basic doís & doníts:
An equalization charge is merely a controlled overcharging of the battery bank. This can be accomplished by using a generator and battery charger or other power sources with the voltage regulation equipment turned off. The object is to bring battery voltage to 15 - 16 volts and hold it there until hydrometer readings in all cells are equal or have stopped increasing, or until all sulfation (white flecks on the plates) has been removed, or both. 15 -16 volts is too high for some electronic equipment, so you should check maximum ratings and disconnect these items as necessary. At these charging voltages, water loss will be significant and water should be replaced as needed. Take care that the batteries do not get hot to the touch (warm is OK) and if necessary reduce charging current or voltage.
Gassing is a normal process that batteries undergo while charging. During the charging process, hydrogen and oxygen are released into the air through the vents on the battery tops, usually along with some water vapor. This can often form a damp surface on the battery that is conductive, leading to corrosion. Remove this film by rinsing with hot water. Water loss through gassing can be reduced through the use of hydrocaps, little catalyst do - dads that recombine the hydrogen & oxygen into water, which drains back into the battery.
Because gassing produces hydrogen (very flammable) and oxygen (which makes things even more flammable) great care should be taken not to inadvertently ignite this (flammable) mixture. Although the quantities produced are small, in a tightly confined space (like in the tops of the batteries) a flame or spark can cause a violent explosion, shattering batteries and sending acid and debris flying about at ridiculous speeds. It is good practice to give batteries the same consideration you would afford to a fuel can or tank in this respect. (unless you are one of those folks who puts out cigarettes in gas cans just to prove that it wonít light)(this is a very, very, bad idea)
Through the gassing process, some corrosion can be expected to accumulate on battery terminals or metal in the vicinity of the batteries. This can easily be removed with HOT water and a scrub brush. Be sure to rinse clean, and as always when working with batteries, wear eye protection. If left unchecked, corrosion can destroy battery posts and terminals, eat through enclosures, and even create dangerous sparks if connections fail. While sometimes fun to watch, corrosion is generally a bad thing and should be held in check through regular cleaning and maintenance. (kind of like teeth)
Batteries should be placed in a vented enclosure that will maintain a temperature of 50-80 degrees Fahrenheit. Sometimes this is simply not possible, but you should do the best that you can. Proper venting of the battery compartment helps to remove the hydrogen and is easily accomplished. Merely venting the highest part of the battery box to the outside is often all that is required. Small battery banks may not require venting, but should be protected from sparks & open flame.
The state of charge of a lead acid battery can be checked in several ways. The first, and arguably easiest method is to measure the voltage of the battery bank. A fully charged 12 volt battery will read about 12.7 volts at 70 degrees Fahrenheit. (double this for 24 volt systems) By the time the voltage reads 12.2, it is 50% discharged, and at 11.9 it is considered empty. The problem with measuring charge in this way is that if there has been any recent activity (charging or discharging) in the batteries, the readings will be highly inaccurate, and temperature can also adversely affect the reading. The second method to determine the state of charge is to use a hydrometer. By measuring the specific gravity of the battery bank, the hydrometer can give you an accurate indication of remaining energy. For example, a fully charged battery may read 1.270, at 50% read 1.190, and at 1.100 be discharged. Hydrometer readings should be adjusted for temperature, and should be performed with the batteries at rest for at least ½ hour. The third and most convenient way to measure battery capacity is with an amp hour meter. These totalizing meters measure energy flow into and out of the battery and keep a running total of available energy at any given instant. Although they may require occasional resynchronization, these meters are very accurate and provide at a glance insight into the state of your system.