A battery’s storage capacity—the amount of electrical energy it can hold—is typically expressed in ampere-hours (amp-hours, or AH) at a certain discharge rate. One AH represents a flow of electric current of 1 amp for 1 hour. A battery is like a bucket—the larger your “bucket” is, the more AH it can hold. Hence, the larger the AH value of a battery, given a particular discharge rate, the more storage it offers.
Often there’s a choice of selecting a battery with either higher voltage and lower AH, or lower voltage and higher AH. How do you know which is most appropriate for your application? In general, limit the number of battery series strings in parallel to three or less (two are better, and one is ideal). This reduces imbalances introduced by having multiple paths for the current to follow and extra electrical resistance created by paralleled battery cables. In applications where more AH are needed, buy lower-voltage, higher AH batteries so that several low-voltage batteries can be wired in series and the number of paralleled battery strings can be minimized.
The denoted AH capacity of a given battery depends on the rate at which it is being discharged and the amount of time it takes to discharge it. Large industrial batteries, i.e. for forklifts, are often rated at the “6-hour” rate, indicating a high current discharge rate, which brings the battery to its terminal voltage (often at 80% DOD) in 6 hours, about the length of a forklift’s working shift. For RE systems, a 20-hour rate is typically used, because that is closely aligned with the more modest discharge rates that bring the battery to a terminal voltage (again, often at 80% DOD) over 20 hours—more closely approximating daily home use before recharging.
For converting 6-hour rates to an RE system’s more common 20-hour rate, multiply by 1.24. Using this calculation, a 100 AH, 6-hour rating offers 124 AH at the 20-hour rate.
Bulk Charge Set Point Voltage. When charging batteries, the goal is to put as much current as possible into the battery as efficiently as possible. But charging a battery too quickly can cause heat to build up in the battery, as well as excessive gassing, and can shorten the battery’s life. To keep from harming the battery during charging, charge controllers used in RE systems limit the charge rate based on the batteries’ voltage. As the cell voltage increases, the charge rate (the number of amps allowed in) is reduced to prevent overcharging.
The initial phase when all available current is allowed into the battery is referred to as the “bulk” charge phase. Once the battery has reached its initial bulk-charge voltage, the charge controller will hold the voltage there for a programmed period of time (often 2 hours)—the “absorption” charge phase. This is done to assure full charging throughout the many cells of the battery. Note that the set points listed in this guide are per cell, so you will need to multiply it by the number of series-connected cells to determine the appropriate battery charge set points. For example, if you were to use four batteries (6 V each, wired in series for a 24 V configuration) and the bulk charge set point voltage range is 2.4 to 2.49 V for your battery’s cells, the ideal battery bank bulk-charge voltage set point would be between 28.8 and 29.88 V (3 cells per battery x 4 batteries x 2.4 to 2.49 V).
Float-Charge Set Point Voltage. After the absorption period, the charge controller ramps down the charging current to achieve the “float” phase, which is a lower voltage that greatly reduces the batteries’ gassing while still keeping the battery full. To continue the example, the float-charge set point voltage range is 2.20 to 2.23 V for each cell. With 12 cells total, the ideal battery bank float-charge voltage set point for this particular battery bank would be between 26.4 and 26.76 V.
Both AGM and gel-cell batteries will not tolerate voltages that are as high as FLAs. The charge controller’s bulk and float set points must be programmed appropriately to avoid damaging these batteries.