Equalization Charge Set Point Voltage. An equalizing charge cycle is a controlled overcharging of the battery bank to make sure all cells get charged, and to remove sulfate ion bonds on the batteries’ plates and to regain battery capacity—before permanent bonds develop. First, the battery is charged to full capacity by completing a bulk and absorption charge cycle. Then the battery is charged for an extended period of time, typically 6 to 12 hours, at a C/20 rate (charging amps equal to battery’s AH capacity divided by 20). By controlling the charge rate at C/20, the battery is kept from harm. (Uncontrolled overcharging can warp the batteries’ plates, causing it to short out and possibly explode.)
Equalizing an FLA battery is essential to maintaining battery life, but can be difficult to achieve with the limited current available from a PV array. In off-grid applications, a backup engine generator is often used to equalize the batteries through a charger. Off grid, the use of household loads is generally limited during equalization to make sure enough current is available. In utility-tied systems with batteries, the grid substitutes for a generator.
Using the example of the four-battery bank (6 V each, wired in series for 24 V) and an equalization charge set point voltage range between 2.5 and 2.67 V per cell, the ideal battery bank equalization charge voltage set point for this particular battery bank would be between 30 and 32.04 V.
It is commonly believed that sealed batteries should never be equalized, yet some sealed battery manufacturers will provide an equalization voltage set point for their batteries. It is important to note that these values are usually the same as the bulk voltage set point for that battery. Typically, equalizing sealed batteries means merely extending the absorption period for a longer duration than normal. Additionally, sealed battery “equalization” is usually done only if the battery is showing signs of premature capacity loss (i.e., not lasting as long as normal on a charge), and is not part of routine battery maintenance. Regardless, equalization is very battery specific, so it is important to find appropriate voltage set points and charge current ranges for your particular batteries.
Dimensions. When you’re designing your battery bank, the size of the batteries—their length, width, and height—determines the size of the containment that you’ll need to buy or build. In addition to considering the dimensions of the batteries, it’s a good idea to leave 1/2 to 1 inch of space between each battery. This will help keep the individual batteries operating at the same temperature and allow them to shed heat during heavy charging regimes.
Weight. Even the smallest batteries used in RE systems can weigh as much as a Labrador retriever—50 to 60 pounds. The really big batteries can weigh as much as a small horse. So, adequate trucks, skids, pallet jacks, and forklifts all become more important in moving batteries safely as the bank grows in size. You’ll need to make sure your floor and/or rack is stout enough to support the total weight of the bank.
Warranty. Manufacturers generally guarantee their products to be free of defects and perform as specified for a set period of time, and will replace defective units during this time period. Many manufacturers offer one-year free replacement with additional prorated warranties for two or three years. During this period, the distributor will replace the failed unit for a percentage of the replacement cost.
Batteries have enabled Christopher LaForge to live and work for more than 20 years at his off-grid, sun- and wind–powered homestead, SunFarm, in Bayfield County, Wisconsin. He is an ISP-affiliated PV instructor with the MREA, a NABCEP‑certified PV installer, and a member of the NABCEP board of directors.