Whether you spend $200 or $20,000 on your batteries, without proper care they may be headed to the scrap yard sooner than you would like. Here are some expert tips on battery installation, charging, and maintenance to make sure your investment is long-lived.
Proper housing. Make sure your batteries are housed in a safe, easily accessible place. Most batteries require an enclosure that is lockable, sealed, insulated, and vented outdoors. Small details such as sloped covers (so things are not piled on the box), clear viewing windows (for easy inspection), and a removable side (for ease in replacing batteries) can make a big difference. See “Battery Box Design” (HP141) for more tips and tricks.
Batteries are dangerous (see “Safety!” sidebar), and they should not be accessible to anyone unaware of proper safety protocols. But we also want the batteries to be accessible when they need maintenance. Cell caps on flooded batteries, and terminals, should be easily reachable. Consider battery layout, as it is preferable not to lean over one battery to reach another—making access easy reduces the chance of accidental shorting.
Interconnections. Some industrial batteries come with bus bars for making intercell connections, but most battery banks need cables for series and parallel connections, as well as cables to connect to an inverter or DC load center. Battery cables should be large enough to handle their maximum continuous current, and be protected with fuses or circuit breakers rated for high amp-interrupt current. Cable size is determined from the inverter specs and/or DC loads that come off the battery bank. For residential-sized systems, 2/0 or 4/0 cable is common.
Using welding cable for batteries was once a common practice, as listed cable was not available and it is relatively inexpensive, flexible, and can handle lots of current. However, it is not designed for this application and is not listed by the National Electrical Code for use in battery systems. Flexible, UL-listed, NEC-approved battery cable is now readily available, and should be used for all battery wiring.
Keeping batteries healthy requires equal charging and discharging across all cells—differences in resistance within a battery bank can lead to premature failure. Poor lug crimps, loose terminal connections, unequal parallel cable lengths, and small wire gauge can all affect the equal treatment of cells.
Wiring. Electrons can follow numerous paths when entering or leaving a battery bank with multiple parallel strings, so it’s critical to minimize the number of parallel connections and ensure they are equal in length. When wiring parallel strings, always make series connections first. Next, parallel the positive ends of the strings, and then connect the negatives. Inverter cables should be connected on opposite corners of the battery bank to keep electrical paths between strings as equal as possible.
A battery’s lifespan is affected by how deeply it is discharged before getting charged back up, and how long it stays in that discharged state. A battery’s state of charge (SOC) is the amount of energy remaining in the battery. The lower the SOC is allowed to drop, the shorter its lifespan will be. Sizing an off-grid battery bank for 50% SOC is common, but remote systems with no backup power sources may be designed to maintain SOC at 75% or above to extend battery life—the fewer times a heavy, unwieldy battery bank needs to be replaced, the better. Backup power systems are often designed to go down to 20% SOC since they are rarely discharged.