The surest way to ruin batteries within a year or two is to keep them at a low state of charge (SOC) for weeks at a time. Active battery material will crystallize, covering the plates, which will become permanently inert. We call this “sulfation.” Ideally, batteries should receive a 100 percent full charge about once a week for good longevity, and more frequently is better. If this takes a full day of backup charging with a generator, do it! Use your monitoring system to know when full SOC is reached. If you don’t have an amp-hour meter, watch for the voltage to reach maximum and the charge current to drop to a low level. This means the batteries are unable to accept much more energy, and are accepting only a “finish” charge.
In winter, some people run their backup generator for an hour a day—just enough to prevent the system from shutting down. Bad idea! It may be better to run it for ten hours, once a week, or whatever it takes to fully charge the batteries, instead of partially charging them more frequently.
Finish-charging a battery bank with an engine generator is an inefficient use of fuel, and results in extremely long generator run times. As a result, generators are typically shut down once the absorption charging stage is finished. But at this point in the charging process, the battery bank will only be at about 85 percent SOC. Since regular, full battery charging is important for battery longevity, make sure that your RE sources are topping off the battery bank after the generator has done the bulk of the charging. Relying on your PV system to provide the finish charge may be difficult during winter months. Another option is to set the inverter–charger to equalizing mode (see below) during generator charging about once a month to ensure that the battery bank is getting fully recharged.
The extreme of undercharging is called “overdischarging.” Voltage should never, and I mean never, be drawn below about 11 V (for a 12 V system), or 22 V (24 V system), etc. System controls and inverters usually include a “low voltage disconnect” (LVD) function. If you have DC loads connected directly to the batteries without LVD, you are asking for trouble. It’s better to lose power than to squeeze out another watt-hour and damage your batteries. Metering is vital here, because if you wait for the inverter to shut down or the lights to go dim, it’s already too late—batteries will likely have lost a portion of their capacity and life expectancy.
Finally, flooded batteries need to be equalized at least four times a year. Exactly how often depends on several factors, including the size of the battery bank in relation to your charging sources and the average depth of discharge during cycling. During normal battery discharging/charging, the individual cells of each battery will stray from a common and consistent cell voltage. Equalization can be thought of as a controlled overcharge of the battery bank that serves to both equalize cell voltage, and provide an aggressive and necessary mixing of the battery electrolyte. Equalization charging can be done with your PV system if your array is large enough, or with an engine generator or the grid. Most PV charge controllers and inverter–chargers have battery equalization functions.
If you remove more energy from your battery bank than you put in, your batteries will suffer. It’s not the batteries’ fault, yet this is the most frequent cause of complaints about batteries “not holding a charge.”
Here is one common scenario: A well-meaning appliance seller or mechanical contractor sells you a device that uses “very little electricity.” Ha! They don’t know about the initial expense of solar electricity. For example, about US$3 will buy you about 40 KWH per month of grid electricity. But adding more PV and battery storage to meet this load could mean an investment of several thousand dollars! Or, without upgrading your system, this would require frequent generator backup (especially in winter). The same blunder also happens when a resident decides it’s trivial to leave a coffee maker or large TV on all day. Even low power loads will add up if they’re running 24/7. When people don’t accept this reality, they overdraw their energy account, and often blame the batteries.
If I had more pages, and I could show the Top 40 blunders, from transportation nightmares to eye injuries to divorce. The lesson: Accept professional advice and service.
Lead-acid batteries are an old but durable technology. They are about 80 percent efficient at releasing stored energy—few high-tech storage systems come close to that efficiency—and they rarely fail suddenly. With good management, you’ll know when to replace them before they let you down. And even then, they are fully recyclable. Give your batteries what they need, and your batteries will do the same for you.
Frequently asked questions and answers about batteries • www.batteryfaq.org
“Batteries: How to Keep Them Alive for Years & Years…” by Windy Dankoff in HP69
“What is a Charge Controller?” by Windy Dankoff in HP72
Using the TriMetric (or other battery system monitor) to Maintain Your Battery System •
Thanks for contributions and photos from: Allan Sindelar & Mark Drummond, Positive Energy, Santa Fe, NM; Nick Lucchese, Sierra Solar, Grass Valley, CA; Roy Butler, Four Winds Renewable Energy, Arkport, NY; Ray Walters, SolarRay, Taos, NM; Tom Elliot’s Alternative Energy Information Center, Cover Mountain Ranch, CO; Matt Lafferty, Universal Energies Inst., San Francisco, CA; Jamie Surrette, Surrette Battery Co. Ltd., Springhill, NS, Canada; Joseph Marino, DC Power Products, Healdsburg, CA; Phil Undercuffler, Conergy Inc., Santa Fe, NM; Todd Cory, Mt. Shasta Energy Services, Mt. Shasta, CA; Richard Perez & Joe Schwartz, Home Power magazine