Batteries allow us to store our renewable energy (RE) for times when the sun isn’t shining and the wind isn’t blowing. They are often called the “weakest link” in renewable electricity systems, but battery problems nearly always are the result of bad equipment choices, installation errors, and lack of attention—the human factor!
In my 27 years as a system supplier, I have seen serious battery-related mistakes made repeatedly, by amateurs and professionals alike (and I’ve made a few myself). The results can be expensive, hazardous, and damaging to the reputation of renewable energy. That’s why I am presenting these classic blunders, and their ready solutions. High-quality batteries can last ten to twenty years, but they can die in one or two years if abused.
Nearly all battery-based RE systems use lead-acid batteries. So this article applies only to them (not to other battery chemistries). It applies to batteries charged by PVs, wind, microhydro, and engine generators, the utility grid, or any combination of sources. It applies to off-grid independent systems and also to grid-tied systems with battery backup.
Batteries are built with a variety of structures and materials, according to the application. If you choose the wrong type, you will get poor longevity.
RE applications require batteries to discharge to below 50 percent of their storage capacity, repeatedly. This is called “deep cycling.” A full-time, off-grid home system will typically experience 50 to 100 cycles per year at 30 to 80 percent depth-of-discharge. Always use high quality, deep-cycle batteries in RE applications. Engine-starting (car or truck) batteries are designed for quick, high-power bursts, and will survive only a few deep cycles.
The batteries used in grid-tied, emergency backup (standby) systems will only be deep cycled occasionally when there is a utility outage. Periodically, flooded, deep-cycle batteries need to be actively charged to mix the electrolyte. This prevents stratification of the solution. Because battery cycling/active charging may be very infrequent in standby applications, it’s best to use batteries that are specifically designed for emergency standby or float service. They might not be good for hundreds of cycles, but they will stay in working order through years of light usage.
Another distinction is between “sealed” (maintenance-free) and “flooded” (liquid-filled) batteries. Most deep-cycle batteries are flooded. They require occasional watering, but tend to last the longest. Emergency standby batteries are often sealed, and they require no regular maintenance. Deep-cycle, sealed batteries are sometimes chosen because they eliminate need for a ventilated space and for easy access. Sealed absorbed glass mat (AGM) batteries are designed for float applications, and are a great choice for grid-tied PV systems that include battery backup. They typically cost up to twice as much as flooded batteries, and require more careful recharging regimens, but are the best battery type for standby applications.
To design a stand-alone renewable energy system, you first establish an “energy budget”—the number of watt-hours you will consume per day. Next, you need to determine how many days of stored energy (autonomy) is required. This variable can range between three and six days (or more) depending on your average daily electrical consumption, the output of the RE charging sources and their seasonal availability, and your willingness to use a backup engine generator.
Most home systems grow larger over time. Loads are added, a PV array is enlarged, but a battery bank cannot be readily expanded. Batteries like to work as a matched set. After about a year, it is unwise to add new batteries to an established bank. If you foresee growth in your system, it is best to start with a battery set that is larger than you need. But be sure you have sufficient charging capability, or the battery bank will be chronically undercharged, which will lead to sulfation and premature failure.