Batteries allow us to, store our renewable energy (RE) for times when the sun isn't and the wind isn't blowing. They are often called the "weakest link" in renewable systems, but battery problems nearly always are the result of bad equipment installation errors, and lack of attention, the human factor!
In my 30 years as a design engineer, I have seen serious battery-related mistakes repeatedly, by amateurs and professionals alike (and I've made a few myself). The results are 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 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 any other battery chemistries). It applies to batteries charged by PV's, wind, microhydro and engine generators, the utility grid or any combination of sources. This applies to off-grid independent systems and also to grid-tied systems with battery back up.
Wrong Battery Type
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" (Iiquidfilled) 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.
Flooded batteries require the addition of distilled water every two to six months depending on battery type, battery temperature, and on the charge controller settings and system usage. Some people forget to water their batteries. The low fluid level caused excessive gassing, and the plates to warp, short out, and spark, ultimately igniting an explosion.
But don't overfill your batteries, either. There is no need to fill them more frequently than required to keep the plates submerged. Fill them only to the level recommended by the manufacturer. Otherwise, during final charging, bubbles will cause excessive spatter and possible overflow, leading to corrosion of the battery terminals and wiring. Though an additional expense, a battery watering system simplifies battery watering.
Many Small Batteries in Parallel Strings
The ideal battery bank also is the simplest, consisting of a single series string of cells that are sized for the job. This design minimizes maintenance and the possibility of random manufacturing defects. Suppose you require a 700-amp-hour (AH) bank. You can approximate that with a single string of 700 AH industrial-size batteries, or two parallel strings of 350 AH (L-16 style) batteries, or three strings of 220 AH (golf cart) batteries. The diagram below shows these three variations.
A common blunder is to buy the smaller batteries because that approach is less expensive up front. The problem is that when current splits between parallel strings, it's never exactly equal. Often, a slightly weak cell or terminal corrosion will cause a whole battery string to receive less charge. It will degrade and fail long before other parallel strings. And because partial replacement aggravates inequalities, the only practical solution is to replace the entire battery bank. One way to reduce or avoid parallel battery strings is to use the highest DC voltage standard that is practical. The same batteries that would form two strings at 24 V can be wired all in one string for a 48 V system (now a common standard). The quantity of energy storage is the same, but the layout is simpler and the current at critical junctures is cut in half.
If you must have multiple battery strings, avoid stacking cable lugs at the battery terminals to make parallel connections. Instead, bring wires separately from each string to two bus bars outside the battery box. This reduces corrosion potential and helps create electrical symmetry.
Single String Large 2-volt cells wired
in a single "string"-literally, one big battery.
Strings Medium-size batteries, two strings in parallel. Same amount
of lead, equal energy storage.
Three Parallel Strings
Smaller batteries, three strings in parallel.
Again, equal energy storage, but much more to go wrong.
# 5 Failure
to Prevent Corrosion
lack of a Protective Environment
Lead-acid batteries temporarily lose approximately 20 percent
of their effective capacity when their temperature falls to 30°F (-1°C).
This is compared to their rated capacity at a standard temperature
of 77°F (25°C). At higher temperatures, their rate of permanent degradation
increases. So it is desirable to protect batteries from temperature
extremes. Where low temperatures cannot be avoided, buy a larger battery
bank to compensate for their reduced capacity in the winter. Avoid
direct radiant heat sources that will cause some cells to get warmer
than others. The 77°F temperature standard is not sacred, it is simply
the standard for the measurement of capacity. An ideal range is between
50 and 85°F (1 0-29°C)
Arrange batteries so they all stay at the same temperature. If they
are against an exterior wall, insulate the wall and leave room for
air to circulate. Leave air gaps of about 1/2 inch (13 mm) between
batteries, so those in the middle don't get warmer than the others.
The enclosure should keep the batteries clean and dry, but a minimum
of ventilation is required by the National Electrical Code,
A battery enclosure must allow easy access for maintenance, especially for flooded batteries. Do not install any switches, breakers, or other spark-producing devices in the enclosure. They may ignite an explosion.
The fluid in flooded batteries gasses (bubbles) during the final stage of charging. When using flooded batteries, a trace of acid mist escapes and accumulates on the battery tops. This can cause terminal assemblies to corrode, especially any exposed copper, which causes resistance to electrical current and potential hazards. It's an ugly nuisance, but it's simple to prevent.
The best prevention is to apply a suitable sealant to all of the metal parts of the terminals before assembly. Completely coat battery terminals, wire lugs, and nuts and bolts individually. If the sealant is applied after assembly, voids will remain, acid spatter will enter, and corrosion will appear. Special products are sold to protect terminals, but many installers prefer petroleum jelly. It will not inhibit electrical contact. Apply a thin coating with your fingers, and it won't look sloppy.
Exposed wire at a terminal lug should be sealed, using either adhesive-lined, heat-shrink tubing or carefully applied tape. You can also seal an end of stranded wire by warming it gently, and dipping it in petroleum jelly, which will melt and wick into the wire. Or, you can solder the lugs. Whatever the method, these connections must be very strong mechanically. Batteries protected this way show very little corrosion, even after many years.
It's also important to keep battery tops clean of acid spatter and dust. This helps prevent corrosion and stray current across battery tops. Keeping battery tops clean is easy if you keep up on the job. A good habit to get into is to wipe the tops of the batteries with a rag or paper towels moistened with distilled water each time you water the batteries. Do not apply baking soda to the battery tops, since it might enter the batteries, neutralizing some of the electrolyte.
Renewable Energy Systems