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 photo shows a system that was ignored for more than two years. 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.
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.
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.