One of the most difficult decisions a family of limited means has to face when dealing with batteries is the decision to reduce a battery bank. When batteries begin to fail it forces owners to make difficult decisions, namely to cut out the bad batteries from the battery bank (and reducing bank capacity) or buying new batteries. While it is natural to resist taking the decision to remove bad batteries, it is vital that you do this earlier rather than later, the longer you keep even one bad battery in a battery bank, the more the harm will spread, – bad batteries make weak batteries worse and eventually you find yourself with several bad batteries and a next to worthless system.
Obviously in an ideal situation you buy new batteries, even fewer new batteries are better than a larger battery bank of weak batteries, but resist the urge to just replace one or two batteries with new batteries, as while this is an option the new batteries will have shorter lives and in the long run is generally counter-productive. The weaker batteries will “lean” on the new batteries and it will shorten the useful life of the good batteries.
What matters most is how bad the old batteries are, the worse the older batteries are the more they will “lean” on the new batteries. You can probably get by adding/replacing a couple batteries on a larger system (24v or 48v) if the rest of the batteries are in decent shape (proper evaluation of a battery can only be done under load), but you really need to weed out the weakest batteries as they can really drag down new batteries over a short amount of time.
We have a 24 volt system and have gone through two battery bank changes (three sets of four 6v batteries), the first time our inexperience contributed greatly to the short life of the battery bank. Had we monitored the batteries closer, been more vigilant and responsible with maintenance (equalizations, specific gravity/hydrometer use) and had the good sense to remove our bad batteries earlier (when you have one bad battery on a 24 volt system you have to remove four batteries, – something difficult to rationalize when inexperienced…) we probably could have gotten several more years out of our first battery bank. We are into our eighth year on our second set. Our first set only lasted five years. That is why maintenance matters.
What new off-grid adventurers who chose to be independent of the power company need to realize is that you need to learn the basics, be vigilant and monitor the batteries routinely and be willing to cut out a bad battery at the first sign of troubles, even if it means reducing a string of four batteries to do it. Doing so is not only necessary, but it offers opportunity too, for one those four batteries are not lost, you do not discard them, – at worse one or two are beyond redemption. Often you can set up a separate 12-volt system to run a fan at night, or light a room or run a radio or television. Better still, the longer you live off-grid and rely upon yourself, the more adaptable you are to energy needs. We use a small fraction of the power we used when we first went off-grid, over time you start to think about what a device will “cost” to run, you avoid devices that produce heat, that have high induction loads or use a lot of power (light bulbs etc..). So, even reducing a string of four batteries after four or five years will probably not be as big a burden as you think, especially when you consider the probability that that string of batteries is actually dragging down the other two strings and you may actually have more usable power once it is removed (it actually is a “device” that is consuming the battery banks available power).