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.