my brother worked at a battery store for many years and a trick he told me about is if you have a weak battery use a charger with a engine start feature and turn that feature on for 10 /15 minutes then charge the battery for 24 hours on a lower setting. the engine start feature will kick the coating on the plates off. the coating on the plates is what kills a battery. once the heat and vibration kicks off the coating, the battery will come back to it's highest settings. BUT, this doesn't work all the time, but if you get lucky you will have a good set of batteries again, it's worth a try. a word of caution, cover the battery with a shop towel to keep the acid from spraying in your face when you have it on engine start. good advice whenever you work on batteries. this can happen even when you jump start a car.