Yes, batteries do self-discharge when left idle for long periods, and the lead sulfate that begins as a spongy material initially forms a hard crystal if the battery is left in a discharged state for weeks, or possibly months. Once that happens, it is extremely difficult to redissolve the sulfate by charging the battery.
I have reclaimed sulfated batteries by slow charging for several hours, then totally discharging by using a headlight bulb or other drain. Recharge- - - -discharge- - - -etc. sometimes as many as 20 times. The sulfate crystals dissolve slowly and go back into solution to for m sulfuric acid. A battery hydrometer and a voltage-controlled charger are pretty essential to reclaim a battery in that condition. Simply starting the vehicle and running it for 15-20 minutes once a week should prevent the crystals from forming, or connecting a low-rate charger overnight once or twice a month for long-term storage would also work.
Jerry
