Leaving a lead acid battery discharged causes sulfation, a reaction which causes a battery to be difficult (if not impossible) to recharge.
Deep cycle lead acid batteries must be recharged immediately after a discharge cycle. They should not be discharged below 1.75V per cell, nor can they be stored in a discharged or partially discharged state.
Sparks and flames must always be kept away from a wet cell battery. NEVER short circuit a wet cell battery as it may EXPLODE.
ALWAYS WEAR SAFETY GOGGLES AND KEEP WATER NEARBY WHEN WORKING AROUND BATTERIES.
Lead acid batteries are not environmentally friendly and must be properly recycled.