Depth of discharge can be very important. When lead-acids discharge, the metal in the electrodes moves into the electrolyte, making the electrodes smaller and

Depth of discharge can be very important. When lead-acids discharge, the metal in the electrodes moves into the electrolyte, making the electrodes smaller and weaker. Couple that with electrodes made very "spongey" to increase surface area to maximize the cranking current, and they can start to flake off and fall apart if discharged enough.