For effective compatibility in off-grid systems, batteries utilizing solar panels need a “deep cycle” battery. Deep cycle batteries are designed for continuous charging and discharging, which is the exact solar application. Starter batteries are designed to crank an engine with momentary high loads and for a few seconds. This means that starter batteries cannot be deep cycle because the emphasis is on immediate power and not capacity.
Deep cycle batteries will have an Amp Hour (Ah) rating, whereas starter batteries might also have an Ah rating or a reserve capacity (RS) rating as well as a Cold Cranking Amp (CCA) rating. CCA refers to the starter battery being able to deliver power at cold temperatures. Starter batteries cannot be swapped out for deep cycle battery and vice versa. The battery chemistry inside a starter battery would fail or dissolve in a deep cycle application.