As cheap and plentiful as they may seem, car (automotive) batteries are unsuitable for use in PV systems. Car batteries are designed primarily for engine starting. Their plates in each cell are made from thin sheets of lead to maximise the surface area, allowing it to deliver the high currents required for starting engines. These batteries excel for ‘burst’ use. Any other slow discharge usage is bad for the batteries, because when the car battery is discharged, their thin plates are much more susceptible to sulphation. The only reason why they last in cars is because as soon as a car is started, the alternator immediately begins to charge the battery. In solar PV systems, a car battery discharged from long-running night lighting will stay discharged until sunrise. In that time, sulphation starts to occur in the battery.
Car battery sulphation is even more of a problem during times of insufficient sunlight. If there are several cloudy days, there will not be enough energy to fully recharge the batteries. If the user is not careful and continues to use night lighting and other loads, the chronically undercharged car batteries will deteriorate badly, often irreversibly.
Deep cycle batteries have thick plates which are far more resistant to sulphation. They can tolerate being in a discharged state better than automotive batteries. High quality deep cycle batteries can tolerate being in a discharged state (no more than 50%) for up to a week with minimal ill effects.