The first step to picking the best 12-volt battery for a particular job is determining what dimensions and electrical capabilities the battery needs; using the wrong battery is potentially dangerous. In addition, small differences in size can cause a battery to not fit.
Fully charging and discharging a battery leads to wear over time and can cause the battery to fail prematurely, so people looking for batteries used in this manner should look for deep-cycle batteries that are better able to handle these scenarios. These batteries don't offer the same peak voltage as standard 12-volt batteries, but they provide better longevity in heavy-use situations. Photovoltaic systems, electric forklifts and electrical backup systems for computers often use these types of batteries.
Those looking for batteries that sit in an enclosed space also need to make sure the batteries they're looking at can fit. In general, it's best to stick with manufacturer recommendations, as small changes can make a battery incompatible with a particular product.
Battery capacity is measured in amp hours, and batteries with more capacity tend to cost more than those with less. However, it's also important to note that a battery with more capacity than required allows users to avoid fully draining the battery, which can improve its longevity.