A500+, A2000, A3000 and A4000(T) have batteries from factory. They do a lot of damage when they leak, so if buying be sure to inspect the battery area for damage.
Non-plus A500s also typically have 512k ram expansions with battery-back clock that needs replacing too. Possibly A1000s too (never seen one). As it's on the expansion port, they are less likely to do damage to the motherboard. So in A500s the usual casualty is the ram expansion, which is easy (and cheap) to replace.
Battery-backed clocked expansions in A1200 tended to have coin cells that don't leak (that I know of).