Battery chargers are usually rated and priced by their power output, either in amps or sometimes this is mixed with typical size of engine they are suitable for. Features to look out for include LED battery condition display, charging status display, polarity protection and thermal overload protection. Insulated copper leads and quality battery clamps.
Many battery chargers are able to deliver 2 types of charge, one fast the other slow, slower charger a battery is much better and less likely to cook or dry out the battery, aalways folow the instructions but usually this will invlove fast charging for short periods only. Also ensure what type of batteries the charger is suitable for, sealed or lead acid, some will charge both so worth looking out for them.
If you own a classic car, motorcycle or just have a vehicle that is not used very often then invest in a trickle charger or battery mainteince charger, they will ensure your battery is fully charged when yo uneed to use the vehicle but they will also prolong the life of the battery.