A Trickle Charger(110537)
Trickle Chargers are used both to charge batteries and to extend their life. They should be relatively cheap to purchase and for the most part draw power from a static mains source. Solar panels can also be used as a power source, perhaps while mobile. They can also be known as battery tenders (there is a very popular firm of the same name – just to add confusion), battery chargers or battery maintainers (older batteries required maintenance; this is also, confusingly something else).