It sounds like you need a battery trickle charger, not a regular battery charger as such.
The trickle keeps the battery topped up by adding when charge is needed and shuts off when charged. If you leave a conventional battery charger on all the time you could effectively boil the battery!
The trickle is a more steady way of going about things.
Probably the best trickle charger on the market is the Ctek mxs 5.0.
Porsche, Bentley, Ferrari supply these CTEK with their own badge on, for much more £££!
I have cars I rarely use that sit on these trickle's, never a problem with the battery.
Lidl do a Ctek style copy which many people use and seem happy with for £13.99, it was a deal a few weeks ago, so may still be in stores?