Kid,
Before spending money on charger have mechanic/engineer check electrical system for a parasitic drain on battery. This condition is usually caused by a faulty component (switch, relay, etc.) allowing very low level of current to flow with the Master switch off.
A simple check is to put a multi-meter inline with the negative connection at the battery and set to read amps. The positive remains connected. If meter indicates amperage (a load) then start pulling circuit breakers one at a time until load is removed which normally will isolate your problem to that electrical circuit.
With today's technology and shelf lives, batteries should not auto-discharge unless it is being electrically consumed or its static capacitance level is below minimums. An exterior cause could also be the aircraft charging system is not operating correctly.
I normally just recommend disconnecting the battery, or at least the negative connection when aircraft not in use. This method has worked for extended periods provided the battery is up to spec. Most aircraft lead acid batteries have some kind of scheduled inspection to check capacitance, etc.
But if you still want a charger I attached a couple links to what some pilots use on my side of the pond.
As for EASA "approved" systems, don't know except these chargers fall below the approval level requirements here, unless you wanted to permanently install system and connect to the main aircraft electrical system--which I do not recommend.
Long post, but have seen too many people spend a lot of money not fix the problem.
Good luck. W1
GILL GC-024 24VDC CHARGER from Aircraft Spruce
Aircraft Battery Charger | Aviation Specific Maintainers
Chargers, Battery from Aircraft Spruce