Mercedes Benz GL450 battery drain

[MantaRay]

I'm currently looking after the above vehicle [off road storage] and have have had it connected to a battery conditioner for a few months without problem. However, it now seems as though the conditioner/charger can't keep up with the current drain so the battery continues to ran down to c.11.5 volts, insufficient to start the engine. I can hear something like a small fan running when I'm inside the car, could this be a ventilation fan for the battery compartment which is situated inside the vehicle under the carpet?

I've tested the battery on a high discharge tester and the voltage holds up once I've topped up the charge using a commercial battery charger.

[mitchmoir]

This vehicle depending on the year has two batteries, The main starter battery and a small auxilliary battery. I suggest charging both batteries.

[mitchmoir]

Also make sure there is no water ingress in to the boot space. Eg remove the floor panel and side panels and reach your hand down and check for wetness etc.

[MantaRay]

The vehicle in question has a single large battery hidden beneath the floor under the drivers seat and the voltage has been holding up sufficiently with the vehicle leads disconnected. My guess is that the battery conditioner is not quite powerful enough [0.8 amp], but I'm still puzzled by what sounds like a small motor running as soon as I reconnect the battery with nothing obvious left switched on.

[MantaRay]

The vehicle is a 56 reg by the way.