flat battery

We have a Malvern, it was first registered in December 2017. We have owned it sine April 2021. We used it reasonably regularly during the summer, not so much come the winter. We went to it just before Christmas, the battery was flat so we jump started it, drove it home and plugged it into the mains all day before returning it. Today we went back and it was flat again. We jump started it and took it for a 30 mile drive. When we got back we turned the engine off, the turned the key again - NOTHING. Completely flat even after a 40 minute drive. We have 2 solar panels so I thought that there would be some life somewhere - not enough to turn on the panel above the door to see how much juice was left in each battery. Are the batteries likely to be dead after 4 years? Is there something that we may have inadvertently left on?? Any suggestions appreciated.

Without voltage readings its hard to say.
However batteries do not like going flat.
I would get an electrical check first to establish fault and budget for 2 new batteries,

Micky

You will probably have an EC500 rather than the EC700 I have but it does sound as though you have left it switched on over the winter when the solar charge is low which has flattened the leisure battery and then the vehicle battery.  As Micky has said, batteries especially older starter batteries don't like being flattened so it is possible you now have a dud vehicle battery.  You will also need to check the leisure battery which may still work but with reduced capacity.  If you did switch your EC500 off, all the solar charge would have gone to the leisure battery and it would probably have been OK but no charge would have gone to the vehicle battery.  Even when a vehicle is not being used, its electrics constantly drain the vehicle battery which is why so many cars had flat batteries after lockdown.  A solution is to disconnect the vehicle battery when the vehicle is not being used for a long time.  This can be done easily on the Boxer by using the red isolation button on the ignition.

The 2017 Malvern is on Mercedes Benz with an EC500 electrical system.  Unless it is hooked up to the mains and set to charge the vehicle battery, that battery will be flat in 2-3 weeks.  The MB handbook says there is a vehicle battery disconnection switch but there isn't.  The battery is under the floor in the passenger footwell and the only way to stop it discharging in storage is to disconnect it.

If storage is outside you should be able to install a second solar panel with a single channel controller connected direct to the vehicle battery and that will keep that battery charged.

If the battery has been fully discharged a couple of times its probably had it.

Battery charging is a function of time and to recharge a flat battery either with the on board mains charger or the alternator will take about 12 hours.  40 mins on either won't touch it.