Help with Motorhome laid up for long term

I don't know and we don't know what the OP doesn't know 🙂

agreed this might be a cheap/quick way to 'balance' the two batteries with flow moving from a healthy leisure battery to a depleted cab battery and having a fuse (switch) to allow/prevent this could indeed be useful..

a 'manual' battery master equalisation system

However, it needs to be understood that the reverse action is also true....if the leisure battery gets low, for whatever reason, it will suck power from the cab battery.

in heavy off grid use, especially in winter where solar help is poor, normally charge would only be taken from the leisure battery, it will now also drain the cab battery at the same rate.

which is why a Battery Master (effectively joining the batteries as you describe) has a relay (electronic 'non return valve') to only allow the flow from the leisure battery to the cab battery and not the reverse route.

simlarly, batteries connected together 'should' be of the same age and capacity, ideally a matching pair.....as in a new/upgraded twin leisure battery situation. 

furthermore, any battery or leisure system can develop a fault at any time (eg short causing fast drain). its bad enough (but not critical) to lose the leisure battery but, if connected to the cab battery without the protection of a BM, this would be dragged down too, meaning no vehicle starting capability.

i realise your post was a 'quick fix' and a versatile one for those who understand what its doing, but i wouldnt suggest this as a permanent solution....for the above reasons.

IMHO, a BM with added protection is a 'better' fix and forget solution.

@ BB .... has a relay (electronic 'non return valve') 

Would your 'none return valve' not be a diode? A relay is a switch ...

but I agree with everything else re similar batteries etc 😊

yes, a diode.....im not an electrician (certainly shows!) but i (think i🙄)understand the principles.....

you mean for my 'non return valve'

Being a Majestic the solar may very well be linked to the van battery as well as your habitation battery. You can check by looking at your controller. It should be a truma controller sd10 and if the cab battery is attached you will have 3 sets of wires going into it. If you only have 2 sets of wires into it then it will not be connected. We have a dealer special Autoquest and I connected the cab battery to to controller myself.

Sorry forgot to say it’s very doubtful that ehh will charge the cab batter either. I understand that the encore range had that until 2019 and no longer has that facility, the encore being the top of the range in Elddis motorhome production

I knew what you meant .... 😉

"Which" has published an article about maintaining little used vehicles during lockdown. Click >here<

peedee

The advice above that reads  “A cheap way to maintain both batteries“ by connecting them together with a 1.5mmsq cable is foolhardy and should not be considered whether an inline fuse is fitted or not!  The current delivered under short circuit conditions from a vehicle or leisure battery under fault conditions is so high that it would not only melt the small cable but could also start a fire. If batteries are being paralleled then the cable connecting them should be no smaller than that fitted by the vehicle manufacturers. If a fuse is fitted AT THE VEHICLE BATTERY TERMINAL the size of the cable to the second battery could be reduced but the required cable size would need to be calculated by a competent electrical engineer. A second fuse would also be needed at the leisure battery terminal to protect the cable from a short circuit from that battery too.  Please don’t attempt to connect batteries ‘on the cheap’. Split charge relays are available or, as others have said, there are proprietary devices available to do this safely.

I find these useful I keep one in the car the other in the caravan, you can buy them for as little as a couple of quid