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Topic: Flat batteries

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MK2025

25/9/2025 at 12:28pm
Location: North East
Outfit: Toyota Alphard

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Hi everyone, I hope somebody can help me, again please.

I'm finding both leisure and car battery are flat. I charged the leisure battery up via a 3 pin plug adaptor to the cable and to my surprise the car battery charged too. Is this normal? I knew the leisure recharged itself from the car via the alternator but not the other way.

In addition, why would both batteries be flat at the same time? I've jumped the car battery and ran it for an hour or so, the panel in the van indicates both batteries are 4/5 charged so hopefully this was just a one off but appreciate any advice you can give a newbie please.

Colin21

25/9/2025 at 12:42pm
Location: East Herts
Outfit: 1992 Elddis Wisp 450CT + X Trail

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It sounds to me you have a wiring fault and both batteries are connected directly together so that if the leisure battery gets drained, the starting battery drains too. If you have a split charging system the relay may have stuck or the contacts have welded closed. The batteries should only be connected together when the engine is running so that they both charge off the alternator. When the engine stops the relay should drop out and disconnect them from each other.

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Colin

MK2025

25/9/2025 at 12:44pm
Location: North East
Outfit: Toyota Alphard

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Thank you Colin, that's perfect. Would I need an auto electrician to have a look at this for me do you think? I was going to take Halfords up on their 'free battery check' but not sure they would look at them both for me.

daveyjp

25/9/2025 at 1:32pm
Location:
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I would find a campervan converter with experienced auto electricians to have a look.

Chances are it may need partly stripping back and rewiring so the correct sequence of charging and discharging occurs. May also be an opportunity to look at a few upgrades to improve the situation.

Monty15

25/9/2025 at 4:04pm
Location: London
Outfit: Lunar Cosmos 524

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I think your problem is way above the skill set of a Halfords shop! They can put a tester on a battery and verify OK or otherwise, but the complexities of a split charging system on a MH/camper is likely outside their scope.

A proper auto electrician may seem expensive, but if they can go straight to the problem source and replace the exact faulty part first time, then that's likely cheaper than multiple 'best guess' and changing parts until the problem goes away attempts by others, also you risk irretrievably damaging both car and leisure battery if they are flattened too deeply too often (lead acid batteries should NEVER be discharged more than 50% of their capacity, it damages them) - that could be in the region of a £250 saving straight away if you avoid damaged batteries and needing replacements!

Capt Lightning

08/11/2025 at 11:32am
Location: Turriff Aberdeenshi
Outfit: Romahome R30 Dimens

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I think that all motorhome / caravan owners should have a basic multimeter. Motorhome wiring, especially in older models is fairly straightforward.

It has been suggested that the split charge relay may be stuck and sometimes this also controls power to the fridge which would quickly drain the batteries.

I've also personally experienced accidentally leaving a light switched on

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Two drifters off to see the world.

I'm tired of reality, so I'm off to look for a good fantasy.

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