Fuses on split charge leisure battery setup

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Well-known member
Mar 21, 2013
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Surrey Hills
My old setup worked fine, with an original regulator off the dynamo, and a direct connection between motor battery and leisure battery controlled by a relay off the regulator.

I had to replace the mechanical orginal regulator with a new solid state one.

The new one is not able to delivery enough current to switch the relay, leaving the ignition light on dimly red permanently.

So I have bought one of these auto-sensing relays, if it senses 13.3v it connects and joins the batterys, anything less it isolates the L battery.

It says you must fuse on both sides, near the battery, which makes perfect sense, if there's no fuse and you have a fault, the wire will burn out and
start a fire in engine compartment.

...but they don't say what value of fuse is needed..

Anyone got any experience on this - the relay is rated at 160A peak, I guess the relay should be a lot lower - but how much ...

Any help greatly appreciated


Well-known member
Aug 20, 2015
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Brixham, Devon
What size is the cable? That's what the fuse is there to protect.
The fuse is there to protect the cable but its the amps flowing through the cable that dictate the cable and fuse rating. The cable may need to be thicker to avoid volt drop over distance, don't just use the cable you already have.
TBF this system all needs to be matched, taking into account the ah of both batteries, dynamo output and length of cable to be used.
Easiest way is to look at the kits available from 12v planet or VWP etc. and compare with what you have. These guys have done all the working out for you. Only volt drop will be specific to you based on cable length.


Active member
Oct 20, 2009
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Year of Van
After 1972
Van Type
Also something I have found and solved recently on VSR (voltage sensing relay) if you have different batteries, like AGM LB and lead acid MB there can be cross over as the LB might hold the relay open (I charge from LB side on shore power and if I disconnected my MB the relay held open creating power on ign circuit.)
Also AGM charges different but without getting into it there is a fix.

Feed the earth wire to the VSR via a 4 (or 5 pin if thats all you've got) relay.
Make the earth feed relay ign switched to cut earth (turn off relay) when parked up and ign off.
This way the lower AGM LB voltage won't hold open the relay.

I hope that made sense. Graham is right though, 50A on each side, as close to battery as possible.

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