Camper Question Of The Week: Leisure battery – charge fuse keeps blowing?

As part of my now nearly done T5 conversion (pics to follow soon 🙂 ), I’ve fitted a leisure battery and have a voltage sensitive relay between the main battery and the leisure battery. On the charge cable from the main battery, before the relay, there’s a 16A fuse. The relay opens up at 13.7V and closes at 12.8V, so only allows charge to the leisure battery when the main battery is fully charged (i.e. when the engine is running).

So, the setup is (apologies if I’m teaching anyone to suck eggs, this is all new to me):

Main battery -> relay -> fuse -> leisure battery.

**EDIT** Actually, sorry, it’s Main battery -> fuse -> relay -> leisure battery.

Now, the fuse has blown twice. The first time I noticed it had gone I assumed it was from when the van was jump started by the RAC last week (too many starts to move it around the drive whilst working on it + no real runs in the van = flat battery + no start, doh!). However, I’ve just been to Halfords to buy another fuse, fitted it in the car park, and somewhere on the twenty minute drive back it must have blown again.

Any ideas?

IHN, the fuse is not big enough. My split charge setup has a 60A fuse in there (and it MUST have a fuse at each end of the wire!!). What’s happening is that the leisure battery is a bit low on charge, so when the split charge switches over to charge the leisure battery, it is pulling more than 16A, and blowing your fuse. Another fuse won’t stop it happening – it will just happen after a day or so of running your fridge (ie. when you don’t want it to blow).

I would recommend: Run big cable between the main battery and leisure battery, with the split charge relay contacts in the circuit, and a big fuse at each end (by big cable I mean 25mm2 (or about 8mm diameter) and a big fuse (60A or so). The fuses must be at each end in case there is a short circuit to ground (-ve), and as there is a big source of current at each end of the wire, both fuses must be able to protect the wire.

what he said above is the gospel -big wire to carry current big fuse to protect big wire from shorting- depending on size of your relay connectors you may need to use it to trigger a solenoid switch to keep the cables big enough – about another £10
http://www.vehicle-wiring-products.eu/VWP-onlinestore/fuses/fuses.php midi fuses from here
http://www.vehicle-wiring-products.eu/VWP-onlinestore/fuses/holders2.php
case here

Okay, I get that the fuse needs to be bigger (although the 16A came with the kit), but I don’t get why there needs to be a fuse at each end of the charge wire?

I assume that we’re talking:

Main battery -> 60A(?) fuse -> relay -> wire to leisure battery -> 60A(?) fuse -> leisure battery

And the charge cable (again that came with the kit) is about a 4mm core. It’s also all in under the floor, and rewiring would be a f-ing nightmare.

OK, short circuit happens in the middle of the main cable. Fuse at one end blows (if there’s only one fuse). You still have a big battery capable of delivering 300Amps down a piddley wire at the other end of the wire. So, wire gets hot, lets smoke out, and you watch the van burn.
Sorry to be dramatic, but I have designed vehicle electrical systems for a few years and know how much damage a hot wire can do.

Oh, and I would question the qualifications of the person who put that kit together. It is suitable for joining PP3’s together, not car batteries.

This is an area you really need to spend some money (probably only £60 or so) as it will let you down if you don’t.

(And you’re description of:
Main battery -> 60A(?) fuse -> relay -> wire to leisure battery -> 60A(?) fuse -> leisure battery

is spot on. The relay should be capable of carrying more than the fuse rating (my relay is 100A, fuses 60A).

Where’s your leisure battery? (if under the passenger seat it shouldn’t take much to rewire – if built in at the back like mine you could do what I did – behind the units, then (in plastic conduit) under the rubber matting floor by the plastic tread plate and up under the dash to the hole through the bulkhead). I had to cut some of the foam away under the rubber carpet to make it sit well, but it works a treat.

Good luck!

I keep singing “Camper Question Of The Weeeeeek” in a Harry Hill style.

Gotcha about the fuses, thanks.

I’m waiting to here back from the fella I got the kit from, but it looks like I may be doing some rewiring…

Oh, and for BigJohn: