Split Charging Problem > Fuse blown

[aknapper]

So last May we got to the campsite only to find that the 075 battery we use as a leisure battery was flat and no charge was going to it . Not really a big problem but we were at a non-electrical site and I needed to spark the fridge to put it on gas. Just put in the starter battery to get the stove going and when we went out for our hill climb dropped the flat battery off at a local garage to recharge it.
I thought it was just the battery being low on charge so recharged it fully at home. No problems on later camping weekends but then we were on electrical hook-ups. But by end Sept the batter was completely flat and I killed my cheap charger trying to bring it back to life.
I thought it was a problem with the Zig system and the online forums really didn't address my particular problem. BUT yesterday I finally had the chance to use the multimeter on the system and yup no charge going to the leisure battery (which was my spare 075 starter battery). It's located at the back and has a fuse and a breaker on it. Both of those were fine. And then I remembered a forum post that said there may be another fuse on the charging cables but near the starter battery. A quick rummage around under the seat, saw it, pulled it and voila! a blown 20amp fuse. Fingers crossed I pulled another one from the fuse box, inserted it and turned on the bus - multimeter verified we had a proper charge to the leisure battery!!!
So, the lesson learned here was to use the multimeter and then look for fuses on the main positive or negative charging cables (mine is on the positive) to see if they had blown or not.
A simple fix that was bothering me for months as I had no time to rummage around hunting down the root cause.
I also bought a proper leisure battery yesterday so the other 075 stays as a spare, plus another gas cylinder for all of our wild camping plans for this CV-19 affected summer.
Andrew

[Mocki]

20amp is far too small .
However it’s correct for your system if it only has a 30amp relay - which is inadequate.

The reason your fuse blew was that your "leisure " battery was too low and tried to take more Amperage than the fuse value Would allow . If you keep the battery 75% charged ( above 12.4v) it will work .
A split charge relay and more crucially its wiring and connectors should be at least 75% of your alternator max output .
Minimum on a T25s would be 50amp . Although the battery would only try to "draw" this much for a few mins when very flat it will over rate the wiring and relay

[aknapper]

Thanks Steve for the additional info. As you said, I suspect the 075 battery got too low(er) at some point, tried to draw more and blew the fuse.
Electrics aren't my strong suit with vans.
How would I know that the 20amp fuse is correct or incorrect? I mean you say it should be 75% of the alternator output, which is great info. But I am so wary with fiddling around with van electrics. I could put a 30amp fuse in...but would that be right? I really don't know if the split charge wiring is correct.

The leisure battery housing is in the back in the 'clothes' hanger area. It has another fuse (must go look to see what amp it is) on the positive line - so that's two fuses in series. Plus it has a breaker on it as well. I don't know if this is typical for Komets... So perhaps the two fuses were designed this way?

Any thoughts gladly accepted so I don't fry the new proper leisure battery

[Mocki]

Look at the s0lit charge relay and check it’s specs .
Look at the wiring , if it’s thin , as I suspect it will be , do not update your fuse , the fuse is to protect the wiring , keep with the 20amp , just don’t let the battery get too low .
There is no point of more than one fuse in a single feed . The fuse should only be at the source of the power .

[1664]

I don't know if this is typical for Komets...

No it's not although the original wasn't much better either. A small relay with integral fuse at 20A under the drivers seat automatically switched by a trigger wire run under the side of the rubber cab mat to a terminal in the glovebox vehicle fuseboard. Then a wire from the relay also ran under the mat to the leisure battery under the passenger seat. My fuse went on a few occasions so I relaced the whole lot but kept the trigger wire (which also did the fridge 12v relay to run the fridge while driving. That relay incidentally is behind the little box section in the lower half of the cupboard behind the drivers seat).

To be fair back in the day of these campers there was very little to run your leisure battery right down, no TVs, laptops, mobile phones etc, just the lighting really and they also assumed you would be driving the camper while you were holidaying so the leisure battery got a regular top up.

It looks like someone has had a fiddle with yours, I don't think Autohomes ever put a leisure battery in the wardrobe.

[ZsZ]

Look at the s0lit charge relay and check it’s specs .
There is no point of more than one fuse in a single feed . The fuse should only be at the source of the power .

If you connect two batteries together with a switch (relay) inline, both sections of the circuit will be powered either the switch is open or closed.
Better to have fuses at both batteries in a split charge circuit.

[aknapper]

Thanks everyone for the feedback. I'm going to have a look at the ZIG manual and see if the split charge wiring looks like it should.
The leisure battery box and holder etc etc does look original to this Autohomes Komet - it's been custom fitted as a separate compartment with same fittings / carpet as rest of the cabinets. So a bit of an enigma.
My wife has to do some authorized essential field work this week on Mull of Kintyre, so we're taking Otto as our own essential worker accommodation and will pitch up with permission on the landowner's property. I'll report back on how the new leisure battery charging etc worked.

[aknapper]

Trip went off without a hitch.  Leisure battery worked fine.  But I will monitor and do some sporadic testing to be sure to be sure.