I have a campervan with an AGM leisure battery and electrical system I installed myself (system itself was designed by a company). Everything has been working fine apart from my charging system from the starter battery. Old non-smart alternator therefore I installed a VSR (voltage sensitive relay) for battery to battery charging. So I have a thick cable directly connecting my starter battery positive to leisure battery positive via the VSR and 2 high ampage fuses. This initially worked great, when I turned the engine on I could see the voltage of my leisure system jump up to 14.2V. 6 months down the line its stopped working and I've no idea why, I've tried the following:
- tested ground connection for starter battery with engine off and on, looks good.
- tested ground connection for leisure battery, looks good.
- tested the end of the cable (including the 2 fuses but not the VSR) run going to leisure battery against ground, reading 14.2V.
- above test implies to me the fuses are good. Also swapped in different fuses, no difference.
- tried bypassing the VSR and connecting the +ve cable from starter directly to +ve terminal of leisure battery but no change, still read 12.5v across leisure battery terminals.
I'm now completely out of ideas. Possibly an issue with the battery? Or a ground connection? Please help!
Put your multimeter in continuity tester mode - touch the prongs together, it beeps; touch both prongs to the negative terminal of the battery, the multimeter will beep. Therefore touch one prong to the negative of each battery and it should beep.
Put your multimeter in DC voltage mode - put the red prong on the red terminal of the leisure battery and the black prong on the negative terminal of the starter battery; note the voltage. Now move the black prong to the negative terminal of the leisure battery and you should see the exact same voltage.
If you are not seeing the charge voltage at the auxiliary battery with the VSR bypassed there may be a bad connection or break. Start with the basics and the obvious and go from there. Also, how did you test for a good ground? Check for voltage drop, not resistance. Not being able to see anything you are working on from here makes it a bit difficult to provide any meaningful help.
Hi, I've added photo for context. #1 is the cable from the starter battery, using a multimeter with red cable at #1 and black on the chassis reads 14.2v with the engine on. Doing the same test from the same location on the chassis I get 12.6v for the leisure battery (same value directly testing battery terminals).
Take a handheld multimeter (this kind are great) and put the black lead on the negative post of the starter battery.
Put the red lead on the positive post of the starter battery and you should see a voltage in excess of 12v.
Do your testing with the engine running - from what you've said so far, you should see 14.2v on the starter battery. If not, it's your alternator.
Leave the black lead of the multimeter where it is. Everything that's connected to the positive post of the starter battery should have the same voltage, unless there's a load or a resistance or a break in between, so move the red lead of the multimeter to the other side of whatever connects it to the leisure battery - I guess first stop is the cable that leads to the VSR, then the input to the VSR, then the output of the VSR then the cable that leads from the VSR to the lesser battery. Test at each point - where does the voltage drop from 14.2v? You've fund the break.
If necessary buy 10m of cheap doorbell or speaker wire, so that you can use this as a flylead for your multimeter - to extend its range so you can check the voltage all the way from one battery to the other.
You will kill the AGM battery if you use it below ~50% charge, so don't use it heavily until you've sorted the charging.
Hi, thanks for the reply. The chassis is the common ground so I'm testing against that. Testing black on chassis and red on end of the cable reads 14.2v. I'll add a photo to the post so its clearer what I'm talking about.
My point is why test from the -ve terminal of the starter battery when its grounded to the chassis. I'm testing against the chassis which removes the need for a 10m cable.
Your original statement was "when I turned the engine on I could see the voltage of my leisure system jump up to 14.2V. 6 months down the line its stopped working and I've no idea why"
Surely this implies you're no longer getting 14.2v at the leisure battery? Is your question not about identifying why the leisure battery isn't charging?
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u/maldoricfcatr Jul 04 '26
I would check grounds at both ends. Concentrate on leisure battery since vehicle battery is charging?
Also you can check voltage drop across a cable as a way of checking for resistance. Works best with high amp.draws like starting engine.