Leisure + Starter batteries - how do they work together (fridge confusion!)

[agentdagnamit]

Hi everyone, this is my first post.

I’m trying to understand (in a simple way) how my starter and leisure batteries work together in my Mazda Bongo. The van also has a Zig unit. The main appliances running off DC (when not hooked up) are a 12v/240v fridge, led lights and the radio.

I’ve had the van for about 7 years, mostly things are fine, but occasionally I seem to manage to drain the starter. I also suspect the leisure battery could do with replacing as it is over 4 years old and gets several months of use each year.

I definitely have some sort of split charger, and the Zig unit is capable of recharging the batteries when hooked up. It can also provide AC power to the fridge.

A couple of things are confusing me:

A few days ago (in 30+ degree heat) the fridge shut down after less than 10 hours (on 12v). But as I write this, it has been running for 14 hours and still going strong. The leisure battery reads about 11.8v

But, the starter battery also reads about the same. It seems like both batteries drain and recharge together, even whilst the van is parked up and not on EHU. Shouldnt the starter battery sit at around 12.4v or more (ie ready to start the van, and then recharges via the alternator). I thought the split charger relay would isolate the leisure battery when the engine is off.

I just disconnected the leisure battery, and the fridge continued to run. This was surprising! Should I be worried? The fridge probably has a voltage based auto cut-off, but isnt that designed to protect a slow discharge leisure battery, not a starter type battery?

The Zig unit has a “Car or Caravan” battery select switch. I’ve taped this down so it always selects the leisure battery. Why would I want to draw power from the starter? It also has a battery indicator (red or green light) – but it only ever shows green (for good).

Sorry, a long message. Maybe this comes down to me trying to understand why the relay isnt blocking off the starter battery when the engine isnt running (so all appliance can ONLY run off the leisure). Or have I misunderstood what the relay is supposed to do. Someone suggested that I check it isnt stuck, but I wouldnt know how to identify it (I cant seem anything other than fuses wired in line with the battery leads).

Thanks

 

[funflair]

Hi

I would suggest that most if not all your assumptions are correct, yes the starter battery should sit around 12.6v with nothing connected and yes the split charge relay should prevent the leisure battery drawing power from the starter battery when the engine is not running, I assume your fridge is a compressor model (no gas option) I might be tempted to do a simple test with a voltmeter by checking the voltage of both batteries with no loads switched on then again with a load on the leisure battery, the terminal voltage should drop slightly on the leisure battery but not the starter.

 

[agentdagnamit]

Thanks. Yes, that's the test I've done. Both batteries read pretty much the same with (or without) the fridge running. If I knew where the Relay was, I'd see if it was stuck....

Any recommendations for a good Sparky in Bristol!

 

[autorouter]

Do you have any solar power? If you have a split charge relay that is the Voltage Sensitive Relay (VSR) type, the solar can sometimes switch it on, depending on the exact wiring.

There will be a thick wire from the relay to the leisure battery, and also from the relay to the starter battery. And a big fuse, maybe 30A or 50A, in each wire near each battery. I would think the relay will be near one of the batteries.

If there's only two thick wires, plus a thin earth wire, to the relay, then it's a VSR. If there's an extra wire then that's the trigger wire from the alternator, that switches the relay on when the alternator is powered up, so it's not a VSR, it's a standard split charge relay.

 

[TheCaller]

Zig units are very basic & are designed for a typical car/caravan setup, not a motorhome or campervan. As such, they don't do what you expect in a m/h.

The Car or Caravan switch is almost certainly wired to be starter & leisure in the Car position & leisure only in the Caravan position. If you have it taped in the Caravan position & it still draws power from the starter battery, it may be faulty, or it may be wired in an unusual way. You usually have to select 'Car' to get the leisure battery to charge while the engine is running.

It is very possible that there isn't a relay, just the switch that you see. I got caught out in a VW camper with a Zig unit back in 2008 - I couldn't believe it was as basic as it was until I had to work out why I couldn't start the van one day!

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[Guigsy]

The Zig on my previous 2007 van used a relay attached to the ignition position on the key. There were no diodes. When the key was turned, both batteries were connected together. I could here the click from the Zig panel when I turned the key.

It was basic, but it did have advantages. At one point I had a weak cab battery, but I could effectively boost it by leaving the key in the ignition position for a few minutes before I started the van to pull some leisure battery charge. I could also leave the key in on sunny days and it'd boost both batteries. The main disadvantage was when the starter motor was running, it pulled a lot of current from the leisure battery and the installer has used pretty thin wiring.