Leisure & Starter battery always reading the same voltage!?

[agentdagnamit]

I've been testing my leisure battery and how long it will power my fridge via its 12v supply. So I've been measuring the voltage every 30 mins or so, with a multimeter across the terminals.

Weirdly (to me anyway) the starter battery's voltage seems to drop at the same rate roughly. As far as I know, everything is wired correctly, excessive fridge use has never killed the starter.

I'm doing all this parked up, engine off and not connected to any AC hook up.

Should I be worried?

[g8dhe]

You don't say what voltages your getting, or if the LB is being charged from EHU in which case a lot of the modern VSR are bi-directional and if the LB is being charged then the relay will operate to allow the SB to be charged as well.

[agentdagnamit]

Not connected to any EHU. Voltage starts at around 12.4 dropping to about 12 when the fridge has been on for 3 hours.

I've not had any issues starting the engine, but the fridge is switching off after less than 10 hours (admittedly, most recently in very high temperatures)

[g8dhe]

It is a bit odd in that case, have you checked to see if the relay is releasing and hasn't welded its contacts together? Perhaps best to also check current flow on the charging leads between the two. A 100AmpHour LB will normally keep a compressor fridge running for between 2-3 days. A coolbox you will not normally power for one night as they are horribly inefficient!

[agentdagnamit]

I think this is probably beyond my level of understanding. I cant see where the relay is (I have a Zig unit, does that do the job instead?). There doesnt seem to be anything else wired into the positive to positive connection (other than fuses).

I just checked the voltage across the 2 batteries with the engine running. The starter battery was slightly higher (13.7v to 13.4v approx), but to me that suggests the relay is doing its job and the alternator is charging both batteries. But once the engine is off, both batteries read pretty much the same voltage again.

I dont want to buy a new LB unnecessarily, but this one is 4.5 years old, so maybe that's the obvious first step.

[g8dhe]

The Zig unit only controls the 12v power coming from the batteries to the various circuits it feeds.
Normally there will be one relay with cables going to each battery and fused close to each battery, if its a Voltage Sensitive Relay then the only other wire will be a small one to chassis from the relay, if its controlled from the Ignition key then one to earth and one to the internal fuse block is normal, but there is more than one way of wiring these up.