Auxillary Battery Woes

[BreakingTrails]

I have recently purchased an 'overland ready' vehicle and to complete it- a dometic CF50 fridge/freezer. The aux battery system did not previously have a car fridge so I'm not sure if the infrastructure they built it with is sufficient... anyway i'm getting ahead of myself. I currently have a couple of problems arising which essentially are making my fridge unusable.
1. The battery does not seem to consistently charge when the car is running- I dare say it is rare to see it charging. Max voltage after 4 hour drive yesterday was 12.5V.
2. When I plug in the fridge the voltage drops dramatically (though very briefly) down to 9.9V triggering the fridge's low voltage battery monitor which shuts the fridge off. After a couple of seconds the battery jumps back up to 11.6 or so, and when the fridge turns off back up to 12.5.

Some more info about the system-
The battery itself is fairly new (1 year, 12000miles)
Car is a 1998 Land Rover Defender
Cables running from crank battery to aux seem fairly small
Battery was performing okay for the first week or 2 with the fridge but has recently capitulated.
System is built with automatic isolator and electronic voltage display
We thought that the battery wasn't taking on any charge at all but noticed a slight rise in the voltage after the last drive
No problems with the crank battery at the minute.

Any tips would be most welcome!

 

[Buddha.]

I would guess your isolator isn't working or your wires to the auxiliary battery are so undersized that you're getting too much voltage drop. Measure the voltage at the cranking battery and at the auxiliary battery to see what the voltage drop is. Can you read any numbers on the auxiliary battery's wires? Something like 10awg or 8awg etc. What battery isolator do you have?

 

[DaveInDenver]

Where are the batteries located? Physically close to each other or are there long runs of wire between them? How long is the wire run to the fridge? To me this sounds like the wiring is undersized and probably coupled with long runs leading to large voltage drops.

 

[Rbertalotto]

Battery was performing okay for the first week or 2 with the fridge but has recently capitulated.

If you are sure about this, then it is a bad isolator relay in the charging system, a bad connection somewhere in the wiring or a bad battery. If the wire charging the battery was working OK for a couple weeks, then the gauge should be sufficient.

Let us know what you find.

 

[luthj]

Time to start investigating with your volt meter. You should be able to trace down voltage drops and/or faulty components. In my experience about half of these installs have woefully undersized components, and about 1/4 have dangerously poor crimp work and wire protection. It is incumbent upon you to verify the state of the install. Or if you can't find a qualified automotive electrician to verify for you.

 

[WOODY2]

Sounds as though you should also investigate wire size to fridge plug?

 

[BreakingTrails]

turns out it was a 2 pronged problem-
#1 was a loose nut on my new starter motor causing only partial charging of my battery
#2 was undersized wiring from battery to 12v socket.

solved rather easily by stripping the plug from my fridge's power cable and wiring it directly to the auxiliary battery! Now to hope my Dometic fuse doesn't blow as they are prone to!