Fridge wiring with dual battery setup

[Vanders Lurker Find all posts View Profile Close Vanders]

I've purchased a Redarc BCDC 1225D that will be setup in the rear of my 2018 Prado to charge an aux battery that will run a fridge, lights etc while free camping.

My question is whilst driving am I better to have the fridge running off the main battery and only run off the aux when camped up? This will allow the aux battery to get as much charge as possible to completely top up whilst driving between sites. Has anyone done something similar that would facilitate this without physically have to change anderson plug connectors from one source to the other?

Cheers in advance.

[dBC Member Find all posts View Profile Close dBC]

I did something very similar just a few weekends ago using a SPDT relay, something like the one below. My BCDC and Aux battery are all under-bonnet which makes wire routing a bit easier. I already had a Redarc Solar Relay, triggered by an IGNITION-ON signal from the wipers as per the Redarc installation guide, so it was easy to add a second relay (triggered by the same signal) to switch the loads between the two batteries, depending on whether the ignition was on. As you say, it ensures all 25A are going into the Aux battery during the bulk phase, and when it's gone into absorption mode, it means the BCDC's current monitoring is actual current going into the battery rather than combined battery + load current.

The only downside I've noticed is that because the relay switches the loads over to the cranking battery before cranking starts, it puts a slightly higher load on the cranker while it's cranking and more importantly, if the fridge is running at the time you crank, the voltage can drop low enough that the fridge's low voltage detection trips. That puts my Waeco into fault mode, but it self recovers about 10 seconds later once it sees the voltage has returned to normal.

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

Why do you think it is better for the fridge to work off the main battery when the engine is running? The energy comes from the same place and all you are saving is the efficiency of the BCDC and add costs of extra wiring and a relay. Not worth the trouble and another point of potential failure in the bush. I have my aux in the back with a BCDC. Solar infeed from the roof when the enhine is off. Works very nicely. Build thread is in my signature if you are interested.

[dBC Member Find all posts View Profile Close dBC]

I've run with the simpler approach for the last 2 years, and I agree it works fine. Downstream of the BCDC you've only got 25A to play with and when your Aux battery is depleted, the more of that 25A you can pump into it, the faster you'll get it back up. Any loads, including the fridge cycling, steal current from that 25A budget. So if the fridge kicks in and draws 4A say, you're now only pumping 21A into your depleted Aux battery. By flicking all the loads across to the alternator once it's running, the entire 25A can be pumped into the Aux battery.

That's when it's in Boost mode, but there's a secondary advantage when the BCDC goes into Absorption mode. It's then monitoring its output current to decide when to switch to Float mode, but it assumes all the current is going into the battery. Any loads that are drawing current will appear to it as if the battery is drawing that current, causing it to delay its switch to Float.

As for failures in the bush, mine is set up so I can bypass the relay and connect all the loads back to the Aux battery in about 30 seconds. Now I just need to find time to go on a touring trip and see if can notice any improvement in recharge times.

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

Ok, I get that and it makes sense. For me it would require running a lot more decent cable so I will pass.

[dBC Member Find all posts View Profile Close dBC]

Yes, I can see it would be a lot more involved doing it down the back. Up front it becomes a pretty trivial job, so I thought it worth a try. I'd actually bought the relay years ago thinking it would be essential and quickly discovered it wasn't. Two years later, on a rainy Saturday afternoon while making some unrelated changes, I discovered I had all the bits I needed, so whacked in the load-changeover-relay. It tests fine in the driveway.... but I need to get out there and see if real world usage justifies the slight increase in complexity.

[Vanders Lurker Find all posts View Profile Close Vanders]

Thanks fellas, I'll give this some more thought but happy to know there's some options.