BC/ DC Chargers and Anderson Leads

[Rob M]

We have had 3 previous tow vehicles all fitted with an Anderson Lead connected to the auxiliary deep cycle battery in the vehicle. No problems what so ever.
However, we have changed our tow vehicle and it has a lazy alternator. My understanding is that once the alternator senses that the cranking battery is fully charged it slows down or similar and reduces the current it produces.
Hence we fitted a Redarc BC/DC before the auxiliary deep cycle battery in the tow vehicle.25 amp to keep the deep cycle auxiliary battery charged. This runs the fridge in the tow vehicle and lots of charging points.
The Anderson lead was fitted to draw power from the deep cycle battery.
The Topaz also has a 40 amp BC/DC charger fitted.
The problem created was that the charger in the Topaz would draw up to 40amps from the deep cycle battery if the Topaz batteries were run down. This flattened the auxiliary deep cycle battery as it could only be charged through the 25 amp charger.
Once this battery is flat the charger in the tow vehicle goes into limp mode and will not charge the deep cycle battery. You have to trick it by connecting the 2 batteries with jumper leads. It then thinks there is charge in the deep cycle battery and commences charging it.
So today I have rewired the Anderson lead to draw directly from the Cranking Battery. This is the recommmended wiring procedure from Redarc. There is a real risk in this if I forget to disconnect the Anderson lead to the Topaz and the Topaz batteries are down.
I am now looking for an auxiliary battery protection type device that I can put in the line to prevent this. I recall I had something like this many years ago to prevent the auxiliary battery draining the cranking battery. It only allowed current to flow when the cranking battery was fully charged.
Suggestions please.

[Lt4vette]

Our Tvan is wired so that the DC-dc charger only draws power when the tow vehicle ignition is on - via an ignition switched signal wire on pin 2 of the trailer plug to the Redcar  charger.
Thanks,
Russell

[Smallfry]

The dc/dc charger in the Topaz will not draw from the crank battery when voltage drops

[Gumb]

What you've done is correct. The Anderson powering the trailer should always come from the cranking battery and as has been suggested, it only operates when the ignition is on. That's how Track wanted mine set up for the Firetail.

[peter_mcc]

I am now looking for an auxiliary battery protection type device that I can put in the line to prevent this.

Buy a 12V 100A relay (1) and wire it so that it is only energised when the ignition is on (or engine running). Then run the Anderson plug feed through the relay.

This will disconnect the power from the Anderson socket when the ignition is off automatically.

You could do it with a Voltage Sensitive Relay (VSR) as well (eg Redarc, Projecta, etc ones) designed for dual batteries but it costs more and doesn't really give any great advantage. This would connect the Anderson plug based on the battery voltage rather than the ignition. I guess it would help if you tended to sit there with your ignition on and engine not running for a while - the basic relay would connect the Anderson plug, the VSR wouldn't.

(1) eg https://www.jaycar.com.au/automotive-relay-150a-spdt/p/SY4073
There are also lots on eBay - search for 12V 100A relay eg https://www.ebay.com.au/itm/253779477644

[Cruiser 105Tvan]

Old School was use a starter solenoid, if wired correctly will do same job.

[Rob M]

Thanks for the advice.  Have been doing some more research and checking. As Rob from Bowen suggested the Redarc BC/DC charger in the Topaz acts as an isolator and prevents the cranking battery being drained.
The problem for me turned out to be the fact that I run a fridge in the back of the tow vehicle which also draws off the deep cycle battery.
It wasn't just the Anderson lead, it was because both were drawing off the deep cycle battery.
Hopefully problem solved
Rob

[Smallfry]

You can use this or something similar to protect your deep cycle battery in vehicle dropping too low, or some of the newer ARB/Engel fridges will cut out at pre selected voltages

https://www.projecta.com.au/power-management-products/intelli-volt-12v-30a-low-voltage-disconnect

[peter_mcc]

You can use this or something similar to protect your deep cycle battery in vehicle dropping too low, or some of the newer ARB/Engel fridges will cut out at pre selected voltages

https://www.projecta.com.au/power-management-products/intelli-volt-12v-30a-low-voltage-disconnect

If you go this route don't stick it under the bonnet or your beer will get warm! We had one - on hot days the temperature under the bonnet is too high (even off to the side) and the unit thinks it is in an overcurrent situation then shuts off. It might work if you have it in the cabin but even then I'd be worried about a hot day as the inside of a parked car in the sun can hit 75C.

[Smallfry]

Yeah mines on a shelf in glovebox, no problems

[Gumb]

My Redarc unit sits happily under the bonnet on a bracket in front of the radiator so it gets good air flow. No problem at all, even in FNQ.