Relay on ground wire of Redarc SBI12

ER,
I presume you have a simple and conventional setup where once the isolator operates the batteries are connected and charged in parallel.

Under this arrangement, when the key is off, any load on the electrical system will draw down both batteries until the system voltage reaches the drop-out voltage of the isolator, at which point the isolator disconnects the batteries.

Your crank battery won't steal from your second battery unless it is faulty and/or there are loads normally attributed to the vehicle, such as lights left on, radio, etc.

Likewise, your fridge connected to your second battery will draw off both batteries until the voltage (of both, because they are connected) comes down enough for the isolator to drop out, whereupon the fridge will run off the second battery alone.

If you want to disable the smarts in the SBI12, a simpler way than adding an IGN-controlled relay to the ground wire is to just remove the Redarc module and hook up the solenoid coil to an IGN controlled source.

If you do this you should add a diode across the coil terminals with the band at the positive side of the coil. This acts as a spike/surge supressor, a function normally done by the Redarc module.

Cheers

G'day, I see nothing wrong with the current set up, it disconnects at a set voltage of the crank battery (12.7Volts), with a delay of 10 seconds, the crank battery is supplying power to the aux battery, not the other way around. Cheers.

I wouldn't worry about it, most VSR type isolators work on the same principal, both batteries
will remain connected till the surface charge drops. Assuming no accessories are running this may take a few minutes but very little power will flow from the aux to the main as the only load on the main will be ECU back up power etc. Your more likely to loose a small amount from the main to the aux if a fridge is running for example but again very little charge as the isolator will drop out fairly quickly and your only talking milli amp capacity not amps. Same the other way around, as soon as you put any load on the main ie turn the car headlights on the surface charge will drop and the isolator will drop out within a few seconds, and you would loose very little aux charge.

I have done what you have suggested in the past. The Redarc isolators used to drop out at 12.5V and while connected, the solenoid used something like 110mA (from memory). The issue I found was not with camping, but when stored at home - both batteries would get drawn down to 12.5V even though they had no load on them. Made it impossible to keep your batteries fully charged.

Redarc addressed the problem about 5 years ago by lifting the cutoff voltage to 12.7V. With wet cell batteries, I am OK with this. But if I had a pair of Optimas (or AGMs) in there which sit at a higher voltage, then I would fit the relay.

I just find that the crank battery voltage slowly decreases to below 12.6v, the deep cycle holds a higher voltage of 12.85 for longer. My isolator disconnects at 12.6v. So the crank battery is slowly pulling power out of the deep cycle until they both reach 12.6, then it disconnects. When the car is not being used for camping & the fridge isn't switched on, would this power transfer be reducing the life of the deep cycle?