+12V 200A Relay Isoator or DC to DC charger

[rarebird]

Hello

I currently run a lead acid 102A auxilliary battery under the bonnet of my Nissan Patrol for the purpose of powering a 70 litre Waeco car fridge.
I intend putting in a second auxilliary battery (AGM type 130A) in the rear of my Patrol to get some more life out of the fridge.

The current system uses an IBS 12V 200A relay to distribute the power and is monitored via an in cabin TJM (IBS) management system.

I was considering changing the Relay to another IBS model designed for three batteries however I get conflicting advice on whether that system will suffer to much current loss over the three metres of cable from the under bonnet relay to the rear battery and am told I will need to use a DC to DC charger instead.

These chargers are quite expensive (a 40A model for three batteries is $600-$700) and would appreciate some more opinions before committing myself to the additional expenditure.

Thanks

Rarebird

 

[Lucien Nunes]

Whether the B-2-B charger offers you much advantage may depend on whether your vehicle has an intelligent alternator controlled by the ECU. Such units, fitted to recent vehicles e.g. Euro-6 compliant models, save fuel by fast-charging the vehicle battery on the overrun and when mechanical power is most available to drive the alternator, and shutting it down part of the time when not. If the auxiliary battery is not wired in such a way as to be able to take advantage of the brief fast-charge periods, it will get left behind relative to the vehicle battery. Voltage drop along the cable to the back of the vehicle is your enemy here. The B-2-B charger not only deals with the voltage drop but carries on syphoning charge out of the front battery into the rear, even when the alternator is idling, so overcomes these problems.

OTOH, problems caused by voltage drop in cables can always be solved with bigger cables. Whether it is economic or convenient to do so is a different question; for an anchor windlass local battery in the fore peak of a boat, 20m away from the engine room, the cable might be prohibitive. As your cable is only 3m long, it seems to make sense to beef it up. If you can run a 35mm² or larger cable to the rear battery, it will probably work fine. We have vehicles with 220Ah auxiliary batteries in the back and they were giving satisfactory service with just 16mm² even with 30A intermittent running load on the rear battery, although these have non-managed alternators so the charging voltage is continuously available.