Isolating DCDC Chargers

The majority of DCDC chargers powering additional batteries are using a wet start battery, meaning the nominal standalone voltage is around 12.8V. Manufacturers make products to suit the majority, and for the most part it works well. Until it doesn’t.

The Problem: DCDC chargers have in-built isolation systems based on wet battery voltages. This means that without any form of hard isolation, you will drain your start battery to this cut-off voltage, worse, with more people moving to lithium start batteries, these isolation values will drain your entire start lithium to within 15% capacity.

Redarc BCDC On / Off Thresholds

When I connected my DCDC charger, I didn’t think about cut-off values. I kept wondering why my additional lithium was always fully charged, even though I have a small constant load connected to it. That battery should discharge 1ah per week for my use. It wasn’t. So I went looking for the why and then remembered the cut-off isolation values.

I don’t want to keep my start battery at 12.7V or slightly below. When I stop my car or charge my battery, I want to maintain a fully charged state without anything drawing from it whilst sitting around.

Solution – a hard disconnect system in the DCDC input.

There are two primary ways to stop a DCDC charger constantly charging additional batteries until its internal fixed disconnect occurs – high current relay OR programmable disconnect. For any charger 40A and below, high current relay is a simple, reliable and a cost effective solution, typically around $10 (ignition switched). For higher currents, a programmable disconnect is required (65A @ $90 – 220A @ $150), yet a programmable disconnect may still be unreliable for a lithium start battery, as its minimum voltage needs to remain within a slim threshold between alternator charge voltage and battery resting charge voltage.

All modern vehicle variable voltage alternators have room for error in this scenario.

Chart showing different 12V battery discharge cycles.

Redarc themselves recommend a high current relay. Two chargers with relays may be a better solution within modern vehicles than one large output charger running a programmable disconnect.

My Situation – I used a high current 70A relay in my own installation, which runs a BCDC25 for my second lithium. The relay is switched from ignition, so if the car is on accessories, nothing happens, but once on ignition, the charger will commence running as per designed. I have a camper trailer in the future which will require 30A for the Redarc Manager 30 system. It too will run the same setup, a 70A ignition switched relay ensuring total isolation between my start battery and additional batteries.

2 thoughts on “Isolating DCDC Chargers”

  1. Hi, I have a 2014 200 cruiser with dual batteries.
    They are isolated, I have a Redarc Bcdc25d charger,
    Connected with a 200w solar panel on the roof.
    At the moment the vehicle is parked outside daily.
    my expectation is that the 200w solar panel should be charging the deep cycle Aux battery during the day.
    But twice when I have run my bushman fridge over a weekend off the Aux, the Main battery has dropped down to 10v…requiring a jump start.
    Both batteries a grounded to chassis.
    Any ideas on what would cause this to happen ?

    • Hey Peter, grounds have no impact on this, as a circuit must be completed. You are correct, the solar should be charging your Aux and the system voltage isolated from your primary battery.

      Isolation is the starting point to fault finding electrical. Isolate the start and aux systems. Remove your ground and place an amp meter inline to see if you have parasitic drain when off. Don’t start the car. It can actually take up to, and beyond, 10 minutes to see a drop in current to normal levels (50mA or less is normal). Youtube how to do it.

      But isolation is how you fault find. Isolate everything from your normal start and run system. If the battery continues to drain, and there is no parasitic drain, then the battery may be stuffed. You then start to connect circuits and check by loading them (if appropriate and correct meter used).

      You can repeat this process with the car off, but aux system connected, to see if the system is charging. The LED’s on the DCDC will show. Once it has isolated, plugin your fridge and see if there is any draw from the main battery.

      It is hard to tell you something like this, without seeing the vehicle directly, because there may be something gone wrong with your car that just happens to be causing parasitic drain under x conditions. But the simple rules are to isolate and begin there. Just be aware that modern vehicles will show drain for a period of time after you close the doors.


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