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.
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.
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.