Lithium starting batteries and DC/DC chargers.

Let’s have a quick chat about the intricacies of lithium starting batteries when dealing with auxiliary/secondary battery systems running a DC-DC charger. For ease of this explanation, we’re going to include setups that have vehicle mounted 2nd battery system (under bonnet, ute canopy, etc) and a caravan/camper setup where the connection is made from car to trailer via an Anderson plug etc.

Background info

Lead acid battery chemistry (flooded, sealed, AGM, etc) are nominally 12 volts and have a fully charged voltage of approx. 13.5v and a ‘flat’ voltage of 10.5v. There is a roughly linear relationship between voltage and depth of discharge (DOD) for these batteries meaning every % of difference between the 13.5v and 10.5v is the same % difference in DOD. Lithium on the other hand does not share this linear relationship of voltage vs. DOD. Have you noticed how your lithium power drill just suddenly loses battery life after working completely fine the whole time before? Lithium also has a very small difference in nominal voltage (12.8v LiFePO4 over 12.0v PbAC). In normal operation as a starting battery, with an alternator charge of roughly 13.8- 14.4v, this poses absolutely no issue and the internal battery management system of the lithium takes care of the battery entirely. A unique set of circumstances do arise when combing with a auxiliary battery system and DC-DC charger though….

DC-DC chargers and VSR’s

In a vast majority of cases, a DC-DC charger for an auxiliary/2nd battery system in a 12v setup will combine the starter and auxiliary battery using voltage level as the connect/disconnect.  Most of the time, this number is set to approx. 13.2v for connection and approx. 12.0v for disconnection.  In practice, this means the vehicle will be started and will charge the starting battery from the alternator until it reaches 13.2v and then allow the DC-DC to connect and start charging the 2nd/auxiliary battery.  When the vehicle is engine is switched off and the alternator no longer running, the system will remain connected (start and auxiliary) until BOTH batteries have reached 12.0v before the DC-DC charger will disconnect from the starting battery and run purely from the auxiliary battery.

If we look at the graph above, we can see that if we have a lithium starter and ‘wait’ until this 12.0v disconnect level is reached, we could run the risk of having the starting battery very low in its charge capacity and potentially not have the power to start the vehicle.

The simple fix

So, not to worry!  The simple fix is to install a relay in the supply line to the DC-DC charger that is only closed/on when the vehicle is running and will disconnect the DC-DC charger from the starter battery when the engine is off.  We recommend getting a robust relay larger than the current of your DC-DC charger to ensure everything stays functioning without issues for the life of the install.  Check out the below schematic for reference.

If you’re running a caravan/camper setup, this can be avoided by disconnecting the power supply (Anderson plug etc) from the car once you’re setup at site.

DC-DC chargers with an ignition ‘sense’ wire

A quick note for DC-DC chargers with an ignition ‘sense’ or ‘trigger’ wire.  This wire is often for use of a DC-DC charger and a smart alternator.  It is a trigger to override the CONNECTION of the DC-DC charger, not the disconnection.  An ignition wire is to turn charging ‘ON’ and we are talking here about turning the charger ‘OFF’ 😊.