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Charging Lithium and lead acid battery at the same time

Hi Philppe,

Can I charge with my alternator a Lead acid start battery at the same time a Lithium bank. I'm using a "multi battery Isolator"? or do I need to replace the start battery with a Lithium too?




Hi Frits,

This is a question many of us have and there is no black and white answer.

First you need to adjust your alternator regulator to charge the lithium battery efficiently (13.8V bulk, no or minimal absorption, and 13.2V float).  Then you have a few options:

1- Use a splitter to connect your alternator to the two batteries (make sure the voltage sense wire of the alternator's regulator is on the lithium battery).  The charge voltage of 13.8V is not enough to fully charge the lead acid battery and that battery will not last long.

2- Use the Balmar "Duo Charge" or equivalent (DC/DC converter to charge the lead acid battery with proper three stages, taking its power from the lithium battery when the alternator is running).  I have one but I am not convinced.  I just unplugged it!

3- Connect the two batteries in parallel - there will always be a trickle charge from the lithium to the lead acid battery.  Maybe not the most efficient!  I connected my batteries like that last week for 48 hours, then after disconnection the lead-acid battery  stabilised at 12.85V = full. I think I will keep them in parallel full time as it cannot hurt the lead-acid battery and I have plenty of energy. Side benefit: if there is an over-voltage situation and the BMS disconnects the lithium battery while the alternator is charging you will still have a load on the alternator... and your alternator has less chance to burn!

4- Install a second alternator on the engine just for the engine battery

Then, as you suggest, you can only have lithium batteries on board and use a splitter.  Like that the same regulator setting works for all of them.  Just be careful when buying a lithium battery to crank the engine that it can supply the required current for the time it can take to start the engine.  Some cheap 12V lithium batteries are an assembly of tens (maybe 100) small cylindrical cells and a 100Ah battery may not be able supply more than 100A continuous - maybe 200A for one second.  Not enough for cranking.  So check the specifications.