I was to replace my 4* AGM leisure batteries for Lithium, but I have *2 additional lead acid starter batteries for my boat’s bow thrusters, on the same bus and charging circuit. Can it be done?
The trouble is lithium batteries have a working voltage of 13.3V when full so will continuously charge the lead acid batteries even when the lithium batteries are not being charged themselves.
The correct way to do this is to split the bow thruster batteries off the same bus as the leisure batteries and fit either a voltage sensitive relay to connect the banks when charging or fit a DC to DC charger between the two battery banks that works when the lithiums are being charged.
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The disconnect with either option will also prevent the lead acid disharging the lithiums as well as their self discharge is higher.
Or vice versa depending on how you set it up.
you want to split the systems i assume there is also some sort of alternator charging the lead acid starting battery’s, that will not like them all in parallel. i personally would use a LI-BIM 225 over a DC-DC converter, i have had better results and you get to keep the emergency start feature, but you get less charge into the lithium from the alternator then a DC-DC converter.