Battery combiner or DC-DC charger to charge the starter battery

[Tenedos]

I am debating whether to replace the Magnum battery combiner that connects my house and starter batteries with a DC-DC charger.

For context, my alternator is connected to house battery. Main Magnum Charger/Inverter is charging the main battery. And a separate backup charger (the original Jeanneau charger it looks) is charging the starter battery (has its own switch). These two battery banks are connected via a Magnum battery combiner (with two 30A fuses on each battery connection). From what I understand, the battery combiner is parallelizing the connection when the voltage on either of the batteries is above 13.1V which is configurable. So when either of the chargers, or the alternator is active, I essentially have 7 batteries connected in parallel: 6 house and 1 starter.

I find this setup a bit confusing. One idea is to replace the combiner with a DC-DC charger, to charge the starter battery from the house battery.

I should also add that, I am thinking of replacing the house batteries with drop-in LiFePO4s at some point in the future that have integrated BMSes. There is an entire thread about protecting the alternator in this setup and one good solution seems to be having a FLA battery connected in parallel to the lithiums. My existing setup essentially provides that since when the alternator is charging, battery combiner is open, so essentially I have 7 batteries in parallel: 6 LiFePO4 house batteries and 1 FLA start battery. So I guess this is a better configuration for future lithium expansion?

[rbk]

If you’re mixing battery chemistry / charging profiles or plan to in the future, then yes a dc to dc like sterling or balmar will be required. If they are the same you can simply install an ACR like blue seas. Both will automatically combine the house and start bank when charging and disconnect when discharging. The DC to DC will do the same but you can also change the charge profile and they cost more money.

[YANDINA]

The Yandina Lithium compatible Combiners and Trollbridges allow lead-acid and Lithium batteries to be combined for charging but otherwise isolated so the Lithium batteries don't discharge themselves into the lower voltage lead-acid batteries.