Question about lead acid to lithium with dc to dc charger

[Allan71180]

I am installing a lithium house battery on my boat and connecting it to my lead acid starting battery with a Renogy dc to dc charger. My question is will it damage my lithium battery at all if they are connecting momentarily before the alternator is running. The dc to dc charger would connect when I turn the key to the on position. If it's a problem I will find something that is hot only when the alternator is running.

 

[sunshine_eggo]

I am installing a lithium house battery on my boat and connecting it to my lead acid starting battery with a Renogy dc to dc charger. My question is will it damage my lithium battery at all if they are connecting momentarily before the alternator is running. The dc to dc charger would connect when I turn the key to the on position. If it's a problem I will find something that is hot only when the alternator is running.

If properly connected, it doesn't work like that. The DC-DC is a charger, not a shorting device. It literally charges the house battery in accordance with the charging program and draws the needed power from the starter battery/alternator.

 

[Isaac-1]

As mentioned above a True DC-DC charger regulates the voltage, just like a modern AC-DC charger does, it will even step up the voltage if you have say 12.5VDC going in, and the charge profile needs 14,1 VDC going out.

 

[B.Rad]

Allan, you should be good. The alternator charge circuit feeds the Renogy DC-DC which outputs to the lithium batteries. Not sure if you plan to install the switch/es for ON/OFF and/or 100/50% charge. I have a large bank in the RV and it is definitely a must have for me. When running at night the headlights can dim, if bank is low, when the DC-DC is 100% on, but is okay at 50% charging. RV has a 150A alternator.