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Flooded start battery and lithium drop-in in same bank?

CarlN

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Jan 4, 2009
577
Ketch 55 Bristol, RI
I'm adding a large lithium drop-in house bank and considering removing the VSR between the house and start battery and just connecting them into one bank. The bank will be set for the lithium specs of 14.1v absorption and 13.5v float which seems to be "close enough" for the sealed, flooded start battery (not AGM). This way there will always be a lead acid battery to absorb a surge, protect the alternator, and provide power should the BMS shut off.

I'm not worried about the single bank being drawn down such that I can't start the engine. There are low voltage alarms and it even texts me on my cell phone. If worst comes to worst, the genset is on its own independent start battery.

Will the start battery's life be significantly shortened by these charging voltages? Am I missing something?
 
Feb 21, 2013
3,629
Hunter 46 Point Richmond, CA
Assume you have a common battery charger. Good question for your battery charger manufacturer. On my Hunter 46 the engine start battery is a Dec 2018 lead acid and the house batteries are July 2020 AGM's. The Victron battery charger is set for charging AGM's so will see what the battery life of both will be.
 
Last edited:
Apr 22, 2011
725
Hunter 27 Pecan Grove, Oriental, NC
To get the max life out of a AGM battery, they should be charged to 100% occasionally. This helps prevent sulfation. To get to 100% requires high volltage of 14.6 to 14.8 depending on the manufacturer.

You could do this with your setup by using a small agm charger while at the dock with a manual switch between the two batteries to isolate them. Kind of a finicky setup though.