How does this not ruin a start battery?

The DC-DC chargers, Victron and others, provide a current limiting function to the amps going into say a house bank from a start battery that is hooked to an alternator.

If a start battery is charged to 13.2 volts and is hooked to a 11 volt house battery bank and a switch is thrown, to join them, what is the current draw?


I am questioning a pure relay from start to house, if the house battery bank is quite discharged and wondering about the math to determine the amp draw between the battery banks.

Assume that the engine is running and the alternator is CAPABLE OF putting out 100 amps to the start battery circuit

 

Well, I use an ACR between my house bank (Just 2 group 31 AGM's, 105ah 20 hour rate each) and a starting AGM. I do use a 105 AMP high output/low RPM alternator from Arco marine. Where I differ from the setup you mention is that I have my alternator connected to my house bank. I try not to deeply discharge the house batteries. But if they do get rather low the ACR keeps the banks disconnected until the alternator has time to bring the house bank to a voltage high enough to combine the house and start banks. There is a time and voltage threshold to combine but I don't know it off hand. I think it's something like 12.9 volts for 2 minutes.

Since the ACR doesn't trip (combine) until the house bank is about 13 volts there isn't going to be much current flow from the start battery to the house bank even if the start battery is resting at a full charge.

Eventually the whole system reaches 14.2 - 14.4 volts off of the alternator but this takes some time especially if there are air conditioning loads on the alternator.

If you connect your alternator to the start bank and your house bank is only at 11 volts there would probably be an issue with the ACR cycling. This article explains it better than I possibly could.

https://www.bluesea.com/articles/527

I probably didn't answer your question but I tried!

MIA