Batteries and Charging

kordokordo Member Posts: 243 ✭✭
in General Boating Discussions
I'm wondering if it's ok to charge my agm house batteries with a lithium battery.  I've read that you can't combine different batteries due to their different charging characteristics but I'm talking about discharging a 12v lithium battery into a 12v agm battery.  The situation arises when my agm house bank is down to 12.2v and instead of running my genny to charge the house bank, I would just like to connect my 12v lithium battery at 12.8v to my agm house bank to charge them.  The bms in the lithium battery should shut it down when the voltage gets too low.  Thanks,
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  • YYZRCYYZRC Member Posts: 5,138 ✭✭✭✭✭
    edited September 8
    @kordo without knowing your exact setup, perhaps a DC:DC charger is what you need? I run a Victron Orion to charge my lithium inverter from the output on my isolator. 

    If you don’t have a common ground you need an isolated DC:DC charger. That’s the case for me and this is the unit I use:

    https://a.co/d/4qVFpN7
    2008 350 EC on Georgian Bay
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  • LaReaLaRea Member, Moderator Posts: 7,763 mod
    That DC-DC charger would be much better than just connecting the two batteries in parallel.  Or, what about a battery selector switch?  When the AGM runs low, switch over to the lithium.  
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  • kordokordo Member Posts: 243 ✭✭
    LaRea, good idea.  But I'm curious as to if I can connect the lithium to the agms short term.  I just want to charge the house batteries with the lithium once in a while instead of running the genny.  I think it would be the same as connecting a third agm to the existing 2 agms to get a bit more power until I was ready to run the genny.  I would disconnect the third battery (agm or lithium) before charging.  Thanks.
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  • LaReaLaRea Member, Moderator Posts: 7,763 mod
    If the Li battery is at 13 volts, that's probably not enough to really charge the AGMs.  Once you connect the Li battery to the AGMs, the loads would basically just draw from the Li instead of the AGMs.  That's why I'm thinking a battery selector switch would be better.  Somebody correct me if I'm seeing this the wrong way ...
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  • PickleRickPickleRick Member Posts: 4,034 ✭✭✭✭✭
    edited September 8
    I believe the Li is at 13.7 vs the 12.7 of the AGM. 

    While I don't think the load/discharge under normal usage would be an issue as that's a very small voltage drop(assuming every battery in the bank is healthy) I think the issue would the the Li charging the AGM during long term storage without a charger of any sorts being connected 


    Not sure it's the most efficient use of the big money lithium battery but many cannot afford an entire Li bank upgrade at once. 

    I'm betting many who report issues with Li and AGM battery mixing have batteries in the bank with dead or weak cells which would hurt the performance of any new battery added to the set.  

    If you're not going to keep the charger going during winter storage disconnect the batteries after a full charge, reconnect before first launch.  Easy fix 

    But I would second adding a second bank for the Li would be the most efficient addition. 




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  • YYZRCYYZRC Member Posts: 5,138 ✭✭✭✭✭
    edited September 8
    LaRea said:
    If the Li battery is at 13 volts, that's probably not enough to really charge the AGMs.  Once you connect the Li battery to the AGMs, the loads would basically just draw from the Li instead of the AGMs.  That's why I'm thinking a battery selector switch would be better.  Somebody correct me if I'm seeing this the wrong way ...
    I think this is correct. That’s why I suggested the DC:DC charger. Not even certain it would work. Might have to do an inverter/charger which would be inefficient, but safe. 

    Battery selector might be risky as you could end up with the alternators trying to charge the lithium directly. 

    How many amp hour is your lithium @kordo ?
    2008 350 EC on Georgian Bay
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  • PickleRickPickleRick Member Posts: 4,034 ✭✭✭✭✭
    His battery has the built in BMS so should not care about alternator charging current from what I gather.  
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  • YYZRCYYZRC Member Posts: 5,138 ✭✭✭✭✭
    @kordo perhaps you could share a quick schematic detailing how the lithium is wired up so we aren’t speculating and giving bad advice?
    2008 350 EC on Georgian Bay
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  • kordokordo Member Posts: 243 ✭✭
    The lithium is 100ah, Kepworth (Amazon.com: KEPWORTH 12.8V 100Ah LiFePO4 Battery Rechargeable Lithium Battery with 100A BMS, 4000-15000 Deep Cycles, Grade A Lithium Iron Phosphate Battery cells, for Trolling Motor, Boat, Rv, Solar : Health & Household).  No schematic.  If my house bank is low and I don't want to run the genny, I turn the house bank off and connect the lithium to my 12v outlet under my electrical panel.  It's just a short term solution.  All my house loads are then run by the lithium.  Tried it last week and it worked fine.  Now I'm wondering if I can just connect it to the house bank for a few hours to charge the house bank since the lithium would have more volts than the house bank.
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  • GrahamuGrahamu Member Posts: 881 ✭✭✭
    If your house bank is much larger than the lithium battery you risk drawing it down to a point that is not useful if connected together. As suggested I think a battery switch would be a better solution, 
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