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Old 06-18-2021, 11:26 PM   #1
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AGM Lithium B2B Charger

I have a question for anyone using Lithium for their house battery and AGM for their engine start battery, with a "Battery To Battery Charger" in between the two. My question is whether you directly connect the negative terminals of the 2 types of batteries, or if you try to keep them electrically isolated from one another except for the B2B Charger connection. Thanks.
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Old 06-19-2021, 08:08 AM   #2
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I have exactly this setup. A house bank that is LFP, and two start banks that are AGMs. The start banks are charged/mantained by BtoB chargers, MasterVolt MAC Plus in my case.


All the DC negatives are common, so the negatives of all the banks are joined. No isolation is required in the chargers.
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Old 06-19-2021, 08:47 AM   #3
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My negatives are common. However one important note if you are shifting over to lithium for your house: review the alternator charging setup. You probably need a DC to DC regulator between your alternate and the lithium. Not only to optimized the charge voltage for the lithium but also to limit the current draw from the alternator. Otherwise you are likely to overheat the alternator due to the very high current draw of the lithium when it starts charging off the engine.
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Old 06-19-2021, 10:13 AM   #4
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Twistedtree: Thanks very much.
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Old 06-19-2021, 10:14 AM   #5
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My negatives are common. However one important note...

An interesting detail I had not considered. Thanks.
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Old 06-19-2021, 11:00 PM   #6
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My negatives are common. However one important note if you are shifting over to lithium for your house: review the alternator charging setup. You probably need a DC to DC regulator between your alternate and the lithium. Not only to optimized the charge voltage for the lithium but also to limit the current draw from the alternator. Otherwise you are likely to overheat the alternator due to the very high current draw of the lithium when it starts charging off the engine.
Agreed, but that assumes that the alternator is charging the house bank directly. Some people charge the house bank from the start bank and direct the alternator charge to the start battery. So it depends on how the system is set up.
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Old 06-20-2021, 09:12 AM   #7
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Agreed, but that assumes that the alternator is charging the house bank directly. Some people charge the house bank from the start bank and direct the alternator charge to the start battery. So it depends on how the system is set up.

I intended to connect my alternator to the AGM start battery. It was interesting to learn that some people connect their alternators to the lithium house battery.
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Old 06-20-2021, 11:59 AM   #8
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I intended to connect my alternator to the AGM start battery. It was interesting to learn that some people connect their alternators to the lithium house battery.

Mine is direct connect to the LFP house bank. The alternators are externally regulated, and set to appropriate LFP values, just as you would set your inverter/charger, or stand-alone charger. That way the LFP gets the benefit of full alternator output to the extent it's available, and regulation follows the needs of LFP, not AGM.


Then the DC-DC chargers take care of the AGMs with voltages and charge cycles optimum to their needs.


A lot of people leave their alternators on the start batteries then use a DC-DC charger to charge the LFP. Personally I think that's backwards.


I think many do it because they don't have or want to install an external regulator, but I believe that's a false economy because a DC-DC charger is going to cost just as much, and often becomes a bottle neck between the alternator and LFP bank because alternators typically have more capacity than the DC-DC charger. It's also done because people don't want to overload their alternator, but every external regulator I know of has the ability to limit alternator output if required.

With an external regulator you get all the requisite control over the alternator and LFP charging, utilize the full capacity of the alternator, and use a lower capacity/cost DC-DC charger on the start batteries where demands are modest.
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Old 06-20-2021, 04:27 PM   #9
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Mine is direct connect to the LFP house bank. The alternators are externally regulated, and set to ...
Interesting insights into the brave new world that my charging system is about to enter. Thanks very much.
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Old 06-20-2021, 06:52 PM   #10
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I recently installed a system following TT's description for the reasons he outlines. Part of that work package was upgrading the stock alternator to 170a externally regulated, so I can happily bulk charge at 130+ amps from the main motor. The DC-DC chargers are rated at 30 amps.

In talking to sailing friends and living with my new system for a month underway I now understand why that might be overkill for some. In my case I also added a solar array, and in fine weather my house bank remains charged by solar. And I can bulk charge from shore power or generator. In practice I don't really need that capability. If I look at the $$/utility for the alternator/regulator upgrade it doesn't score nearly as well as solar, for my case at least.

With older/smaller motors that don't have serpentine belts it's hard to install a high amperage alternator, so I get the strategy of living with an OEM setup and limiting charge current through a charger. Make sure the charger isn't rated for more than about half the your existing alternator rating though, or you may cook the alternator without heat regulation. You should also be able to set the charger to start only when the starting bank is charged to guard against peak loading on the OEM alternator, if that's a concern.
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Old 06-21-2021, 11:16 AM   #11
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I recently installed a system following TT's description for the reasons he outlines. Part of that work package was upgrading...
Your post makes a lot of sense to me.

My last sailboat had a Ford Lehman with a v belt. I put a new Balmar alternator on the engine, and it immediately snapped the belt. My current Marine Trader also has Ford engines with v belts, so some kind of serpentine mod may be necessary if I upgrade the alternators.

I watched a pretty good online video of a guy who upgraded to lithium house batteries. He left the alternator charging the agm starter battery, and using the B2B charger to charge the house bank. I suspect he did not want to spend the money to upgrade his alternator.
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