cassiopeia avatar image
cassiopeia asked

Mixing AGM and lithium

On our boat, we currently have AGM batteries for the house bank (3 ea), start battery (1), and the bow thruster (2). We want to upgrade the house bank to lithium. We are replacing our alternator with a 170 Ah high capacity in preparation. We have a Centaur 12/100 charger currently charging all the batteries.
Is there a way to design the system to charge the AGM's for the start and thruster banks, and lithium for the house bank?

battery chargingLithium Battery
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3 Answers
kai avatar image
kai answered ·

I'm assuming that you're looking at keeping the banks separate (1 AGM bank for starter, another AGM bank for bow thruster, and 1 Li bank for house loads).

My thinking would be to use an AC charger for the house bank - since that'll most likely be the one you seriously draw down on and need high charging capacity. Then use a suitable DC-DC converter to top up the starter and bow thruster banks.

I skimmed the specs for the centaur, and it looks like you may have to do some research on whether you can match the voltage settings on it with the lithiums you're intending to get.

You'll still have the design the system, above is just an outline that you may find useful.

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ejrossouw avatar image
ejrossouw answered ·

@Cassiopeia Here is a thread you may find of interest and possibly overlooked when you searched the forum first? There is mention of a company that makes a LE or Lithium extension battery to use in such a setup. The fact that AGM has a substantially lower DOD to manage than e.g. LIFEPO4 at up to 90% and both have different charging characteristics, I personally think it not worth the hassle and potential setup challenges with at least multiple BMS's etc. There is not a tried and tested setup from what I can see.

PS - Worth noting from this setup is that the Lithium becomes the primary source of power with the LA / AGM serving as backup and maintained at float. That could be the challenge to overcome.

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Elimac avatar image Elimac commented ·

Hi @ejrossouw.

I've been doing a lot of reading on this issue. I agree with your last sentence "There is not a tried and tested setup" (yet) and also that it seems likely the lithium becomes the primary source. So, to protect the LA, we will clycle very much the lithium. But it could be desired to be this way...

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ejrossouw avatar image ejrossouw Elimac commented ·

As per the other post, check out rvgeeks and the professionals who installed their system. They opted to charge the AGM off a separate charger. It makes perfect sense as unless you have deep pockets for gear or damaging batteries, have the time to experiment and don't fully depend on the system and are willing to risk losing the AGM (I think they are most at risk given DOD) if things go wrong ... then you may just be that pioneer everyone is waiting for.

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cpt-pat avatar image
cpt-pat answered ·

I have been using this two-bank setup for 7 years with good results:

Bank 1: 100 AH Odyssey 2150 battery (AGM type).

Bank 2: Three 100 AH LiFePO4 batteries in parallel.

The Odyssey battery requires an absorption charge to 14.7 volts and floating at 13.6 volts.

The "house load" is supported by both Bank 1 and Bank 2 through a battery combiner (I built my own using Schottky diodes, but commercial combiners are available). Both Bank 1 and Bank 2 are charged from a VE IP22 charger with "Lithium" settings (14.4 V absorption/13.5 V float). I then have a separate 7A VE charger set to 14.7/13.6V charging only Bank 1 to provide the 14.7/13.6V charge voltages for the Odyssey battery. The Schottky diodes prevent backflow to the LiFePO4 battery bank.

Underway, off shore power, the Odyssey battery terminal voltage rapidly drops to 12.8 volts, and the LiFePO4 batteries (Bank 2) pick up the entire load until a transition occurs at 12.8 volts, where the Odyssey battery begins to take up the load. At 12.8 volts, the LiFePO4 batteries are substantially depleted.

Why did I do this? Because my inverter draws 100 A - which is connected to and drawn from only the Odyssey battery. I prefer not to abuse my LiFePO4 batteries with that 100 A load. I would have the same preference if I had to start an engine (I have no engine to start on my sailboat). Also, if the LiFePO4 batteries ever reach low-voltage-cutoff, I won't have an abrupt loss of all power since the load will be shared by the Odyssey battery.

Under normal conditions, the Odyssey battery has nothing to do except provide occasional 100 A draws when I run my microwave. But it's there to "hold up" the load voltage whenever the LiFePO4 bank becomes depleted.

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