Solar -> LiFePo4 -> DC-DC Charger -> Lead Acid

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gardner asked Dec 17, '21

Solar -> LiFePo4 -> DC-DC Charger -> Lead Acid

Hello,

I have 450AH of AGM lead acid on my boat. I am looking to add a 310AH LiFePo4 battery to the house bank. I am mounting 450w of solar.

From my understanding, LiFePo4 will take a complete charge much faster than the AGM. To me it makes the most sense to connect the solar to the LiFePo4 using my Smart 100/50 solar charge controller; and then to put a a 12/12 30A DC-DC charger from the LiFePo4 to charge the AGM. This would keep the AGM at float charge most of the time and I can keep the LiFePo4 between 20-80% SoC. In this situation the AGM would still service the house loads. From my understanding the AGM can deal with heavier loads than my LiFePo4.

My question is: Is there a reason more people don't do this? Is this a bad idea?

Most info that I can find online has an alternator charging a lead acid bank and a DC-DC charger topping up a LiFePo4 bank which is used as the house battery.

Thanks

MPPT SmartSolar Lithium Battery Orion DC-DC Converters not smart

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Answer 1 · 2021-12-17T05:19:38Z

kevgermany answered · Dec 17, 2021 at 05:19 AM

I'd do it that way. But you need to rewire so the DC loads are split. Some from AGM, others from lithium. Better would be to service all the loads from lithium, but your DC:DC might be spending a lot of time at full power and get very hot.

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Answer 2 · 2021-12-17T07:06:24Z

Stefanie (Victron Energy Staff) answered · Dec 17, 2021 at 07:06 AM

Hi @gardner,

this is not a bad idea. In the meantime many others do it that way, me included except that I have only a small 110Ah AGM (starter battery) to be charged from Lithium (via Orion Tr smart 12/12-18) and my Lithiums (400Ah) get the charge from Solar, Shore power charger and/or Alternator.
I think this is extremely efficient. The downside is the much greater installation effort ie. to protect the alternator.

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Answer 3 · 2021-12-27T17:44:25Z

jimj answered · Dec 27, 2021 at 05:44 PM

Hi @Stefanie

I like what I am reading here as it affirms my idea of using two 24V torqeedo batteries connected in series for 48V as a source to an Orion 48-12/30A for charging output to my 3 x12V AGM house batteries. The Torqeedos will be charged with 2 PV's and two Torqeedo MPPT controllers. The Torqeedo batteries also provide power to an electric propulsion motor on my boat.

I wish to monitor this system on a Raymarine Axiom 7 MFD. This is where things get fuzzy and I would appreciate comments. I think to monitor everything on the MFD requires the installation of the Victron Cerbo GX but maybe not. Thoughts?

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Stefanie (Victron Energy Staff) ♦♦ commented · Dec 27, 2021 at 06:16 PM

Monitoring the system on a MFD is a completely different topic. There are plenty of threads here in the modification section about it already. But sure I can help as I'm controlling my system also via an Axiom+ MFD.

I suggest you start a new question so that things do not get confused with the topic of this thread.

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Answer 4 · 2021-12-28T19:28:41Z

jimj answered · Dec 28, 2021 at 07:28 PM

Sure. That's a good idea

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