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I currently have this setup in my van:

Alternator -> Car Battery -> Voltage Sensitive Relay -> Leisure Battery (AGM)

I'm thinking of changing out my AGM battery with a LiFePO4 battery instead, which I know has a very different charging profile to an AGM and also has a low internal resistance compared to an AGM. Can this be problematic?

Thanks.

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    If you consider fires and explosions to be "problematic", then yes it can be problematic.
    – Elliot Alderson
    May 21, 2021 at 15:07

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Yes it will be problematic.

A LiFePO4 battery charged by a car alternator regulator will most likely fry the alternator. The sustained low resistance of the battery will pull a high current from the alternator and it will overheat.

If the alternator does not fry you will most likely wreck the battery. LiFePO4 batteries are easy to charge however they have a set of charging requirements that are not met by a system intended to charge lead acid batteries. In particular sustained overcharge or float charge will very quickly degrade the battery.

You would need a charging system designed to charge LiFePO4 and then a method of topping up your start battery, a battery to battery charger is a good idea here. This would mean changing or modifying your alternator and installing an external charge controller.

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You can't use a bare LiFePO4 battery anyway. That would be madness. They aren't docile lead-acid batteries, they will catch fire if overcharged, drawn flat, charged below freezing, and all sorts of common abuses. Energy density isn't free.

ALL such batteries need a battery management system (BMS) in front of it, as well as a cell balancer. If you get a better BMS you can tell it not to allow charge at an excessive rate. Since all current in and out is going through the BMS, one would think the BMS would want to do that anyway for self-protection of its power semiconductors.

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Not sure why everyone is making stuff up. I happen to make my own batteries in both Li-ion and Lifepo4 for E-board, bikes, automobiles and I'll be making them for solar applications in the future.

Lead acid batteries suck. They constently lose power requiring float or trickling.

A lifepo4 battery won't catch fire... They are nothing like li-ion. When a battery gets full it simply hits thermal load and disconnects worst case. A battery pulls what it needs and regulates its self based on voltage and Lifepo4 batteries have a resilient voltage threshold. Your accessories will likely pull the most power and depending on how many Ah your battery is it will pull 3A per parallel connection... So 25Ah batteries may pull like 12A... A 100Ah battery would pull up to 50A and that's if it's low.

All you need is a bms, which a good lifepo4 should have anyway. It will keep cells balanced, keep it from over over heating, charging under low Temps and over discharging etc. The limits on charging on a decent bms are like 100A but this depends on the battery and the alternator shouldn't have much of an issue keeping up unless you have a big sound system In which case that's a whole other problem.

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  • This post is simply dangerous. LiFePo4 will happily catch fire if the BMS has an issue, or they are bumped hard, or all manner of seemingly minor issues. I have had quite a few fires from these - and that's charging them correctly, not from an alternator designed for lead acid batteries...
    – Rory Alsop
    Sep 20, 2022 at 16:00

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