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What happens if I try to charge a LiFePO4 battery from the car alternator?

I have a X30 LiFePO4 battery (https://cara-tech-24.eu/en/electric/278-powerxtreme-x30-battery-lifep04-battery.html) LiFePO4in my caravan. Currently it's only being used for the mover (electric motor to move the caravan to the exact spot) caravan mover

It is not used for the internal 12V lights in the caravan, which can be powered from the car or from mains from a SwPsu in the power distribution.

I want to connect this battery to the internal lights; the simple method is to wire it in parallel with the power from the car, either directly or via a DC/DC-converter to boost the voltage to compensate for the wiring losses.

But would that be bad for the LiFePO4?

Should I settle for a switch (relay) to either feed the internal lights from car or LiFePO4, and only charge the battery when connected to mains?

EDIT: A test showed the battery in two hours with all lights on only used 25% of the battery. As our use case would never be 8 hours in caravan with full lights on, but no mains power, I decided only to charge the battery from the built-in charger, skipping adding a DC/DC-converter for around €120.

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2 Answers 2

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Quote from manual:

Never connect this battery in series or parallel to other batteries.

Another quote:

Optionally available is a charger which uses the car voltage to slowly charge the battery whilst driving, without the need to install thick cabling

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  • \$\begingroup\$ Ugh! I'm pretty certain that a lot of user's manuals for luxury/lifestyle products sold in The West these days are written by people who would never buy the thing for themselves, and who have only a vague idea of how it's supposed to be used. Where would the "thick cabling" go if not to the terminals of (i.e., "in...parallel with") the vehicle's engine starting battery? \$\endgroup\$
    – Solomon Slow
    Jul 28, 2021 at 18:56
  • \$\begingroup\$ @SolomonSlow Excuse me, but didn't you notice that's a LiFePo battery? It needs to be charged with a special LiFePo charger (just like any other lithium battery), with a CC/CV profile up to 14.4V. It even comes with a built-in mains inlet for charging. You can't just parallel it with vehicle engine starting battery, most likely the result would be somewhat spectacular, unless safety mechanisms kick in. That's why you can buy a charger, which takes in 12V from your main battery, and can charge the LiFePo battery safely with CC/CV profile and stop charging when it is full. \$\endgroup\$
    – Justme
    Jul 28, 2021 at 19:05
  • \$\begingroup\$ On the plus side, it wouldn't be a 'spectacular' failure... LiFePo don't explode like other lithium chemistries. (MUCH safer). That said, you're right, bad idea and don't try it. \$\endgroup\$
    – Kyle B
    Jul 28, 2021 at 19:16
  • \$\begingroup\$ @Justme, I'm not the person who wrote that user manual. \$\endgroup\$
    – Solomon Slow
    Jul 28, 2021 at 21:30
  • \$\begingroup\$ @SolomonSlow, a caravan lighting system is not just throwing some lights and switches and a battery together. There is a central power supply which takes care of switching from "land power", i.e. mains, car power or battery power. Care is to be taken not to feed the power from caravan battery (or mains) back to the car, and to have a separate 12v for the refrigirator, only active when ignition is on or better if generator is running. \$\endgroup\$
    – Lenne
    Aug 5, 2021 at 8:39
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LiFePO4, strictly speaking, is not a kind of "battery," it's a kind of battery chemistry. Your "battery" is a module that has some number of LiFePO4 cells, plus protective circuitry to stop it from catching fire if you short it out, plus it's own in-built battery charger. It apparently is meant to provide power to a camper/caravan, but whether or not it can be charged through its low-voltage terminals or, only through its mains port, is a question for the manufacturer.

There appears to be a link to a user's manual on the web page that you linked to. Maybe you will find the answer there.

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