Charging a LiFePo4 House Battery from your Alternator

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Charging a LiFePo4 House Battery from your Alternator

Postby bdosborn » Sun Dec 08, 2019 11:26 am

A LiFePo4 battery will accept as much charge current as your alternator will output. So your TV alternator will charge it but it will be at 100% output for hours, which it is not designed to do. It will overheat and fail prematurely.

Vehicle manufacturers know the alternator will only be delivering its full output for short durations, so they cut every corner they can. It is not uncommon for a 160 amp alternator to have a continuous duty cycle of less than 90 amps. Under the extra load imposed by high power mobile applications, the alternator and/or the interconnect wiring may over heat. This is especially true of low content (minimal accessorized) vehicles.

Also, you'll need 60A worth of wire (or whatever your alternator is rated at) to connect from your alternator to your battery. Then there's the issue of trying to combine a start battery with a LiFePo4. I don't combine my start battery with the house battery, the two different batteries are hard to combine and charge correctly since they are at different voltages (12.8V vs 13.5V). So the question isn't can I a charge a LiFePO4 battery directly from the alternator but how long will it work until the alternator burns out?

I attacked this problem by using a Renogy DC to DC charger. Now you're only drawing 20 or 40 amps from your alternator and you also get a LiFePo4 charge controller so you don't have to worry about overcharging your expensive battery. You're also not combining batteries, the Renogy won't back feed the start battery from the house battery.

I didn't want to run huge wires through my truck to charge the battery or risk overheating my alternator so I'm using the 20A model. I've only used it twice so far but each time it's worked as expected. Renogy isn't know for high quality products so YMMV. The Renogy doesn't sense state of charge to turn on, it needs a +12V signal. You could wire it to an accessory that comes on with the key but I wanted to be able to control mine. I have it connected to my running lights so it charges when I turn them on.

I also have a 220W PV array and a Victron MPPT charge controller connected to the battery. I can charge at 35 amps or so when the truck is running and the sun is out. No special wiring is needed, just connect both the controllers to the house battery and it just works.
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Re: Charging a LiFePo4 House Battery from your Alternator

Postby RJ Howell » Sun Dec 08, 2019 2:29 pm

I'll using the same B to B charger next spring with my new Lipo4 battery.
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Re: Charging a LiFePo4 House Battery from your Alternator

Postby MtnDon » Sun Dec 08, 2019 7:16 pm

cool! good idea.
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Re: Charging a LiFePo4 House Battery from your Alternator

Postby MtnDon » Mon Dec 09, 2019 8:56 am

This Redarc unitcould be an alternative to the Renogy. Made in Australia. There are three models; 25, 40 and 50 amp. The info page illustrates hookups with lead-acid and LiFePO4 batteries.
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Re: Charging a LiFePo4 House Battery from your Alternator

Postby KTM_Guy » Mon Dec 09, 2019 9:22 am

MtnDon wrote:This Redarc unitcould be an alternative to the Renogy. Made in Australia. There are three models; 25, 40 and 50 amp. The info page illustrates hookups with lead-acid and LiFePO4 batteries.


I had the CTEK charger and it flaky at best. I think it didn’t like the heat we have here in PHX. Due to poor design and stupid mistake on my part I fried it and needed to replace with something. I now have the 25 amp Redarc and it has been flawless. I wish I went with it from the start. I would be money ahead that’s for sure.

I can wait to hear more on the Renogy charger, it’s at a great price.

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Re: Charging a LiFePo4 House Battery from your Alternator

Postby bdosborn » Mon Dec 09, 2019 8:00 pm

Victron came out with a version but I don't think it's made it to the US yet. I'll probably look for one if the Renogy fails as I like their stuff.
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Re: Charging a LiFePo4 House Battery from your Alternator

Postby Rainier70 » Mon Dec 09, 2019 10:11 pm

I am curious about your wiring and mounting set up. How long of a run did you have for the wires, what gauge you ran, and did you use the chassis on the negative side? Thanks.
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Re: Charging a LiFePo4 House Battery from your Alternator

Postby bdosborn » Mon Dec 09, 2019 10:40 pm

I upsized the charge wire on the truck to a #6AWG to try and minimize the voltage drop back to the trailer. The stock charge wire was a #18AWG, much too small to charge with. From the truck, the charge wire runs through the trailer connector. The connector is the pinch point as the wire is about a 14#, which is undersized for 20 amps, much less the 30 or so amps the input of the DC converter requires. The wire from the trailer connector to the Renogy is a #10awg. I've got my fingers crossed that the #14 doesn't burn up, but if it does I'll have to run a dedicated charge wire. The trailer connector wire is why I didn't go with the 40A version. I always use a negative wire, using the chassis to charge is just asking for troubles.

The truck is 18' long (#6awg), the trailer hitch about 5' (#14awg) and another 7' (#10awg) to the Renogy.
Here's what I'm used to wires being rated at:
#6 - 60A
#10 - 30A
#14 - 20A
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Re: Charging a LiFePo4 House Battery from your Alternator

Postby KTM_Guy » Tue Dec 10, 2019 8:33 am

When I had the CTEC mounted in the camper I ran 10 ga all the way and I think that was part of the problem. With the Redarc their wire size chart starts with 6 ga and goes up well into the welding cable gauges. (0,00,000) I used 6 ga now and I’m at 15’ another foot or two and I would be at 4 ga.

I called Redarc (great customer service) about the ground wire and got a great reply. He said when people call with problems they ask the wire size and if the ground wire is ran. If not, they will not go any farther until those are fixed. He said probably less than 5% call back.

It is cheaper to do it right from the start (ask me how I know :? )

Todd

PS avoid the copper clad aluminum crap wire all over on Amazon. If it seems like a good price it is CCA wire and junk.
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