LiFePo4 starting battery? (1 Viewer)

[DirtNap]

I hope this belongs here since it is derivative of the solar battery/generator work going on.
Since my 200 series is heavy AF, even before much of a build, I am interested in places to safely shave weight, i.e., the battery. I see that the Headway 38120HP has 200A discharge. Since the 200 series has a 2kW starter motor (150-160A) why couldn't one drop in 4 of the cells in series for a 3.2 pound battery? Even with some ancillaries it'd be ballpark 5 pounds. What am I missing (besides all of the details on how to make it work!)?

To keep this on track, let's keep this focused on starting battery. Assume any non-alternator power for accessories would be handled by solar and/or a separate house battery. And that I would still have a jump-pack and jumper cables for backup.

Thanks,
DN

P.S. If this is not a dumb idea, I'd love any suggestions for how to incorporate it. I've been studying the YT videos (e.g., the OH8STN channel does a number of LiFePo4 battery and/or solar generator builds for amateur radio for manpack / field deployments). So I am a little familiar with the battery wiring and need for battery controller.

 

[spressomon]

IMO the biggest challenge with using LFP in the engine bay is its ~125°F upper temp limit.

 

[DirtNap]

Thanks, good point. It prompted me to look at the specs again. It was interesting (to me at least) that while the "working temp" limit was 65C/150F, the charging temp was much lower (45C/113F). I suppose that makes sense. Ironically the battery starting would tend to be when the engine is cold(-er) so working temp <150F may not be an issue. But recharging means engine is running so temps could be above 113F pretty quickly, so that could indeed be an issue.
I know: relo the battery to the 200's armrest cool box!

 

[Fisher23]

or the cargo area. Many cars are OEM with the battery in the trunk. Just put some breakers on each end of that long run.

Most of the new marine engine batteries are now LifePo4.

 

[DirtNap]

Good point.
After doing a lot more self-education. One challenge that may be more complicated to over come is the different charging behaviors of lead acid and LIFePO. Lead acid does better if it is float charged once it is fully charged, so it stays connecoto re alternator. LiFePo4 should be disconnected after it is charged. Float charging can cause damage to the cells over time. Disconnecting the LiFePo4 battery from the alternator might help the battery, but will fry the alternator (well actually it's voltage regulators). There appears to be a way to trigger something to cut those V_regs out. I guess you could disconnect the alternator, and it's VR's, run the car from the LiFePo battery, and then reconnect everything periodically to recharge the battery. Sounds like the juice isn't worth the squeeze.

I can recommend this article though, which explains nearly everything. About use on boats, yes, but the main difference is the size of large boat battery banks -- e.g., 300 Ah.
LiFePo on boats
(Irony: they get over the alternator issue by having a lead acid battery stay in the system. Fine for a big boat, but would kinda defeat the purpose I was after!

 

[Fisher23]

A blue top optima AGM says 50C = 122 F. Doesn't really give much other info.
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LifeP04 seems to be about 45c = 113 F charge and 122 discharging so not much diff.

I know some guys running the LifeP04 in the engine bay .. though they all agree elsewhere would give better performance. Though it does seem to work in the engine bay, just not at peak performance. Even if not a peak the same size LifeP04 is about 140 ah vs an AGM 65 ah at best.

 

[Shoredreamer]

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