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Old 06-19-2018, 03:35 PM   #1 (permalink)
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Lithium-ion 12v Battery Replacement

I've discussed replacing the 12v lead acid battery with LiFePO4 before since the voltages (in 4s configuration) and chemistry lend itself to direct replacement. However, the downside is the inability to charge when temperatures are below freezing, higher cost, and lower energy density when compared with other lithium-ion chemistries.

This thread by Luno got me to wondering if Lithium-ion batteries could directly replace a lead acid. It may be safe to charge these batteries without a BMS considering a 4s pack is well below full charge at the standard ~14v alternator output.

Has anyone experimented with an unmanaged (no BMS) lithium-ion 12v battery replacement? What are the potential issues with this? Would LiPoly be better for any particular reason?

There are 5 Ah batteries available for $30 online that would be a fairly cheap way to experiment with.

https://hobbyking.com/en_us/turnigy-...se-pack-1.html

Even if there is no way to get around needing a BMS, it should be a fairly simple matter to combine a Lithium-ion battery, BMS, current limiting resistor, and supercap into a 12v battery replacement that should last a long time and provide quick starts in any weather.

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Old 06-19-2018, 04:05 PM   #2 (permalink)
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One member over on InsightCentral had his commercial LIon battery catch on fire and nearly burn his car to the ground. The BMS didn't catch it. I was seriously considering it until I saw this. Instead, I think I'll go with an AGM battery.

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Old 06-19-2018, 04:31 PM   #3 (permalink)
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Drawing hard on a battery - such as pulling hundreds of amps from it to start your car - is generally a quick way to get your cells out of balance. However near identical each is, there will be tiny differences in internal resistance, and, more likely, in the wiring between each cell.

Of course, if the caps are doing the starting, and you’re drawing reasonable currents, that’s a moot point.

Since there are only 4 in series, it wouldn’t be hard to put some zener diodes and a resistor across each string so that it would bleed them all down to the same voltage and keep them balanced. Then again, a simple, cheap bms could be set up strictly for charging (bypassed for heavy draws) and save you oh the bother.
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Old 06-19-2018, 04:50 PM   #4 (permalink)
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Yeah I have on my been doing this since 2011.

Unless you can keep the battery free from engine compartment heat which usually means you will be relocating the battery and plan to remove the battery when it gets cold don't bother.
LiFePO4 batteries suffer from very bad cold numbing.
Aside from weight, and durability there really isn't any advantage to replacing a lead acid with lithium.
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Old 06-19-2018, 10:08 PM   #5 (permalink)
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Lead acid batteries are spendy these days. If I can replace a car battery with a $30 lithium, I'm all about that. How about for motorcycle use? Since it isn't run below freezing, would it be a good candidate for an unmanaged lithium ion? In other words, is there any reason why LiFePO4 would be a better replacement than other lithium ion chemistries?
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Old 06-19-2018, 10:47 PM   #6 (permalink)
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A $30 battery isn't going to be able to start the car during the winter.
4 cells of LiFePO4 is perfect for lead acid replacement based on system voltage. It doesn't over charge but gets a near full charge.
They are supposed to be more abuse tolerant, less catastrophic failure prone.
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Old 06-20-2018, 12:52 AM   #7 (permalink)
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Is there any particular reason you want to go "unmanaged"? PCBs are pretty cheap. Like $10-20 cheap. For that much, it seems like good insurance.
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Old 06-20-2018, 01:08 AM   #8 (permalink)
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When I stated using lithium there were no cheap bms units available.
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Old 06-20-2018, 12:13 PM   #9 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
A $30 battery isn't going to be able to start the car during the winter.
4 cells of LiFePO4 is perfect for lead acid replacement based on system voltage. It doesn't over charge but gets a near full charge.
They are supposed to be more abuse tolerant, less catastrophic failure prone.
I'd pair it with supercaps and a resistor I already have.

Quote:
Originally Posted by Stubby79 View Post
Is there any particular reason you want to go "unmanaged"? PCBs are pretty cheap. Like $10-20 cheap. For that much, it seems like good insurance.
Not sure how that would work since starting requires several hundred amps. If I remember correctly, the BMS has a decent bit of phantom power draw too. If I'm undersizing the battery, I can't have large phantom loads.
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Old 06-20-2018, 03:50 PM   #10 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
In other words, is there any reason why LiFePO4 would be a better replacement than other lithium ion chemistries?
Not bursting into flames when they're angry is something I like about LiFePO4.

It wouldn't hurt to fully charge and balance the battery occasionally. Maybe have low voltage alarms that are only connected when the ignition is on.

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