this post was submitted on 30 Nov 2023
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Sanity check for LiFePO4 Charger Design (voltage.vn)
submitted 11 months ago by Saigonauticon@voltage.vn to c/askelectronics@discuss.tchncs.de
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So, there are these great 32700 LiFePO4 batteries that showed up in my local industrial market. For like USD 2$!

However, there are no LiFePO4 chargers available. The vendors assure me I can "totally use" a 4.2V Li-ion charger, but I don't believe them (although the cells test as being in good shape).

I whipped up a 5V system with a buck converter managed by an MCU. It turns off the buck converter that charges the battery, measures the battery voltage, and if it's under 3.6V it enables the buck converter. Repeats every few 100s of milliseconds.

Did I overengineer this? Could I have just used a linear voltage regulator that outputs 3.6V (or a Zener), and a current-limited 5v power supply?

Charge speed is not really important in my application. Anything under 4 hours is great. Frankly, I'm just trying to phase out the less safe kinds of lithium cell in my lab.

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[–] Saigonauticon@voltage.vn 2 points 11 months ago

Makes sense!

I'd order online if there was a LiFePO4 charger on the market. However, in my country I've been unable to find one, and importing (excise & duties, paperwork) is more work than building it myself. I'll also likely need a design that I can cheaply include on custom PCBs for manufacture (not for sale to end-users, but for internal use by maintenance technicians).

I gave it a test on a cell today and it seems to charge fine and at a reasonable rate -- but in a sudden flash of brilliance, I forgot to physically connect the ADC pin to the battery, so it couldn't shut off. Well, that's what testing is for I guess.

Anyway after fixing this, it looks like I can call this a win and move on. If it undergoes destructive optimization, I'll report back here with a warning to others.