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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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[–] tofubl@discuss.tchncs.de 3 points 11 months ago (1 children)

Take a look at Battery University, I think it's a great resource.

On the topic of lithium polymer charging, it has this to say:

Charge and discharge characteristics of Li-polymer are identical to other Li-ion systems and do not require a dedicated charger. Safety issues are also similar in that protection circuits are needed. Gas buildup during charge can cause some prismatic and pouch cells to swell, and equipment manufacturers must make allowances for expansion. Li-polymer in a foil package may be less durable than Li-ion in the cylindrical package.

[–] Saigonauticon@voltage.vn 6 points 11 months ago (1 children)

Battery University is indeed a great resource!

However this is not a lithium polymer battery, and as it's a 32700, it is not a prismatic or pouch cell either. It is a lithium iron phosphate (LiFePO4) cylindrical battery in metal housing. Battery University does have them listed in their table of chemistries (in case you're curious), but they don't seem to have much detailed information. Enough to build a charger though :)

https://batteryuniversity.com/article/bu-216-summary-table-of-lithium-based-batteries

Also some more detailed information here:

https://batteryuniversity.com/article/bu-205-types-of-lithium-ion

Anyway, thanks for your reference in any case! I'm not responding to criticize you, only to improve the utility of this conversation in case someone else finds it on search :)

[–] tofubl@discuss.tchncs.de 3 points 11 months ago (1 children)

Oh my bad. I'm no battery expert by a long shot; just meant to contribute a good source of information. But you're way ahead of me anyway. Carry on! 😊

[–] Saigonauticon@voltage.vn 3 points 11 months ago

No worries! I appreciate that you were just trying to assist!

[–] Eheran@lemmy.world 1 points 11 months ago (1 children)

Get a board with a TP5000. Like this one here.

Don't charge LFP to 4.2 volt! The crude "check voltage and if below 3.6 V keep charging" is okay too as long as the maximum current is within battery spec. But measure while charging, don't turn that off to measure the open circuit voltage.

[–] Saigonauticon@voltage.vn 2 points 11 months ago (1 children)

An international parts order is too complex for such a small thing. I'm not in the USA or China. So no TP5000 for me, got to work with what I have.

I agree, no charging at 4.2 volts. The current charger I built seems to work well enough. I ran some tests and it charges within spec. The reason I turn off the charger to measure cell voltage is because otherwise I'll mainly be measuring SMPS noise.

Anyway it beats the charger available in the local market, which is clearly unsafe, no matter how much they assure me that it's 'totally OK'.

[–] Eheran@lemmy.world 1 points 11 months ago

Perfect.