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Sanity check for LiFePO4 Charger Design (voltage.vn)

submitted 11 months ago by [email protected] to c/[email protected]

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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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[–] [email protected] 3 points 11 months ago (3 children)

There are really charging curves that should be followed to avoid damaging the cells.

You want to charge up to a certain voltage at a current (related to the battery capacity I believe) and when the cutoff voltage is hit, you switch to constant voltage (the max voltage of the cell) and then slowly drop the amperage as the battery is topped off.

I’m not sure about your cells, but some LiFe are 95-98% full at 3.45-3.5 volts, but the problem is that the voltage curve is really flat from 40-50% charged up to 95%. So you need really accurate measurement if you want to charge to 95%. The last 3-5% is when the battery ramps up to 3.65v and really is the riskiest part of the charge. It’s also the highest wear part of the battery use, if you can avoid charging it all the way up to that your cells will last much longer.

Decent video explaining charge/discharge here, though he’s using big LiFe prismatic cells.

[–] [email protected] 2 points 11 months ago

My normal lithium battery charger automatically slows its charging speed as battery voltage nears its capacity. I could set it at 1000mah and it will step down to less than 1/10 of that before charging is complete.

[–] [email protected] 2 points 11 months ago (1 children)

LFP cells have excellent cycle life anyway (2000+ cycles); is it worth worrying about staying at 95%?

[–] [email protected] 0 points 11 months ago (1 children)

It’s just that it’s really tricky to charge the final bit because the middle 80% is such a flat voltage curve. They have a 1000x life when they’re taken care of.

[–] [email protected] 1 points 11 months ago (1 children)

It is in no way tricky. Simply apply a constant voltage.

[–] [email protected] 1 points 11 months ago (1 children)

It’s tricky to stop at the right point, because lithium iron only have a very small voltage increase between like 40% and 90% and they ramp up to full voltage right near the limit of their capacity.

[–] [email protected] 1 points 11 months ago

How is it tricky? You keep the voltage constant. If the current exceeds the maximum, you first keep the current constant.

[–] [email protected] 1 points 11 months ago

What you are talking about is nothing special at all and not following a charging curve. The curve automatically looks the way it does when charging CCCV. Constant Current -> Constant Voltage.