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.
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.
It is in no way tricky. Simply apply a constant voltage.
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.
How is it tricky? You keep the voltage constant. If the current exceeds the maximum, you first keep the current constant.