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.
Just based on my experience and knowledge of these batteries, which is not that much really, don’t take any of this as gospel.
Constant voltage current-limited power supply is probably the easiest way to charge it, a regulator + current limited supply like you mention would be one way of achieving that, will waste a lot of the power and charging speed will be limited by how much heat the regulator is able to dump (which for a typical TO-220 regulator without heatsink is like a watt or two? So you’d have a max charge current of like 0.5-1.0 amp)
Your current approach probably also works just fine, but i’m not sure how good it is for the batteries in the long run
You can find charger for sale online though, at least modules that you can solder up to a battery holder or whatever, i would go for one of those if you plan to use these batteries for a longer period of time
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.
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.
LFP cells have excellent cycle life anyway (2000+ cycles); is it worth worrying about staying at 95%?
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.
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.
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.