Sanity check for LiFePO4 Charger Design
Sanity check for LiFePO4 Charger Design
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.
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.
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 :)
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.
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'.
Perfect.
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.
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.