My little story with diy ebike battery charging. I was trying as an experiment to charge my Hyln cell 36v 10s li-ion battery with a diy power supply. Just a 42V 3.8A transformer and then a rectifier and the capacitor. In a month of successful charges, it would charge quickly till 40V and after that a lot slower. In an hour max, it was full at 42V, as measured with a dm. Once it was left charging for lots of hours and since then the max voltage dropped to around 40.3V. Eventually the bms fried and I wonder if the batteries can still charge to their max. Could this have been avoided by stopping the charging in time, or by using a smaller transformer or anything else other than the obvious suitable charger solution.
diy battery charging
- Thread starter papangua
- Start date
What specific brand and model cells are you using? Good high discharge cells can handle a maximum 1C charge rate, but not recommended if you want the cells to last. 0.2C to 0.5C is recommended. If you are charging fully in an hour, you're charging at 1C, so if you don't have cells that can handle that charge rate, you will kill them pretty fast. Even if the cells can handle it when new, they won't as they degrade. Are you checking the temperature of the pack while charging it?
I also fried the new bms that arrived, it had opposite polarity. I didn't take it for granted as I did not realise that current from the battery is passing through the bms, pfff
nothing else is printed. I did not monitor the temperature. Only by hand, touching the pack sometimes, batteries are just sealed with paper and thin plastic, without the the hard plastic casing. It did not feel warm not even slightest, contrary to the trafo and the diodes!
If it's the 1200mAh cell, looks like the Discharge rate is 1C, so the max charge rate will be less. A 1.2A max discharge rate is pretty low for an 18650 cell. How many cells in parallel do you have?
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If there is no current limiting (which lowers the voltage until the current is below the limit), then you can have too high a current that can damage things.
Sometimes these are called "bad boy" chargers for the reason that they don't have any current limiting, and also no voltage regulation.
Also, remember that 42v AC is not the same as 42v DC, which is what you need on the charge output for a 10s pack. What is the actual voltage, unloaded (not connected to the battery), for your "bad boy" charger?
IIRC, the rectified DC voltage will end up about a third more than the peak AC voltage. So if the 42v transformer is 42v AC p-p, then it'd be around 55v DC after the rectifiers at the output.
If your BMS FETs aren't rated for that, then as soon as the BMS turns the input off for any reason it'll have too high a voltage across the charge FETs and they'll probably be damaged or destroyed. Usually the damage is a type that shorts across the FET source/drain, essentially permanently turning that FET on, so that the BMS cannot prevent cell overcharge. In this event, the balancers (if any) will stay on all the time trying to drain the overcharge of the cells, but they can't stop the overcharge. The cells will then charge up to around 5v+ each, which is probably damaging them in ways that can lead to a fire (even if the overcharge is drained off).
Exactly what the damage could do depends on the cells, the exact damage, etc. But I could imagine scenarios in which it can cause the cells to no longer charge correctly, or fully, and cause internal "shorts" that cause voltage drop even while charging, or once the charge source is removed.
docw009
10 MW
You built a simple transformer power supply with a diode and filter capacitor? Applaud your ingenuity but it was bad idea. Unregulated power supply. No constant current-to-constant voltage transistion, Horrible for charging,
I see amberwolf posted while I am typing, I'm just going to say, get rid of it amd buy a lithium charger/
You're lucky the first BMS died and didn't short itself out, That would have been like having no BMS at all and you would have had a fire eventually.,
I see amberwolf posted while I am typing, I'm just going to say, get rid of it amd buy a lithium charger/
You're lucky the first BMS died and didn't short itself out, That would have been like having no BMS at all and you would have had a fire eventually.,
yes i was a lucky bad boy! 58V out of the capacitor and no fire. I did like 1300km - 800miles like that. Maybe I'd have fire if i hadn't disconnected things in time. I did not build the power supply, I combined one from a dead audio amplifier and hooked a trafo i found at the boiler house of an abandoned mansion. However, I had no 42v regulator laying around. And i have bought a charger since then, it's surprisingly light. These days i will check the cells's condition. I' ll seek on ways and maybe i' ll need advice again.
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