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I have a 500mah 1S lipo battery. I wanted to charge it with my lab bench power supply.

Easy, I thought: I want to charge at 2C (which is 1A) so I simply set my power supply to 4.20V and 1A limit and done.

What actually happened is that the battery charged about 0.9C and it was slowing down the closer it got to 4.2V. This means that to charge at a constant 2C current the whole way I have to use voltage much higher than 4.2V, but a fully charged lipo cell must not be over 4.2V or it does the FLAME ON thing.

So my question is, how do normal chargers do it? Do they simply bump up the charging voltage until they reach the desired current (so they work in constant current mode) and they every couple of seconds the stop charging and measure the battery voltage to find out if it is 4.2V and if it is, then they stop charging, and if it is not they jump back into constant current mode?

Thanks

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    \$\begingroup\$ go to BatteryUniversity.com, or read the data sheet of any professionally sourced LiPo. \$\endgroup\$ – Neil_UK Jan 30 at 8:33
  • \$\begingroup\$ "What actually happened is that the battery charged about 0.9C and it was slowing down the closer it got to 4.2V" - what do you mean by this - that the total charge accepted was 450mAh, and current was reducing before the voltage reached 4.2V? \$\endgroup\$ – Bruce Abbott Jan 30 at 10:23
  • \$\begingroup\$ Sounds like there was excessive resistance in the wiring. DO NOT EXCEED 4.2V but beef up the wiring between PSU and cell. \$\endgroup\$ – Brian Drummond Jan 30 at 12:07
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To charge any lithium based battery you generally should not be using a bench power supply unless it has a special "Charge lithium based cells" function.

But in a pinch (there's no other option) I would consider slow charging an option, set the voltage to 4.2 V and limit the current to C/10. So for a 1000 Ah battery, charge with 100 mA up to 4.2 V. So on the supply: V = 4.2 V, Imax = 100 mA. That will slowly charge the cell and charging will become slower when the cell reaches its full charge. I would not do this on a regular basis though, only when there's no other option and you must have a charged cell.

To properly charge Lithium based cells, read this article.

Most chargers use either voltage regulation, current regulation or even both at the same time depending on the state of the battery (usually derived from its voltage and temperature). Fast charging gets even more complex as the full charge current should only be applied when the battery is between 30 % and 80 % full (those percentages can vary depending on who you ask).

This means that to charge at a constant 2C current the whole way I have to use voltage much higher than 4.2V

Charging at 2C is fast charging, you should not be doing that "the whole way", see my remark about fast charging. Even when fast charging the current must go down when the battery is getting fully charged. If you don't, the battery will overheat and might start smoking but for sure you will stress it and limit its lifetime.

The "FLAME ON" thing doesn't happen that easily, you can "safely" charge a LiPo battery to slightly more than 4.2 V but that does stress the battery and that will limit its lifetime. The 4.2 V is a decent compromise between lifetime and battery capacitance.

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  • \$\begingroup\$ "1000 Ah battery, charge with 100 mA" Do you mean 1000mAh? Anyway, the charging with my bench supply works fine at 1C and the battery doesn't even get warm, but it was too slow for me as it slowed down the more the battery was charged, but after some reading, charging (not battery) voltage should also not go over 4.2V, so I guess it will have to do for now \$\endgroup\$ – Askerman Jan 31 at 8:28
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Normal chargers have three phases to charging. A precondition charge is done at a constant current (usually 0.1C or less) if the cell has been discharged below the normal discharge termination voltage.

Once the battery is above the normal discharge termination voltage, the battery is charged at constant current until the voltage reaches the CC termination voltage (4.2V typically but it varies by exact chemistry).

Once the cell reaches 4.2V, the charger switches to constant voltage mode until the charge current goes down to the charge termination point (0.1C or so) and charging stops.

Chargers also implement cell temperature sensing and timers for each of these phases to stop charging if something goes terribly wrong with a cell.

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