I'm trying to understand exactly how CC/CV charging of a LiIon battery works and I tried to replicate it with a cheap bench power supply (RD Tech DPS3003) and a Samsung INR 18650-25R. The bench power supply is capable of both CC and CV. Here is what I tried:
- Discharge the cell to 3.1V
- Set the bench power supply CV to 4.2V according to the battery data sheet
- Also set the PSU CC to 1.250A or 0.5C, according to the data sheet
- Connect the PSU's positive lead to the battery's positive terminal; same for negative
What I thought would happen:
- The PSU will supply as much voltage as necessary to make the set CC of 1.250A flow into the cell
- As soon as the cell voltage reaches the set CV of 4.2V, the PSU switches to CV mode and the current will decrease steadily
This is what I saw instead:
- When I measured the voltage at the battery terminals while charging, the voltage went up only a little, to around 3.3xV and slowly rising as the battery charges
- The PSU display shows around the same voltage and only a current of around 530mA
- The PSU displays that it's in CV mode, rather than CC mode.
Clearly my understanding was wrong and I'd love to understand better. Why did I not see the set CC of 1.250A? Were my settings wrong, e.g. do I have to allow for a higher voltage? But then how will the PSU limit the cell voltage to 4.2V and switch to CV mode later? Or could it be that my cheap bench power supply does not behave as it should?
Additional information: I also did a quick check with my Liitokala Li500 battery charger. I set it to charge the same battery (by then at around 3.3V cell voltage) with 1A and measured the voltage at the contacts. It was around 3.5V. Unfortunately I could not easily measure the current. I'd be very glad for any insights on how my understanding was wrong.