# charging 4 Li-Ion cell in parallel with a bench top power supply

I got a Li-Ion battery which has four INR18650MJ1 cells connected in parallel. Since I don't have a charger dedicated to custom batteries, I'm trying to use a bench top power supply.

I saw from internet that it is optimal to charge Li-ion cells in Constant Current mode first and then in Constant Voltage mode. Also one needs to switch the mode when the voltage reach at 4.1V or something.

But here I have a questions. What does the 4.1V mean? Is it the output voltage of power supply, or the voltage at the node between the + terminal of the battery and the + terminal of the power supply? With an output voltage of 4.2V from the power supply, I have 3.5V at the node. In order to maintain CC mode with a charging rate of 0.5C(1.6A), I think I should use 5V or a higher voltage until the actual terminal voltage of the battery goes close to 4.1V. Is it correct?

4.1 Volts with negligible current means it is full.

Basically the steps are:

1. Charge Constant Current. Using constant voltage here would give you an enormous current, heat and eventually smoke.
See the datasheet for the maximum charging current, otherwise use the rule of thumb: 10% of capacity.
The moment you "switch" from CC to CV is when the supply no longer limits on current.

2. Charge Constant Voltage. The last bit is in constant voltage, since the battery can't take more current without exceeding the voltage rating and overheating.

Basically you can set the power supply to a maximum current (eg: 10%) and voltage, (4.1 V) and connect the battery.

Depending on the size of the battery you might not want to hot-plug it. Use digital soft on-off. Else set the end of the fine trim to be 4.1 Volt, turn fine all down, attach battery and turn fine up again for manual "soft" on.

During charging keep an eye on:
- Time. If voltage or current do not change within expected time, stop.
- Temperature. If you can't touch it, stop.
- Bulging. Stop and bring it outdoors.

Lithium-ion/polymer batteries can explode violently, respect them.

• Thank you for your reply. The PCM attached to my battery has a limitation on input current that is 2A. So anyway I may not charge the four cells with more than 2A in total. This makes me difficult to make CC mode. With an output voltage of 4.2V power supply generates already less than 2A. So I was wondering if it's okay that I increase the output voltage of the power supply. Commented Nov 14, 2017 at 12:26
• NEVER increase the output voltage of the PSU, you can charge cells at low current Commented Nov 14, 2017 at 12:35
• @Nownuri See point 1. Increasing voltage above the ratings of the battery would increase current and heat until bulging, smoke or explosion. If you are not sure of charing rate, use 10% of capacity. For 2 Ah battery this is 200mAh. Commented Nov 14, 2017 at 12:43

I found the actual problem in my setup and would like to answer to this question. When the power supply generates 4.1V in CV mode, the terminal voltage of the battery connected to the PSU should be 4.1V. Therefore, it's just good enough to set the desired voltage of PSU to 4.1V and set the current limit to certain value.

The reason why I had only 3.5V was the cables that I was using. The cables caused large voltage drop.