Battery Stops Charging at 4.0 V (Expected 4.2 V Termination, 20% Termination Current)

[mhni]

Hello Nordic team,

I’m using the Mikroe nPM1300 PMIC Click Board (based on the Nordic nPM1300 PMIC) with an ESP32 over I²C to charge a Li-ion battery.

Setup details:

• Battery: 3.7 V nominal, 950 mAh Li-ion

• Configured charge current: 800 mA

• Termination voltage: 4.20 V

• Termination current: 20% of charge current (~160 mA)

• VBUS current limit: 1500 mA (set manually via I²C)

• NTC monitoring: Disabled

• Power source: 5 V adapter connected to VBUS

Issue:
The battery stops charging around 4.0 V, even though the termination voltage is set to 4.2 V. Once the battery voltage reaches ~4.0 V, the charging current drops close to zero, and charging does not continue to 4.2 V.

Questions:

1. Could any internal safety or thermal mechanism cause early termination near 4.0 V?

2. Does disabling NTC monitoring affect charging accuracy or limit the termination voltage?

3. Are there specific status bits or event registers I can read to determine why charging stops early?

4. Could voltage drops on VBUS or board-level limitation cause this behavior?

5. Are there additional parameters (such as recharge threshold or termination comparator accuracy) that could affect this?

Any guidance on ensuring proper full-charge behavior up to 4.2 V would be appreciated.

Thank you,

[Tomi Koskela]

Hello, 

Does the voltage after charging period increase on each subsequent charging cycles? What is the maximum specified charging voltage for the battery? Increasing the termination voltage above the maximum charge voltage might cause also the battery PCM to prevent overcharging of the battery. 

Do you have picture of the test setup? Long wires for the battery can cause considerable voltage drop. 

Could you also share the datasheet for the battery? 

After charging is completed at 4.2 V as per datasheet, the battery voltage naturally relaxes to a slightly lower value (e.g., 4.13 V) due to electrochemical relaxation. This is a normal behavior and does not indicate a loss of capacity or partial charge. Only if charging is terminated at very tiny currents would it stay very close to 4.2 V. 

[mhni]

Hello,

Thank you for your response. I didn’t get the datasheet for the battery.

Since I don’t have the official datasheet, I am assuming the maximum charging voltage is 4.20 V, as this is typical for 3.7 V Li-ion cells.

I The test setup photo is attached above.

I don’t have the matching battery connector, so I have connected the battery positive to the VBAT pin, battery negative to GND, and used a 10 kΩ resistor at the R9 position for the NTC input.

[Tomi Koskela]

Hello, 

Note you should check the battery datasheet for the charging current spec. It can be 0.5C for instance and not 1C. The jumper wires+connectors can easily add extra resistance of 20-30mohm which will cause ~20mV difference between charger and the cell voltage. So this will add to the difference seen between charging voltage and the battery rested voltage. Shorter wires directly soldered to the board is recommended in this case.