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,

Ok this sounds normal to me. Battery voltage will be lower when charging has ended compared to when you charge it with 73mA (which is roughly the set 10% termination current). The higher the charging current the bigger difference you would see. 

The fuel gauge accuracy and value depend on the selected battery model and how well it matches with the used battery. 

[mhni]

Hello,

I also tried with another battery to verify the behavior:

• Battery: 3.85 V, 1200 mAh Li-ion

• Charging current: 800 mA

• Termination voltage: 4.35 V

• Trickle/Fast charging threshold: 2.9 V

• Charging termination: 10% (default)
  

• The first attached screenshot shows the charger operating in Constant Voltage mode, with a battery voltage of 4.37 V and charging current of 111 mA.

  
  

• The second screenshot was captured after charging completed — the application reports 100% full, but the actual measured battery voltage is 4.26 V, while the expected set termination voltage is 4.35 V.
  
  

• Could this early termination be related to the 10% termination current threshold, or possibly a voltage drop between the charger IC and the battery terminals?
  I would appreciate any suggestions or guidance on how to ensure the charger reaches the configured termination voltage, or if there are recommended settings or additional tests to verify this behavior.

[Tomi Koskela]

Hello,

This is due to battery behavior and internal ESR and it is completely normal to have higher cell voltage with 111mA charging current vs when there is no current sourced to the battery. The battery voltage will also drop slightly when rested due to the battery chemistry. This is considered already on the fuel gauging during the battery modelling that the rested voltage is not the same as the termination voltage. 

The higher the charging current the higher the termination current and that makes the step bigger when charging stops.

[mhni]

Hello,

When using the IP2312 Battery Charger IC, the charging process stops at around 4.32 V in accordance with the termination voltage setting.

In contrast, with the Nordic charger IC, the charging does not terminate at the same voltage, indicating a difference in termination voltage behavior.

[Tomi Koskela]

Hello, 

Not knowing the full details on the IP2312 charger I would assume the difference is in the termination current. It seems it does not have specific termination current, but it relies on timer function to stop the constant voltage charging. So yes it is different in that sense.