today I did a charging experiment with a fully depleted M5Paper battery. After about 2 hours the charger IC switched from charging to standby (according to the two LEDs I've soldered to the corresponding charger IC outputs).
I used a multimeter (connected directly to the battery) to measure the battery voltage. The voltage from the internal ADC, read via
M5.getBatteryVoltage(), was about 5% higher than the voltage read by the multimeter for voltages below 4.2V. Above that the two values were pretty close. The reason for that is the voltage divider (3k / 11k) which when fed with 4.2V produces about 3.3V to the ADC input which is about the maximum a GPIO can take.
The highest voltage I've seen on the multimeter was 4.31V before the charging stopped. After which the voltage dropped back to 4.25V. I think that means the charger IC actually is setup for a 4.35V battery, but for some reason doesn't go all the way up to 4.35V.
Update: a possible reason is explained in the datasheet (Google translated):
_______________Charge termination voltage setting The default full termination voltage V FLOAT set inside the chip is approximately 4.35V, but due to the large charging current, the internal resistance of the battery and the line Loss will cause the actual full-charge termination voltage to be lower than this value, resulting in battery failure The law is full enough. SLM6635 by an external resistor RPV to increase V FLOAT of Voltage, used to compensate various losses, or to satisfy different applications Special requirements for voltage.The compensation voltage can be calculated by the following formula:
Delta V = I bat (Standby mode) * RPV
If there is no need to compensate the V FLOAT voltage, it is recommended that RPV be set to 1kΩ.