I've read somewhere that constantly charging a Li-ion battery, even if it's fully charged already, is safe and does not hurt the battery. Is this true?
I just bought a DS3231 breakout (ZS-042), later I found a topic about it having a "circuit" that constantly charge the battery.
I found no schematic for the ZS-042 breakout board, but most likely the charging is done the same way as with the RTC Tiny DS1307 modules: The charging is not done with 5V from VCC, but with a voltage divider that divides the 5V by using resistors into a 4.2V/0.8V voltage.So the LIR is not "constantly charged", but it is only "charged until the cell voltage is 4.2 volts", which is the same as "fully loaded".
The battery is fed a constant current from Vcc, via a diode and resistor in series. So the LIR is constantly charged.
I don't believe it will last decades, because, unlike DS1307 modules, the high current needed for the temperature conversion that is done every minutes, and alarms, also self discharge and probably other factors... I think 5 years is closer to reality
The resistor seems to be 200k or similar, so it's a kind of trickle charging (which is not recommended for Li-ion batteries like the LIR2032)..