I do not now categorically rule out the possibility that the device has a pure measuring function that is supplied by the battery voltage itself. This is technically feasible. But that would be a unique feature of a charger and would also be clearly advertised accordingly.
However, I am not commenting to answer the first part conclusively, but regarding monitoring. If that is the sole purpose, then a charger is the wrong tool anyway.
Perhaps there is something more suitable:
https://www.digitec.ch/de...
Otherwise you can also use a voltmeter or multimeter.
https://www.digitec.ch/de...