However, this month’s project also has a reversed battery polarity detect system that contains an alarm and shutoff network intended for accidental wrong connection of the charger with the re-chargeable battery so whenever the battery is wrongly connected to the charger an internal alarm will sound and the charger will be shutoff or no output charging voltage will appear between the charger clips.
https://foxtrotpaparomeo.wordpress.com/2022/10/28/02052022-updatesbasic-zener-diode-discrete-gate-network-applied-automation-in-battery-charger/