When using a 12v battery charger would it delete charge if charger was switched off at power but connected to battery still?

May be possible if one of the diodes is bad in the charger. If so it also means the charger is not charging correctly and your battery is not getting charged. A voltmeter should read around 14 volts when charging if not toss the charger. FAR more likely is a bad battery

"Delete charge" ? A computer term for an analog situation?

I guess U mean, will it Discharge the battery? No. . The diodes in the charger prevent this. U can leave it on forever , unplugged, it wont drain the battery.

Ive Never seen one that did-! If so, it would not charge and Smoke!

REF: Electronics tech since '79, fixed power supplies 8 years.

It shouldn't do that.

Maybe a cheap one will, but decent ones shouldn't.

Not sure, but if any resistance is present as a load to the battery, it will drain the battery (a little or a lot).

Connect your ammeter/milliammeter in series and measure the current flow (if any). Then you'll know.

Depends. Take a sensitive DMM and check the current draw when connected to the battery. Many "smart" chargers have voltage-sense circuits that use a (tiny) amount of current. But the simple fix is to disconnect the charger from the battery instead of guessing.

Depends on the charger. It can take out the charge eventually. Depending on the internal wiring or the charging system.For an hour or two not enough to worry about. Connected for MONTHS a different possibility.

No the circuit is opened with the charger off.

No. Circuitry won't backfeed 12 VDC to 120 VAC.

Some do.