In trader_4 writes:

Any diode worth using will have so little 'backflow' it would take
years to drain a car battery.

+1

I guess things have improved since my days working the
lab with George Steinmetz...

The other thing Danny thinks the battery is draining over many days,
while the OP clearly said it happens overnight.

My impression was more that it started off (for illustraion)
on Sunday with 100 pct charge. Not driven at all. So Sun eve
it's at 95 pct, Mon morning at 90.

(Using "20 percent" as the cutoff needed for starting)

Driven a bit Monday, so Mon eve 92 pct, Tues morn 85.

Drops down a couple more percent each day.

Finally, the next week.. Mon morning 21 pct, mon eve 23,
Tuesday morning... 18 percent. So... nada...




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