I use this Kencove charger that runs on 4 D cells. I've had them going for a week without running down. I have no idea how long they'll actually last. A charger uses very little juice until it's touched. Then it dumps a whole lot in a hurry. 4 D's only total 6 volts but the charger multiplies that by 1000 times or more.

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“In a time of deceit telling the truth is a revolutionary act.”
― George Orwell

It's not over when you lose. It's over when you quit.