I would bet that the electric wind spring driven clocks we had back in the old days from the 1930’s through the 1960’s consumed more power than the power required to charge a cell phone battery. These clocks would rewind about every 2 minutes. We certainly didn’t leave the engines running so that the car battery would stay charged to keep the clock wound up. I often while traveling let the car battery recharge my cell phone battery overnight. I’ve never had a problem.