Huh. But the batteries aren't rechargeable. I had read that the batteries drain at the same rate whether the machine is on 24/7 or if it's storage — not true?
Many slots nowadays use the Ni-MH (Nickel–metal hydride) batteries to retain the machine's settings and internal accounting numbers.
Those ones are definitely re-chargeable cells and are only really "ON" and being dis-charged when there is NO power to the machine....otherwise, they are trickle charged to a specified voltage and an embedded chip switches off the charging voltage.
There are several types of re-chargeable cells in different electronic slot machines...older platforms in the 80's -90's such as the IGT S+, used Lithium cells before the Ni-MH batteries came out.
If you stick in non-re-chargeable, regular alkaline cell batteries in a slot machine that has a charging unit, you run the risk of overheating the cell and having it leak, explode, or catch fire.