I used to take those 9-volt battery connectors, wire them up together and then recharge a dead 9-volt with a brand new one until the tester strip thing showed they were both even. Surprised they never popped because they would get really hot lol
Those chargers are really dangerous. It is technically possible to recharge a disposable alkaline battery a few times, but you're never going to get more than a half charge, and it will fuck up the internal chemistry turning each battery into a tiny potential pipe bomb.