It flopped because it doesn't really work, and most stuff we stick batteries on these days already do a good job of extracting all the energy out of the batteries. It's mostly already built-in.
EEVblog made a few videos on it:
- https://youtu.be/Z-m-7jex88k?si=KhEtCmvGkeyEtj4h
- https://youtu.be/Tl90m2KHbNk?si=0LK6Op_E32w3tMSO
- https://youtu.be/dnXiLBabSTU?si=DufjcVHCvlLSGi5Q
Dead old batteries are dead, if there's almost no energy in them they're done. Best case you might be able to stick them in a remote or an LED candle or whatever. But it's a physical limit: almost dead batteries just can't supply high currents because of internal resistance.
Another way to look at it is those potato batteries: yeah, you can measure a voltage off them. Yeah, you might be able to light up an LED. But you will never be able to power something higher power, not even for a couple seconds. It's physically not able to provide much current.