Not an e-biker - my knowledge is more about cars, so perhaps somebody could help me out. Sometimes a car develops a parasitic drain on the battery that slowly bleeds the charge off of a perfectly good battery over time. A classic example is the failure of a switch that leaves one tiny light bulb somewhere on 24/7.
(This can be a bit of a nightmare because the driver may repeatedly try to replace batteries under warranty.)
In this case, if OP has every reason to believe the battery is OK, I would consider checking the bike before it kills the next battery, or at least not storing a battery in this particular bike.
Edit: Break out a multimeter. Put the new battery in the bike and set the bike up like you stored it for the winter. Check for any current flow at all. I would expect there to be stone cold zero - the bike doesn't even have accessories like a clock that a car might have. Measuring amps requires a way to break the circuit and put the meter into the circuit.
Try to get a measurement near the battery so you are checking all possible circuit paths at once.
Last edited by ScottCommutes; 04-22-24 at 09:05 AM.