So you're saying this issue can be mitigated if you unplug quite soon after reaching fully charge, but if you leave on for "too long" once fully charged then the battery begins to discharge to around 85% - does it then stabilize or keep discharging?
To be honest this almost sounds like a design feature - batteries shouldn't be left at 100% charge when stored, so presumably if you take it off the charger after it gets to 100% then you are "just about to ride" however if you leave it plugged in for days, I can imagine this is not the best situation for the battery to be in, and there is an automatic discharge feature to protect it. For example if you forgot it was plugged in and ignored it for a month and it stayed topped up to 100% this would be bad for its health.
RC battery chargers for example has a storage function to discharge or charge batteries to a safe level for storage. It sounds to me that this is a design feature, given that it is not a battery problem, i.e. it is not an issue with the battery failing to accept 100% and topping out at 85% - it gets to 100 but then drops if left on charge too long.
Is my understanding of the "issue" correct and therefore does my guessed explanation make sense?
This link says:
5 Electric Bike Charging Tips for Long Battery Life
Tip No. 4: Don’t Overcharge an Electric Bike Battery
Don’t just leave your electric bike battery on the charger for long periods of time — think several days or more. When you do, you can create a situation wherein the battery will discharge leaving it at perhaps 95 percent of capacity. The charger then goes to work, topping off the battery. This cycle of minor discharges and topping off continues creating a series of poor charging cycles.
Instead, use a timer on your mobile device to remind you to take the battery off of the charger.
