[br]
I had the same issue and looked for a solution for months. I left my bike over the winter and when i tried to charge it, i couldn't.
It seems the battery went into "sleep mode", something lithium batteries do if they remain fully drained for more than a few weeks. The reason the charger remains green is that it doesn't detect the battery due to its low voltage. When i tested the battery with a multimeter i was getting 1.5v rather than a healthy battery's 40v.
SOLUTION:
Put the battery in the bike. Lift up the front wheel and spin it backwards, as if going in reverse. The white LED lights on the battery should blink. You are basically forcing a charge into the battery through the motor. You need to keep doing this until you get the voltage up to around 18v. At that point the charger detects the battery, turns red, and charges as normal.
The reason this can't be done via the charger is that the charger is "smart". A chip deactivates it if no battery is detected. You can hear the deactivation click after a few seconds of plugging in the battery. Until that click your battery was charging.
I used that as a shortcut to spinning the wheel which takes ages. I keep plugging in the battery, and unplugging at the click. If you have the multimeter connected to the other end of the battery at the same time you can see the voltage slowly increase. Keep at it until the chip detect the battery and turn the light red.