Thanks for your reply! Well, I hadn't used it in 1-2 years (can't remember), so I'm pretty sure the battery is simply drained. After charging it, using it, and charging it again for 3-4 times, I can say the battery lasts for around 15 minutes (not straight through, as I only give the bike power for a few seconds at a time sporadically). Then it makes a puttering sound, and then dies. After a few mintues, it has a few seconds left of energy, then eventually makes the same sound, and then dies again. What do you think? (and, thanks for your help!)
Pammy