I also ended up here as I had a set of batteries die (and puff up). The original batteries are made by Synergy Taiwan under their Advanced Hybrid line (AHB). Original batteries are part # AHB682828 - 2.1watt hour or 550mAh. This means it is a lithium polymer battery, 6.8mm thickness, 28x28mm in dimension. Most lipo packs like these are coded similar so you can find any other generic pack 27mm square or smaller, and 6.8mm or thinner. The extra wire is a thermistor.