I would think that a charger to properly and safely charge a high voltage Lithium battery costing $500 would cost a LOT more than $40. I suspect its just an over simplified wall charger that is not adequately charging your battery. As you use the battery in the bike it is not having its cells properly balanced and soon it will fail or be virtually useless. Im basing this on my use of some fairly expensive Lipo batteries in model aircraft that have very exacting charge specs and fail in a spectacular fashion if not treated properly. Let us know how it turns out