Undervoltage protection. One or two more battery packs. Pay attention to the charger to match, or use a timer to calculate the time to charge. In principle, it is possible to charge the low-voltage battery pack with a high-voltage charger, pay attention to the current limit, but do not charge for too long, calculate the charging time with the capacity of electricity and current, too not only damage the battery, but also may be dangerous. The controller does not understand the bad tone, and I do not know that you are brushless, no bad make do with it.