It is recommended to charge a 4S 6500mAh LiPo battery at a rate of 1C, which means charging it at 6.5A. With a 1C charge rate, the battery should take approximately 1 hour to fully charge. It is important to never charge a LiPo battery at a rate higher than 1C as it can lead to damage or even cause the battery to catch fire. Always monitor the battery while charging and follow all safety instructions provided by the manufacturer.
