The recommended charging rate for a 4s 6500mAh LiPo battery is typically around 1C, which means you should charge it at a rate of 6.5A. Charging at a lower rate is also acceptable, such as 2C (13A) or even as low as 0.5C (3.25A) for a slower, more gentle charge. It is generally not recommended to charge LiPo batteries at a rate higher than 1C, as it can potentially damage the battery and reduce its lifespan.
