Cannot be interchangeable. Because the voltage of 208vf lithium battery and 21v lithium battery is different, it is not compatible. Battery voltage is one of the basic parameters of the battery, different voltage of the battery has different output capacity and power, can not be directly replaced. Therefore, when using the battery, the voltage battery should be used in line with the requirements of the equipment to avoid damaging the equipment or causing danger. When purchasing batteries, ensure that the required voltage meets the device requirements.
Of course, the battery voltage is not the same, the voltage is too high will be bad battery voltage is too low.