The quick answer is no, they are not interchangeable. The reason for this is that while both companies make 40-volt lithium-ion batteries, the Ryobi battery is slightly wider, making it incompatible with the Greenworks charger. In addition, the Ryobi battery has a higher voltage, meaning that it would not work with the Greenworks tools. Ryobi 18V batteries are not compatible with the 40V string trimmer, but the 40V battery will work in the 18V string trimmer. ... though, expect around 30 minutes of runtime from a full charge. Are all 18 volt Ryobi batteries interchangeable? Most 18 volt Ryobi batteries are interchangeable, but there are a few exceptions. For instance, the.