Unfortunately, there are two APC900 models - old and new. The new models need no. 164, the old ones need no. 123. Unfortunately, I ordered no. 123 first and then had to send the battery back. Tip: simply remove the old battery and compare the photo. If it is split in two, it will be no. 123. If it is a single large block, it is no. 164 (just look at the photos at Galaxus).