The 500-CARRIER makes it possible to replace the TAC-500/550s rechargeable battery, with non-rechargeable batteries. If the rechargeable battery on the TAC-500/550s were to run down, the user could insert a pair of CR-123 Lithium batteries into the carrier and then use it in place of the rechargeable battery. Requires 2 - CR-123 Batteries which are not included. Not compatible with TAC-400/450 Series.