This is the optional battery back that will power your PT2 or PT3, in the event that you loose power in your observatory and you need to lower the pier to close the observatory roof. You can also use this battery to operate the pier continuously in the field, or if AC power is not available at your site. The battery has a audible alarm that will sound when it needs to be charged. The dovetail feature on the battery pack shown in the picture slips into the receiver on the controller. Depending on the load on the pier, you can operate the pier for days until it needs recharging.   This battery pack is compatible with the new 2021 Pt2 or PT3. Not used for any piers sold before 2021.