Your explanation does not specify whether the battery is a cell or a battery. If it says 14.4V and 1.5Ah on the outside casing, the watt-hour rating is 21.6Wh. Your explanation does not say whether the item is packed with the rivet gun or whether the cell/battery is already installed in the rivet gun. Let’s say if it is a cell, it exceeds the 20Wh limit, so it would have to be shipped as Class 9 dangerous goods and use PI 966 Section I if packed together, and PI 967 Section I if already installed. If the item is a battery, the limit is 100Wh, therefore this 21.6Wh battery can be shipped as Not Restricted.