When you first purchase your device, you should charge each battery overnight using the docking station provided. Then completely discharge your batteries by leaving the tracker on and running for a full 7 days each battery. Do this 4 times to help extend the life of your batteries for years to come.
Its important that you solely use the docking station to charge your batteries. The AC adapter that connects directly to the GPS tracker does not adequately charge the batteries as well as the docking station does. The tracker is designed to track, the docking station is designed to charge.