This MINI USB Charger will take about 20 minutes to fully charge the battery. With your USB Charger plugged into a power source of no more than 5V 2 amp output, the charger's power light will remain off. With the depleted battery installed now, the USB power light should come on solid red until finished charging when it will turn off. It should take about 20 minutes to recharge, not 120 as mentioned in the Start Guide. If it takes more than 90 minutes, stop charging, something is wrong.
Please don't pull on the wires when unplugging the battery, they will break easily. Unplug by pulling on the two connector halves. Your MINI should move at up to 12 MPH with the fully charged battery.