What's inside?

Tux has a 4 cell NiMh battery included. There's a charge IC inside tux so the battery can be recharged from a 7.5V DC transformer.

How to recharge?

Just plug the transformer at the back of tux and switch it on. The charging time may take up to 5 hours of the battery was completely discharged. It's still possible to use your tux when charging, only the spinning is disabled to avoid messing up with the charging wire.

How can I know it's charging?

You can use the battery monitor gadget and check the charging voltage over time. If you unplug then replug your tux, you should also hear a yummy sound indicating that the charge has started.

How does the voltage change when charging?

Here's a chart of a typical charge from a flat battery. I had to merge 2 screenshots in order to get that chart so the vertical rulers are not accurately positioned.

1. Turned on the motors to discharge the battery; there's an option to avoid measurements done when the motors are on as the voltage is always lower in that case. The option was activated so the graph wasn't updated during that period.
2. Turned off the option so now the voltage is correctly displayed.
3. The voltage was low enough so we plugged the transformer and the charge started.
4. It takes between 4 and 5 hours for the voltage to rise to it's maximum around 6V.
5. The voltage reaches a maximum and then slightly starts to decrease (few mV, barely noticeable on the graph). At that time, the battery temperature increases a lot and the charger IC detects the end of charge from the thermistor. The charger switches from fast charge to trickle mode, and the battery voltage suddenly drops for a few hundreds mV (the drop just below the 5 on the graph.)
6. The trickle current maintains the battery fully charged but the voltage will slowly drop before stabilizing around 5.5V.