Most battery-life extending pack tend to be brain dead, they automatically charge when they're the needed. The Wi.U is a bit different. You can set the date, time and percentage you want the charger to activate.
The battery is supposed to provide battery power for up to 3 days at a time, but when it does eventually run dry you can wirelessly charge it on Wi.U's charging pad. Sadly, it's a Kickstarter project for now, but if it does go to retail it'll go for at least $130. [Kickstarter]