If your Nintendo Switch battery pack also needs to charge your smartphone or tablet, buy the UNU Superpak battery.
The UNU Superpak has enough power to fully charge an iPhone 6s four times and a cable that changes from MicroUSB to Lightning in seconds. Pair the charger with a USB Type-C cable and you can use it to recharge your Nintendo Switch when you're away from home. The battery itself charges through MicroUSB. Status indicator lights let you know when it's out of power.
Amazon sells the UNU Superpak in black for $23.99. A white model costs $30.99.