Note: This course was created in 2016. Battery capacity has increased since then and modern phones will usually not charge from this design anymore. But it will work with other USB devices that use up to 500 mA.

Have you ever experienced that your phone’s battery dies when you need it the most? Maybe on a hiking trip when you want to take a beautiful photo. Maybe when you’re trying to find your friends in a festival full of people. It’s usually at the worst possible moment.

With this portable USB charger, you can recharge your phone on the go. It uses standard AA batteries that you can buy in any supermarket, or you can modify it to use other battery types, solar power, or even power adapters not really meant for your phone.

The charger works like a 500 mA iPhone charger that was common around 2016. Newer phones need more current and won't be able to charge from this.