5V devices like mobile phones, tablets, iphones require portable chargers that have an input and output of 5V and 500-1000 mA. There are several types of portable chargers of this type. The battery capacity varies and the type of battery is Li-ion type. You can opt for usual portable external chargers or solar chargers that work on solar energy. The reason to select a 5V charger for a 5V device is the voltage compatibility. It is advisable to use a charger that is compatible with the 5V device.