It's pretty annoying when your smartphone's battery dies during a long trip. One solution is to take a power bank, which is a kind of second battery able to refresh the smartphone while you're on the go. There are plenty of options on the market but there are a few tips worth considering.
Capacity: One rule of thumb is that generally the more expensive the power bank is, the more charge it can hold. This determines how many times a smartphone can be recharged from a full power bank. This capacity is indicated by the unit milliampere-hours (mAh).
Around 6,000 mAh means the smartphone can probably be recharged twice from the fully-charged power bank. "Anything below this value is usually not enough," says Daniel Molenda from the German telecommunications portal teltarif.de.
Connections: The more connection ports the power bank has, the more devices can be recharged simultaneously, an advantage if you need to charge a smartphone and a tablet at the same time.
Accessories: Not all of the devices come with their own power adaptor. Often the adaptor for the smartphone can also be plugged into the power bank. One gimmick to avoid is built-in solar cells. These require putting the power bank in sunlight - which damages the battery.
Display: The power bank should have at least one LED to show the charge status. Otherwise it's very difficult to estimate how much power it's holding. "Best are models with more than one light," says Molenda.
Charging current: For smartphones and other devices to recognise the power bank as a power source, it needs to have a certain minimum strength of electrical current. For mobiles this is 1 amp, for tablets it's 2. This should be explained on the packaging.