Everyone is familiar with that stressful feeling when your phone is about to run out of power and there seem to be no plugs in sight.

Your handy-dandy power bank always comes through when your smartphone decides to run out of battery while you're out and about.

To make sure your power bank stays effective and reliable in your time of need, however, it's important to take special care of the device.

First, keep power banks away from heat, high temperatures and direct sunlight. Just like smartphones, the optimum operating temperature for most external chargers is between zero and 35 degrees Celsius (32 degrees and 95 degrees Fahrenheit) says Germany's Tuev Association (VdTUeV), a leading technical agency.

When power banks overheat – because they short-circuit or due to external heat – that can trigger an internal thermal reaction that can at the very worst lead to an explosion. So avoid leaving your power bank exposed to the sunlight in the car or on the dashboard.

Also, take your phone out of your pocket while you charge it with the power bank – otherwise, the device, when coupled with hot weather and high temperatures, could potentially lead to burns on your leg.

If you drop the power bank or it looks damaged in any way, then be extra careful, the experts say. If you spot areas where the metal looks tarnished, or the casing is melted, then be sure that you dispose of the device in line with the manufacturer's guidelines.

A further danger sign is if your the power bank grows warm even when it isn't connected to anything – stop using it if that's the case.

If the battery runs out much faster than usual, that also indicates a defect.

You want to avoid leaving your phone unattended while charging with a power bank – and check the temperature every now and then. If the power bank heats up so much that you can't hold it, disconnect it immediately. Also, don't let it charge for too long – say overnight.

When you are buying a power bank, there are a few things to bear in mind: For those who are outdoors a lot, check if the power bank has IP67 certification, as it means it is dustproof and waterproof, says VdTUeV. To get the certification, the device has to be able to survive being dropped in up to 1 meter (around 3.3 feet) of water for up to 30 minutes.

Meanwhile, if you are buying online, take care and ensure the CE mark is not false or misleading: Check that CE stands for "Conformite Europeenne" – and not for "China Export," as VdTUeV has spotted with some goods from China.

Buying a no-name brand also requires caution, and if you can't see who manufactured the item, then it is better not to buy it, says the association. It suggests that you treat products that are extremely cheap, coupled with promises of high performance, with skepticism.

If you buy a power bank, make sure the output voltage is the same as the device you are charging, as otherwise, you might damage it.

Often, power banks have ports that charge at different amperages.

Some models have an intelligent charging mode and can detect the device they are charging themselves and adjust accordingly.

And if you wind up not using your external charger for a while, avoid letting it run out of charge completely: It will last longer if you partially recharge it from time to time.

More about: #powerbank