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Sometimes there isn't a power outlet conveniently at hand to give our devices a top-up, and that's when the best power banks come in handy.

Whether you're trekking in the wilds, or strolling between coffee shops, a power bank can make the differences between staying connected or going silent.

Here is a selection of great power banks to keep your devices charged up when you're away from a power outlet.

Everything about the Omni Ultimate is designed around power and ruggedness. The exterior ABS/rubberized shell is capable of taking a pounding, the ports are well-designed, and the screen offers everything from at-a-glance information, to a deep dive into what's going on.

Flight safe: Required airline permission

If you want a big power bank with plenty of power and the ability to charge four devices simultaneously, the Zendure SuperTank Pro is the power bank for you.

Dimensions:  4.9 x 2.9 x 1.8 inches (124mm x 74.5mm x 45.5mm) | Weight: 20.5oz / 580g | Capacity: 26,800mAh | Cells: 8 Samsung high-density electric-vehicle battery cells | Flight safe: Yes (rated at 96.48 watt-hours) | Total DC Output: 138W | Firmware upgrade:  Yes, requires PC | Display: OLED Screen | Material: Aluminum shell with ABS & polycarbonate midframe

A portable, everyday power bank, more than enough to keep your smartphone and tablet going for an extra day or so.

Dimensions:  4.5 x 2.8 x 1 inches (114mm x 71.1mm x 25.4mm) | Weight: 10.8oz / 306g | Capacity: 15,000mAh | Flight safe: Yes (rated at 54 watt-hours) | Total DC Output:  48W | Material: ABS & polycarbonate

Is it a power bank or a wall charger? It's both!

Dimensions:  3.2 x 2.8 x 1.2 inches (80mm x 71mm x 30mm) | Weight: 8.6oz / 243g | Capacity: 4,850mAh | Flight safe: Yes | Total DC Output:  5V/3A | Material: Polycarbonate

I have a lot of power banks, but it's rare to come across a power bank that claims to be water-resistant. It's even rarer to come across a power bank that is actually water-resistant.

Dimensions:  4.1 x 2.0 x 1.2 inches (104mm x 50mm x 30mm) | Weight: 6.6oz / 187g | Capacity: 6,700mAh | Flight safe: Yes (rated at 24.8Wh) | Total DC Output: 5V/3A | Material: Rubberized coating

These are all products that I've tested and being using for several months. I always have a power bank or two with me when I'm traveling, and this means that they get a prolonged testing.

Here are some of the criteria that I look for:

Tough question. If you want an emergency power bank to act as a backup, then a small 5000mAh power bank will do the job. If you want a lot of power, and perhaps the ability to charge up a laptop, then the bigger the better (well, going as big as you want to carry!).

Power banks up to 100Wh (100,000mAh) can be carried without approval, and for power banks that fall between 100Wh and 160Wh you will need permission from the airline.

Not everyone uses their power bank at home or in a coffee shop. Power banks ban be great for keeping camera equipment and other gear such as smartphones and GPS receiver topped up when off grid.

I have yet to come across a good one. I'm also not comfortable baking rechargeable batteries in the sun for extended periods.