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(Pocket-lint) - The Upp portable USB charger is now on sale across the UK in Apple Stores and it provides something a bit different for those who require a lot more charge than a mere phone top-up.

It is a hydrogen fuel cell charger that can provide up to a "week's worth of power", so can be ideal when you are away for an extended period without a power outlet nearby.

We took an Upp on a trial run in Berlin recently, but wanted to wait until it was actually available in the UK until we posted our thoughts and we must admit it proved invaluable on our travels.

There are two elements to the Upp charger. The main part is the fuel cell, which has an on/off switch and USB port. The other part is a replaceable cartridge which contains the hydrogen needed to recharge the cell.

You plug one into the end of the other and can actually start charging a phone immediately. However, we advise that you charge the fuel cell part of the device in a conventional way first, using a connection to regular power socket as the way the Upp works is it draws on the cartridge as its own stored battery power is waning. That massively extends the the amount of charge it offers, we found.

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Upp claims that is not entirely necessary, with the instruction manual saying that you only need to charge it via a wall socket when the device has been dormant for a long time in order to kick start its fuel consumption, but we found it to work more efficiently for us. So that's what we did.

We charged the main fuel cell through a hidden micro USB port before we travelled and then attached the cartridge when in use in Berlin. It happily restored a dead iPhone 5S back to full power - well, as much as we needed in the time we had available before a meeting.

It works in about the same amount of time it would take to charge a device through a plug socket and because it was fully charged originally, didn't drain the fuel cartridge too much before being fully charged itself again.

Even without ever charging the main cell section in a normal manner, the fuel cartridge will keep it topped up enough to charge a phone up to five times from zero to 100 per cent. That could be essential on a camping holiday or at a festival, for example.

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We didn't manage to test it that many times to find out for our hands-on, but it worked many times on our phone without ever needing to be recharged through a wall socket. Indeed, we were even supplied with a second fuel cartridge in case and never once needed to use it.

This all sounds great but there are caveats. The Upp charger is a weighty and large beast. Together, the cell and cartridge weigh 620g - that's more than five times more than the iPhone we were charging. It is also the size of a burglar's cosh. This would not be something you can carry around discretely in a bag or pocket ready to top up your phone when needed.

It also lets out a weird hissing sound and a faint odour that we couldn't quite place until we remembered stink bombs from our school days. Neither is that bad though and only really noticeable when you hold it up to your ear or nose respectively.

The other minor concern is that, as it is new to this country, there aren't that many vendors that can swap the fuel cartridges with full ones (which cost £5.95 a pop). There are plenty in London - Shell petrol stations mainly - but few outside the borders of the M25. We're not sure if Apple will swap the cartridges, but there are more cartridge exchange points being signed up all the time.

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A dedicated application, which also shows how much charge there is available on the cell and has been redesigned a bit since our hands-on session, has a exchange location finder built in. There's similar functionality on the Upp website at beupp.com, so it'll be worth checking if there's something near you before you make a final decision.

It's also worth noting that it's a fledging technology that will no doubt get better over time - hopefully lighter at least - and more widely supported. For now though, if you plan a long trip up a mountain or something, you might find the £149 starter pack (Upp cell and cartridge included) a useful addition to your gear.

Writing by Rik Henderson.