Travelling to the US? Make sure you've packed your gadgets' rechargeable batteries properly, as new rules governing what power cells you can take with you are now in force.

The bottom line is don't keep batteries in luggage you plan to check in, unless they're installed within a device. Spare batteries must be packed in your hand luggage not in checked baggage.

In fact, the US Department of Transportation recommends that all batteries, in and out of their host gadgets, be taken on board aircraft in hand luggage.

Lithium Metal batteries can't be brought on board at all if they contain more than 2g of lithium, while the Lithium Ion batteries of the kind usually found powering laptops are limited to no more than 25g of lithium overall, to allow you can take one 25g lithium battery or two 12g batteries on board, for example.

The DoT reckons all mobile phone batteries contain less than 8g of lithium, as do most laptop batteries. According to the DoT, 25g is approximately 300 Watt-hours (Wh), while 8g is roughly 100Wh.

An 5200mAh Eee PC battery - as we have one handy - delivers 7.4V and so yields 38.48Wh - just multiply the voltage and current values - and that's well within the 300Wh limit imposed by the DoT.

You can see the full rules here.

Spare batteries should be packed to prevent any of their contacts touching metal within your bag, which could lead to a short-circuit and the risk of fire. Basically, keep spare batteries tighty wrapped in a plastic bag secured with a non-metallic take. Only one battery per bag, please.

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