Running out of battery power is a constant worry when using a notebook computer. The only way to extend the running time is to carry a spare battery, but changing it can be disruptive as it means shutting down the notebook to make the swap.

An alternative solution that offers uninterrupted use is the APC battery pack. Weighing just under 1Kg it's not too heavy, although this weight is in addition to that of the notebook.

The battery pack comes with a selection of power supply adapters for different notebooks. It is a reasonable collection but be sure to check that your particular notebook is supported; the list can be found on the APC website here. 

We had mixed results when using notebooks not on the list. The voltage at which the power will be supplied can be adjusted and it's possible to damage equipment by using the wrong voltage. To be fair, APC uses bright yellow warning stickers to point this out.

The battery worked very well in our tests, giving an impressive extra seven hours to our test notebook's battery life. One thing to be aware of is that the notebook will assume it is on mains power and may not employ some power-saving measures to increase battery life.

However, the APC battery is expensive and it might be cheaper to simply buy another battery from the manufacturer.