This is a very difficult product to categorise. It claims to increase the life of any Lithium-Ion battery and takes the form of a simple sheet of black film with some attractive metal flecks, which you stick to the outside of the cell. There's no power involved, no electronics nor chemicals, in fact nothing except the film, which you can cut to size to fit your particular battery. The company that manufactures the film claims a typical battery life increase of 25 percent for notebook use.

There's a maxim which says 'If it seems too good to be true, it usually is', so just exactly how does Batterylife Activator work and what kind of improvements does it offer? The first part of this question is quite difficult to answer as even the manufacturers don't seem to have a full understanding of the mechanism by which the Batterylife film works.

The makers say it has an effect on uncharged metal particles, parts of the Lithium electrodes, produced within the Lithium-Ion cell during the charge-discharge cycle. By reducing these waste products, the charge retention, as well as the charge time and overall battery life, are all improved. The nano-ceramic Activator film does this, so the makers claim, by itself ionising its surroundings from a background level of around 250 Ions/cm3 to around 7,500 ions/cm3.

So that's the principle (hmmm...), but does it work? We took an old Thinkpad 770 with a replacement battery bought new a year ago and attached the Batterylife Activator film. This is very easy to do, as it's just a plastic, self-adhesive film which can be cut to size to fit the battery pack.

We then repeatedly charged the notebook up and discharged it, running a simple task designed to keep the machine active, without dropping into suspend or hibernation modes. Without the Activator, it ran for 91.5 minutes. With the Activator, it ran for an average of 87 minutes over eight cycles, slightly less than without the film.

We went back to the company with these results and were told they'd had problems with IBM notebooks and their batteries before - something to do with the cells being double-sealed. So, we asked for another Activator sheet and stuck it to a three month old battery from an Evesham notebook. Without the Batterylife Activator, this machine ran for 140 minutes. With the Activator, it ran for an average of 140 minutes over eight cycles. No improvement. Zero. Zilch.

Now it may be that we happened to have two batteries with which the Batterylife Activator is incompatible. Other reviewers on other sites have noted measurable improvements with the mobile phone version of the Batterylife Activator and there's a report from the German TUV testing organisation graphing an improvement. We can only go by what we found, though, and that was that the Batterylife Activator had no noticeably positive effect on the battery life of our test machines.

If somebody offered to increase the battery life of your notebook from two hours to two and a half hours for just £25, you'd be interested. But if somebody offered to make the battery in your notebook look pretty by sticking a £25 sparkly plastic film on it, you probably wouldn't be. Based on our tests, Batterylife Activator appears to be the second of these two.