Some backstory:

They offer an attachment that you put over a standard AA battery to extend its life by a claimed "up to 800%". The device is meant to achieve this by boosting the output voltage of the battery with a tiny step-up converter, so that it always provides a stable 1.5V, instead of dropping down as the battery is discharged. They claim that most devices stop working when the voltage drops below a certain threshold (1.35V, according to them) and by adding that step-up converter to the battery, the entire energy stored in the battery can be used, instead of that initial 1.5V - 1.35V part. They even provide a real research paper claiming that a lot of the batteries found in random recycling bins in Switzerland were in fact disposed of while still having significant charge (30% on average, with 10% of the batteries being practically new)

That's all fine and great, BUT the study was made in the year 2002. They are solving a problem that existed 13 years ago and is extinct since then. Most battery powered electronics - ESPECIALLY those they show in their promotional material now feature such step-up converters built into the device. In which case the "batteriser" will produce absolutely no benefit at all

For a detailed debunking, I recommend this episode of the EEV blog

^{wow that sounded like an ad but it's a very informative show :P}

What pains me most in their campaign is the blatant lying. At the bottom of the page they show this test with a wireless keyboard. The tested battery measures at 13%, and after installing the batteries the reading goes up to 100%. Very convincing, bravo. The problem is that the capacity of the battery is measured solely on the voltage that the battery provides. Boosting it up to 1.5V doesn't add any capacity to it, it just messes up the readout! This is literarily like pouring water to your gas tank just to point at the gauge and say "WOW a full tank of fuel for free!". It will NOT extend the actual life of the battery in that keyboard by any significant time. If anything it will decrease it, since that converter must have an efficiency of <100%. Sure there are some devices that will be helped by the bateriser. I can imagine TV remotes, simple LED lamps and such to show some improvement. But not most devices. Not by 800%. In fact, I will eat a questionably distasteful meal of your choice from my local Chinese place, reddit, if the bateriser improves the life of that wireless keyboard, measured in hours of typing.

/rant.

edit: fixed a link