I found that with full and correctly charged battery, this thing sucks. In the positive sense... it does pull up dust and, in its "dubl duty" sense, water as well. I can't believe how much this power this thing sucks, it's huge for the batteries it has. But as the batteries die down this becomes really terrible really quickly. The original pack is 3.6V and 1300mAh, and mine looks like it's made by Sanyo, and they're Sub-C cells. They say that the unit should last 7 or so minutes off battery power. At this rate of consumption, it's consuming 8.5C, or 40 watts. This is immense for a battery powered device - the cells need to be of very low internal impedence to do this. Granted this is very low compared to a AC powered vacuum cleaner, some consuming at least 120 watts for a handheld, and 300+ watts for full sized cleaners. You're not going to get that much power out of this thing compared to a full sized cleaner, obviously.
Actually taking measurements, it looks like startup is a whopping 20A and steady state is drawing around 6.5 A when unloaded. Probably more when it's actually in a case, this was tested out of case (but with impeller attached). I tried powering with a Li-ion 26650 3.5Ah cell with protection and it wanted nothing to do with this cleaner. Looks like NiCd wins this bout.

This vacuum cleaner uses three internal NiCd cells that are virtually permanent - they can't be replaced by user. However with a screwdriver you can replace... Removing the four screws that holds the two halves together and slicing through the labels on the bottom, you can open this thing. The two halves are not quite the same, and the "bottom" half where the power button switch is attatch will hold onto the motor and fan blades. A rubber seal prevents dust and water from entering the electrical chamber and likely will be the main thing that keeps the two halves from separating after the fou screws are removed. To remove the batteries you will need to open the device and then slide the motor unit out that holds the cells. Be careful not to break the tab on the top of the device as I did on mine when extracting the motor unit - you just need to pull it out enough to access the battery. I epoxied the tab back in place, without the tab, the disengage button's spring will have nothing to back against.

Looking at its guts, it is charged with an AC adaptor through an apparently cheap 1A 1N4001 diode. The battery is connected via small quick disconnect tabs - smaller than F1, but sizeable - they need to pass several amperes. You'll need to locate a three Sub-C NiCd cell pack with these tabs, as well as the fact that you need "tool" or "racing hobby" quality cells that can supply 10C load to be able to have proper suction power from the vacuum cleaner. If you opt for the 2200mAh upgrades make sure they support 10C discharge or they will die very fast.

That extra red wire that connects to a plate and hole on the bottom of the handle portion of the vacuum cleaner appears to be a motor bypass. Using the DC charging plug's barrel and this connector, you can power the motor externally. At around 10 amperes, that's a lot of current to pass through to power the motor.