General Battery Impact driver

HarryMann: These come in expensive and cheap varieties and work somewhat differently

The cheap ones work by revving up a spinning mass and then engaging a pawl, so it transfers its energy to the socket. The one below takes about 3 seconds to apply its impact

They do work though, and can be quite useful, especially for removing bolts where you can't get a large powerbar onto, or where you can't stop the thing turning e.g. a crankshaft pulley nut. A msall 12V battery is quite sufficient, I use a small light motorcycle battery

This is a JCB cheapie from a newspaper offer, about £20 I think...

The expensive ones are what a lot of mechanics use these days, instead of compressed air tools, being more protable, much like a battery drill, or winder, but built from the outset as Impact drivers - good ones are not cheap (£150 upwards), Hitachi would be a good make for instance...

Tip: High on the list of things not to leave lying a round for more than 5 seconds in public view - it'll dissapear