I'm thinking of using this on another project and was wondering if any of you bright sparks (pun intended) know what it actually does.

I'm looking at building a lithium battery pack to give 12 volts. The problem I have, is the batteries are 3.7v and around 4.1 when fully charged. If I use 4 in series I'm up over 16v which sends my inverter into over voltage.

Very difficult to tell from the photo what you actually have there, but given the number of wires it's not just a simple voltage stabiliser. It could be a DC to DC converter with multi voltage outputs.

Most standard voltage stabilisers would have only 3 connections, positive input, common negative, and positive output.

A quick Google shows you can buy a 12V voltage stabiliser for between £10 to £25 depending on what current you need. You would be wise to build your battery pack with a fully charged voltage of 20V if you would like to get the most output. At only 16V a voltage regulator will be unable to supply 12V once the batteries discharge to around 13V.

The part on the picture is a Bosch voltage stabilizer for Start/Stop systems.

It is basically a variable voltage --> fixed voltage DC-DC converter. The purpose of the voltage stabilizer is to prevent voltage fluctuations during operation of the starter motor which for example cause electronic modules to reboot, lights to flicker or display interruptions.

The voltage stabilizer has the following characteristics:

Input Voltage 7 to 18V.
Output Voltage 12V (±0.5 V).
Maximum output power depends on the exact version of the voltage stabilizer but is usually between 200W and 350W

Many thanks for your answers @unofix and @JW1982

The reason I was thinking of using this is it looks a pretty robust piece of kit compared to the amazon ones and I'm pretty sure my local scrappy will only want a fiver for it. 

Might give it a go anyway and report back. Cheers again.