I would be cautious about this, if you mean drawing starting current with a small capacity slave battery in parallel with the main battery. The current will be significant, and drawn from the higher voltage source. If its capacity is relatively small - a few ampere hours, its voltage will drop and the internal heat can warp or collapse the plates. Also have a mind to the capacity of the cables between the slave battery and the car battery for the same reason.

A

this is of course not true. It is like linking two buckets of water. The level in both buckets will eventually be the same. So you can of course charge a battery, but not fully, from another battery. You will even the charge on both.
However you can leave the battery connected via the cigar socket, and the car will charge this and the car's own battery fine.
It will in essence work as if you had two batteries installed in your car, which is indeed the case i some cars.

Unfortunately you fail to understand how batteries work...

Batteries are not buckets. They fill up very quickly with volts, then the current and charging stops. The difference between a flat battery unable to crank and engine and a fully charged one, can be as little as one volt. Two batteries in parallel will quickly equalise their voltages, with little actual current charge being transferred. The 12.7v of the small battery, will at most put maybe 2 to 3% back into the car battery; 13.8v will charge a battery to 90%; 14.5v will put a full charge in. So very different from two buckets.

If you read the reviews for these gadgets, most say they don't work. Those few who have had success, have had success because the main battery has almost enough power in them to start the engine and just need a fraction more. If your car battery is just lacking that 2 to 3%, then you are in luck.

If you need to insure yourself against a flat battery, then the only way is to carry a decent sized, fully charged spare battery and some jump leads with a good amount of copper in them. The 'good amount of copper' means that the starter motor can draw current from the charged battery, rather than the flat one.

Instead, I carry a pair of home made jump leads which have a reasonable amount of copper in them, but nowhere near being able to crank the car over. I would hope to be able to use them to be able to jump start mine, from a car with its engine running. Idea is that the other car would be putting both volts and current into my battery, to enable it to start. In 30 years, I have never needed them.

The tiny difference in voltage between the batteries, plus the resistance of the cables plus probably a fuse, will limit the current flow.

Like I said before, you're not supposed to start the car with this system attached. Nor does it restore the battery in your car to full power.

When the battery goes below a certain point, it won't have enough power to turn over the engine, making starting the car impossible. By connecting this device for a good 10 minutes, it gives the battery in the car just enough juice to start a few times. But you don't try to start on this device. You connect it, wait for 10 or 15 minutes, disconnect it and try to start the car.
If you're unlucky, and the car won't start after a few attempts, the battery in the car will be drained again and your still in trouble.

If something weird happens (like something drained the battery without your realizing it), this can be just enough to get it going again. But nothing more.

I was wondering because there is a system of switches, fuses, ECU's etc. between the battery of the car and the socket. That system was built on the idea that electricity (for example: ) goes from the battery to the ECU, then the BCU, then the fusebox and then the aux socket.
It might not be designed for any electricity 'flowing' in the reverse direction.

Two batteries of the same type and condition connected, will perfectly equalize their charges given time. They will behave exactly like two buckets of water connected with a pipe at the bottom.
It is physically impossible to have anything else.
Current will flow until the charge density is equal in both batteries, at which state the voltage will be the same.

No, they won't, they will only equalize their voltage. Once the voltage in each battery is the same no further current can flow ( a potential difference is required for current to flow, . For a typical lead acid battery the voltage remains constant from about 20% capacity to 80% capacity: so for example your flat car battery will come from 18% charge to 20% and your auxillary battery from 100% to 80%, and that's it. (take my figures with a pinch of salt, but you get the gist of it I hope).

There is a way of using a 12v battery to charge another much more than that, it needs a voltage step up circuit to increase the voltage from the charger. (that's one for the electronic engineers), I've no idea why none of these powerpacks or plug in 'emergency' chargers don't use such a system (or perhaps some do?).

The MG Rover wiring diagram shows two accessory sockets, labelled 'front' and 'rear'. I'm not sure what these are but they're both supplied by relays with the coils being energised by the car's ignition switch in the 'auxiliary' position. Neither the BCU nor any other ECUs appear to be involved. Do you have a different circuit Vinnie?

Simon

From what I've seen the later cars had the cigar lighters continuously on - probably another bit of cost cutting.

I don't know where you have got it that the voltage stays constant. It really doesn't. It stays fairly level, but by no means constant.

The difference will cause a current, and it will flow from higher to lower until the voltages are equal.

http://www.scubaengineer.com/documen...ing_graphs.pdf