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MG Midget and Sprite Technical - Alternator replacing dynamo: additional power wire

Apologies but I have just spent an hour trolling through the Archive and decided it's easier to ask the question directly! Why aren't threads more usefully titled?

I have found some good details on modifying my control box prior to fitting a 55amp alternator. However, I see there is reference to an additional charging wire from the second connector on the alternator back up to the battery. As the alternator is chucking out potentially 55 amps compared to 20 or so from the dynamo, the existing cabling presumably needs help to carry the extra load.

So my question is about sizing that extra wire. All references just say "thick" wire. The absolute safe bet would be a 55amp wire presumably, but I guess some of the load would be shared with the existing wiring so it could be reduced? Also, if I don't have the requirement for 55amp around the car, will the alternator ever have to produce that current in the first place?

My issue with the 55amp cable is that is will be fairly "unpliable" and dificult to manipulate. Could use two half size I suppose.
G Williams (Graeme)

Graeme, read this.

"Also, if I don't have the requirement for 55amp around the car, will the alternator ever have to produce that current in the first place?" Nope, it won't.

2012 technical archive, "another alternator wiring thread".
Lawrence Slater

Graeme, if you follow this link you will see a chart of useful wiring sizes and applicatons.

I used two lengths of 44/.03 cable (in a subtle brown hue suitable for charging systems) to run from the alternator plug to the solenoid live side.

Not to the actual battery terminal though, generally the battery terminal has enough work to do without additionals.

I used thinner brown with a blue stripe for the ignition w/l feed from the alternator.



Assuming all other things are equal, ie the cable terminations are good and the connectors are clean, the current will be shared between the 2 cables in direct proportion to cross sectional area of the cables. Eg if you had a 2.5mm squared cable in parallel with a 10mm squared cable, the thin cable would carry 1/5 of the current and the thick cable 4/5.

Regarding your question do you need this if you donít have a requirement for this much power around the car, the answer is yes. Although the total load of headlights heater etc may not add up to 55A, if the battery charge is low itíll draw the extra power available to charge it.

R.A Davis

Bob: so are you saying that if I have 55 amps available from the alternator and I am using say 5 amps with the car running, 50amps could be used in topping up the battery? Isn't that going to fry the battery unless the control gear in the alternator holds it back?
G Williams (Graeme)

Bill: in a table I looked at earlier either the table (or me!) confused the measurements of cross section against diameter. 44/03 I would imagine is reasonably flexible. The table you link to is much more help... thank you.
G Williams (Graeme)

these might help -

Alternator 1

Alternator 2

Battery 1

Battery 2

there are also videos on dynamos too
Nigel Atkins

Heck with the uprated amp ....I want to see that mega powerhouse sound system and the million candle watt head lamps you have installed....

That is the reason you have gone to the mega amps altenator Im assuming ???

Prop and the Blackhole Midget


I'm not saying the alternator would be charging the battery at 50A, although for a short period this wouldn't fry the battery, what I'm saying is, let's assume it's a dark rainy night and you car has been standing for a couple of weeks without being used. The headlights at 55Watts each will be consuming approx 10A, the sidelights 4 X 5watts approx 1.5A, heater approx 5A, wipers 4A ignition 4A, dash lights 4 x 3Watts 1A, brake lights (when used) 2 x 21Watts 3.5A, radio (if fitted) 2A, this gives a grand total of 31A. This leaves 24A to charge the half charged battery that's been standing for a couple of weeks. It wouldn't charge at this rate for very long, let's assume 5 or 10 minutes and as the battery voltage comes up the charge rate will reduce.

Murphy's Law says that if it's going to fail it'll be on this dark rainy night miles from anywhere when your mobile phone is flat. Personally I'd install the extra cable in an attempt not to become a Murphy's Law statistic.

R.A Davis

Bob: you have appealed to my sensitive side! Confirmed my worries. Thicker cable it is!
BTW, I don't go out in the rain, let alone in the dark. But I know, delays... weather turns bad.... motorway hold ups.... night draws in.... and the fog!

Prop: Have an 8 speaker system with the boot (trunk!) turned over to twin power amps. Let's face it. with the roof off and 140 bhp powering me down the AutoRoute to the Med this summer I will need that electrical oomph. The aircon really hammers it too... particularly with the roof off. It has a habit of freezing the inside of the windscreen and of course the electrical heating in the glass takes 20A on it's own!
G Williams (Graeme)

Cunning plan:

Me to Mrs: "Let's go to Brighton tomorrow!"
(Thinks: "..... and pass the mg spares store on the way. THey have an alternator in stock!")
G Williams (Graeme)

I won't say hello if I see you in Brighton then Graeme. ;) lol.
Lawrence Slater

This is the back of my alternator. The group of three spade connectors I understand but what is the fourth for? This is the one upper left by the phillips screw.
I thought it may have been earth but it has a high resistance back to the body and also back to any of the other three.

G Williams (Graeme)

there are videos that tell you such

I would tell you but I can't remember and I'd have to watch the video to find out so you might as well to save me telling you wrong or make it unclear

links in previous post
Nigel Atkins

Looked at the videos Nigel. I must be missing something because despite a detailed explanation of half a dozen style of back cover, he doesn't say what connects where.
I don't want to guess at it, could be damaging!
G Williams (Graeme)

Some Bosch/Lucas alternators have a small tapping that lets 7v pass through to the heated screens on small Ford motor cars, which looks quite like that one if my memory serves (no guarantees, I don't stay current any more. You probably noticed my 'soddit' humour there)

You don't need to worry about any except the two big, one small at the main plug hole

You want two power outs one power in as in there

So, the other one? Ignore it.


in my memory he does detailed drawings and mentions the fourth connection, it might be across videos and not just in one

perhaps I'm wrong and it's in another video not linked here but I thought I'd seen it recently and those vids are recent
Nigel Atkins


As has been said, ignore that terminal next to the screw, it's probably a tapping for driving a tacho on a diesel.

All you need to do is connect up the three other terminals correctly and it should work fine.

SR Smith 1

this drawing shows all you need to know

not sure whose it is, might be Bob's, but as it shows you use the two large terminals to power the battery and the small terminal next to them in the socket to feed the ignition light circuit via the ignition switch

ALL OTHER TERMINALS are unnecessary, so you can ignore them

I wouldnt advise doing any thing to any other terminal on the back

Both Bosch and Lucas used the standardised terminal sockets as did Motorcraft for Ford cars

These are the best ones to use as the fittings are pretty well standardised too

Yours will be fine on the three terminals in the socket


Thanks everyone for assistance. I'm going to ignore the "spare" connector. I have some 27amp wire which I will run in parallel to the existing power supply. If that cable is rated at say 20amp I won't be far short of the 55amp rating for the alternator. If I can get a second 27amp run in (fitting it into the spade-end connector might be an issue)it will be over-egging it but perhaps not a bad idea.
G Williams (Graeme)


Glad you're running in another wire. I'd run it straight to the battery wire on the stater solenoid if I were you, only a short run as well on an A-series car.
SR Smith 1


Definitely run a second wire from the alternator to the starter solenoid. Ideally a 65/0.3mm size (35A). It will go into Lucar 35A push-on terminal and give you the total capacity that you need.

Richard Wale

All done! Thanks for advice.
The PO-1 had connected the dynamo up the wrong way round, it turned out. Instead of using different size spade ends on the wires up to the RB340 he had matched them to the dynamo end. Fortunately the heavy wire was taking the output but is was connected to the ignition light connection as that was the one calling for the 3/8th connector. I wonder whether it ever charged properly or the fact that I connect up to a battery maintenance/conditioner kept it going.

G Williams (Graeme)

This thread was discussed between 24/05/2013 and 01/06/2013

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