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MG MGB Technical - 6v to 12v conversion

My 74 died on me over the weekend. I suspect that it maybe an issue with the batteries. When the car broke down on the road I took a quick look at the batteries.I had never seen anything like it! What a mess. So this maybe the time to convert to a single 12V system. From what I have seen, it looks pretty simple, but would appreciate if someone could post a step by step process to do this. Do I need to run new cables etc? from where to where etc.


GG Ginty

Hello Gerry,

1- The most difficult part is to find a 12V battery that will fit in the box. I decided to use right box for weight distribution purpose. Look at the archives for the right battery model. Be cautious when you fit the battery as fuel pump is partly in that box

2-Simplest connecting procedure: connnect together 2 battery terminals in left battery box, now unused. This will feed your negative wire in the right box and you could return easily to original set-up.

Jean Guy Catford

I definitely wouldn't advise 2. Get rid of the extra connection and avoid anything flapping about and shorting, simply move the ground cable that is the left box to the right box. You also need to clmap down the single battery somehow, you don't want it bouncing around, which will shorten its life and could short out on the lid.
Paul Hunt


Paul advice on securing battery is a good idea. For his comment on #2, you do not risk nothing as only slight possibility ground could bounce. A negative ground does not need to be isolated. No problem of shorting as many cars have unprotected ground cable. My #2 suggestion is a way to avoid replacing cable and worse drilling hole for new negative cable
Jean Guy Catford

Used right side. Put a ground wire hole in the upper edge of the battery compartment. Bought a commercial plastic battery tiedown holder. Drilled two holes in the lower battery bracket to attach the metal rods for the tiedown.

Put one of the plastic storage compartments in the left side.

Should not take more than an hour or so.

Check out the Westco Miata Battery. It's the smallest 12v available, and will fit the
one 6v compartment.

Bernie Lowe

This thread was discussed between 30/03/2009 and 31/03/2009

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