Moss Motors, Ltd

I have a 1971 B roadster, and I'd like to convert my dual 6-volts to a single modern 12-volt battery.  I have done my own bodywork and mechanical, but will admit electrical isn't my area of expertise (although, I'm more than willing to give this a try).

I understand the polarity of my "B" will probably be different than most modern auto batteries I pick up (right?)...so what does this conversion entail?  Perhaps new starter even?  Will the guages operate correctly after switching the polarity?

Also, any recommendations as to specific batteries that will fit in one of the B's (small) battery boxes?

Chris 

Older Bs are "positive earth" meaning they use the negative side of the system to power the "load" (all your electrical components) and they run the positive side to ground.  I believe the conversion point was in 67.  You're newer than 67 so you don't have to worry about that.  I may not know the exact point that it switched, but I can with confidence say that a 71 has negative earth (the right way to do things, even for Lucas)

I hear that a Group 26 battery fits, but I'm in the middle of my own 12V conversion and I made a cardboard mock-up battery representing the largest thing I can fit in the battery box.  Now all I have to do is take it to the store and find a battery that fits in my cardboard box.

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Phoenix | 72 MGB

12v Group 26 battery will fit.  71B is neg ground.  No changes required except neg bat cable.  Drop in the battery and go.

Hi Phoenix, Im a few miles away in Mburg. 

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Lee Miller
USA PA
1966 MGB
1967 MGB
1971 MGB
www.miller-garage.com

Ignorant question (I may have mentioned I'm no electrician..haha):

 How much of this can be accomplished using existing cables?

Ref Battery fit, The battery size can vary slightly due to lips etc. I do know that the Wallymart group 26 will drop nicely into aMmoss battery box and it does have a handle to make removal easier.

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Alfred, Maine
63B Waiting for resto
63B Donor
67B Ours
73BGT Mine
73BGT Hers

Almost all of this can be accomplished with existing cables.  The hard part is, at least.  Right now, you're set up like so:

You're converting to:

You're really just eliminating the connection between the batteries and thus, you may need a slightly longer ground cable to reach your existing ground spot.  No more than $5 at any local auto parts retailer.  The positive terminal to starter can and should be reused.  Good luck!

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Phoenix | 72 MGB

Thanks Phoenix...that's exactly what I needed! 

Woohoo! 

I might have just bought myself a few more weeks of driving!

Interesting point on year.  I resently purchased a '74 GT.   I has a 12v conversion with POISTIVE GROUND.   Haynes manual says and show 67 and newer 18V enginened cars with NEGATIVE grond.   Make me wonder what is going on!    Right now the battery is dead (it was running and well) and I need to at least charge the thing!!

 Jim in NM

I just converted back to twin 6v from the previous owners single 12v conversion. The largest 12v I can get to fit is 41Ah, but I can get twin 6v 70ah for the same price. As far as I'm concerned its a no brainer. Why pay the same money for 30 less Ah?

 I know the later cars have bigger battery bins, so size isn't so much of an issue.
 

Ok...never got this done last fall.  Instead I chose to repeatedly charge my failing 6-volts!  I think today would be a perfect day to get this done (65 degrees, sunny, and no wind).

Obviously, the Neg would be the best (and cheapest) to replace.  How long should I get and what gauge should I use? (I have the new battery, but the car's 38 miles away...tough to get a look at)

 Edited to add:

 I'm probably just going to mount the new ground/earth cable to whatever box I put the battery in.  If I'm changing the layout of the battery box/cables, I can drill an extra hole to mount the Ground.

When i bought my B I tow dollied it from Indianapolis home.  The first thing I did was replace the batteries with a 12volt so I could get it inspected & drive it.  As I remember, I used the existing cables since the ground was on that side.  Or perhaps I moved it??  Anyway, I know I didn't have to buy a cable.  I put it in the right side box without a liner.  Later, I got the plastic liner for the left side.  Makes a good place to put an emergency tool kit or an extra quart of oil ect.

As a side note, I remember going to an autocross once & the tech inspector wanted to see the battery hold down.  After I expalained where the battery was and assured him it was properly fastened, he said OK forget it. LOL

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72 MGB 80 TR8
Pittsburgh PA

I did the 12 volt conversion this weekend having used the diagram from Phoenix in an earlier question about this topic.  It could not have been easier, I used a Napa series 26 that fit perfectly in the battery holder of my 70 MGB.  With a 30 inch wire to attach to the original ground it took all of about 10 minutes.  Works great, glad to get rid of the 6 volts.

be sure to get a top post only , the dual post ones can short out against the frame work for the batt, its happened to me once...even with the little plastic caps on the unused posts