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MG MGA - Two 6V batteries to one 12V


Right have finally had enough of the two 6V batteries. 

There at the end of there life and now aren't taking a charge. The previous owner had them a number of years and replacements are simply too exorbitant and not really justifiable right now.

Planing to replace with a 12V that fits ~ Exide 063 te Premium Car Battery.

Have read the MGA guru pages and a few other sites but am looking for a total idiots step by step guide!

Am going for now (and ease) to place the new battery behind driver (rhd).

Is it a case of just connecting existing long wire to the positive terminal and the shorter (currently on the second battery, passenger side) to the negative.

And lose the connecting wires between the two current 6V batteries but keeping any earthing wires.

Am just wanting The Old Lady to start up without the help of the AA.

Will put in a cut off switch but that'll be my next project. So advice on that would be appreciated too.

Many thanks, Matthew. 
M Elliott

You are wise to change to a single 12v. Bob West has advised me that he will no longer supply 6v batteries as he has had so many returned in under 1 year under warranty because the quality is so poor. I replaced mine a year ago at 9 years old. Bob remarked that this is exceptional and normally the 6v do not last anywhere near that long nowadays. I bought a Varta B35 390 battery from him, though you will be able to get one from any one of many suppliers.

The Moss price for 6v batteries is around 60 EACH, expect to pay a similar amount for the Varta 390. The Varta 390 fits perfectly into the MGA battery cradle without modification to the cradle itself.

There are just two small modifications needed, firstly to the earth lead. This depends on whether you have a battery cut-out switch mounted on the back of the heel board. If you do not have a cut-out then it is an ideal time to fit one. If you do have one, you may get away without the need to modify. You presently have two batteries connected by a cable. In my case I was able to straighten this cable to reach the input terminal of the cut-out. It depends on the length of the cable. If you do not have a cut out and do not fit one, then you will need a longer cable to connect the 12v battery to earth.

The second modification is owing to the 12v battery being a little less tall that the 6v. This means that the clamp is not at the right angle to secure the battery. All that is needed is to lengthen the threaded section on the two clamp rods so that you can then put the clamp across the top of the battery and tighten it down securely. This is possible on the 12v because there are no cell screw stoppers to get in the way. See the photo of mine in place; the two red wires are for my CTEK battery conditioner, a worthwhile investment.

A bonus in fitting the 12v is that you then have a spare battery cradle into which you can fit a box to hold spares for touring holidays. The fitment of a 12v is, of course totally reversible for originality purposes.

Mike Ellsmore

The advice above on the varta battery did not come from Mike Ellsmore, it came from me, Stuart Mumby. Obviously some strange glitch in the system.
Mike Ellsmore

Stuart (and Mike!)

Many thanks for the advice. It all seemed a bit too simple so was just needing the reassurance that it was!

The previous owner had installed a cut off switch between the two current batteries. On a kind of spider (but only 4 legs) arrangement so I think Earthing cable and everything else are long enough to move about.

Will install a proper cut off switch on the heel board once The Old Lady fires up.

No need to adjust holding down clamp as I've none.  Will temporally secure battery with cable ties.  Looks like I've  another project to add to the ever growing list.

Looking at your image, my battery cables seem to be somewhat larger diameter ?

Many thanks, Matthew.
M Elliott

Hi All
The Varta fits perfectly.
RHD car, battery behind passenger seat.
Positive earth - so positive lead to cut off switch and from there to frame.

A.M. Kunz

That's my battery ordered.

After going to view possible wedding using wedding cars tonbridge venues on Saturday. I think I'll be allowed to get to work on The Old Lady Sunday and hopefully not fry myself in the process !

Next weekend all going well I'll hopefully put in a cut off switch and be getting her ready for the summer runs.

M Elliott

It sure would be nice to have these batteries available in the U.S.... The Odyssey PC925T-A single 12v will fit, and probably last a long time, but it sure is expensive! Will need a new (longer)ground cable and modified hold down as well.

The amazon picture is not correct in that the PC925T comes with Brass screw in SAE terminals.

Now if I can just ease a bit of money out of the budget...

Russ Carnes

That Odyssey battery is quite small. CCA is about equal to twin 6's when new, but storage capacity is less than half of the original specification for MGA. If you leave the lights on when parked it will go dead quicker. It would likely do better if used with an alternator instead of a generator.
Barney Gaylord

Many years ago I fitted single 12v batteries to both of my 1600's and also a small plug in the "ramp" behind the front seat so that I keep them on automatic chargers during their long periods of "storage". I made a pineboard box on the other side with a jack and wheel hammer in case I ever have a flat tyre. I have also fitted the largest 12v battery that would fit into my daughter's restored MGC GT (with the aforementioned plug). I top up occasionally with distilled water but the batteries are still OK after many years.
Barry Bahnisch

How can we get Varta etc to the USA?
Russ Carnes

I use Interstate Batteries here in the USA and hey fit perfectly in an MGA and my 100 Healey.

Cheers, David....
David Honness

David, which Interstate battery fits?

...100 Healey eh?...!

Russ Carnes

Battery arrived today and earth wire is too short to so need to get one ever so slightly longer.

I did however prop up the battery and connect it temporarily, fuel pump ticked into life and horn toot tooted.

I the closed my eyes and pulled on the start and nothing ! Tried again a spark from the battery

Am now extremely worried that Ive gone and done something silly, like accidently reversing the polarity and have fried something !!!!!!

Any suggestions of what I can do to check I've not hurt The Old Lady while I get a new wire made up so I don't do anything totally stupid ?
M Elliott

If you had a spark from the battery terminal, it's just a loose connector and no harm done. The starter motor is not polarity sensitive. If the fuel pump ticks it is okay too. The engine will still run even if the ignition coil is in wrong polarity, just makes spark jump the wrong direction across the spark plugs. Incorrect polarity could damage a radio, or a diode fuel pump, or any modern electronic accessory.
Barney Gaylord

Oh' my my, when I saw an answer from Barney I thought I was in big trouble and had lots of work and things to replace.

Thankfully it looks like it was just a problem with the temporary set up, phew and another phew!

Will update you all with progress. Tomorrow I will have a good look over The Old Lady to see if she's any morden electronics, pretty sure theres none but I have spotted something for another post...

Again, many many thanks Barney.
M Elliott

The Interstate Battery I have in my 100 Healey is a I-26 65 months 500 CCA battery. It is the same size that I had in my MGA's. The battery is a 12 volt. I stopped using the twin 6 volt batteries years ago.

Cheers, David
David Honness

Thank you, David. I'll check it out. The length of 8.00" looks like a tight fit. I'll go measure to see.
Russ Carnes

A battery size chart resides here:
I use a 35 battery it is larger than a 26 but will fit. Other specifications being equal a larger battery always performs better than a smaller one. A 51 resides in my coupe much smaller but was free out of a Honda being salvaged.
R J Brown

"....a larger battery always performs better than a smaller one".

Except for the additional weight.
Barney Gaylord

Hi anyone in the Bath area, looking for a battery, might check out Ebay item 150758193945.

Ross McIntosh

I've fitted an Odyssey 925 to my MGA. My brother ran the same battery on his 1971 Porsche 911S, so I thought if it can handle a 2.4L, it can handle an MGA. It fitted nicely into the existing battery tray, just had to modify the clamping arrangement.

I hope your conversion to 12V is going well.

all the best,

Grant :-)
G Hudson

Evening to everyone.

That's The Old Lady running with 12volt power, yey!

Overall the conversion went well. Most of the time was spent checking and re-checking which way round the original two 6V batteries were.

An unforeseen problem was the need to enlarge an eyelet on one of the replacement battery cables by a few millimetres. Lacking appropriate power tools I ended up working by hand. Was just like whittling wood, although somewhat slower and with a shower of golden metal glinting in the sun.

Anyway, after all that was done and battery wired up, she thankfully started with a punch. Way better than ever. So we promptly went out on a short drive before I spent the next two and a bit hours cleaning up the garage and adding another coat of polish while my better half continued to knit her socks.

Many thanks to everyone for the advice and support.

I will be back shortly with some more questions about a mystery fluid seepage. Shall first look over the service manual for identifying the parts involved and possible likely course.

Again, many thanks to all and hope to see some of you on runs this summer.

All the best, Matthew.
M Elliott


Thanks for the heads-up.

That's my battery sorted - just need to get the engine and 'box back in...and a few other little jobs!
Dave O'Neill2

I installed a Sears size 26, 60 month, 525 power level "Plus Start" on the passenger side. "Everyday low price of $49.98." I'll keep a trickle charger on it and do have a cut off switch installed already.

I'm curious to see how long it lasts and will be happy with anything in the five year range.

Best Regards,

Ted Persons

This thread was discussed between 28/02/2012 and 27/03/2012

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