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MG MGB Technical - 12V Battery Conversion


Can someone briefly explain (or direct me to a site which does) how to convert my 1971 mgb roadster to a single 12v battery.

Many thanks

Alex Davies


The principle is very straightforward, at the moment you have two 6volt batteries that are joined together by an interconnecting cable that effectively makes a 12volt battery. All you have to do is remove these batteries and replace it with one 12volt battery. My suggestion would be to put the single battery behind the driver so you can use the existing positive (+) battery cable to go to the new battery and either move the existing earth (-) cable to the other side or buy a new one. The trick is to get a 12V battery that fits the existing battery tray. If you have the old lead battery terminals on your existing battery cables you may wish to replace them with the much better clamp type. This would also be a good time to fit a battery isolator switch.
Bob Davis

I dug all this out of the archive when a V8 here was being converted. Yuo can even get the case sizes if you check the archive.
Stan Best

Apparently the 202 is a very good fit and this Varta with a 4-year guarantee and 390CCA looks like good value.

I'm planning to buy one soon...and it's only in Cannock!
Dave O'Neill 2

On my 1974 V8 I have fitted a single Unipart GBA4063 or a RAC 100 these are both 063 batteries but higher output. Both available from Unipart (Partco ) branches throughout the UK.
All you need to do is cut off the clamping ribs at the bottom of the battery and then it fits in original right hand side 6v battery box with no need to modify the hole. The only mod needed is the right angled retainer in the bottom front of the battery tray, this needs to be cut out and replaced further forward to allow for the larger battery.
The 063 battery is the cheaper option than many others that have been suggested in several other threads. I intend to put another battery in the left hand battery box in the near future as the V8 needs more amps when the engine is hot the 1800 version would only need one.
a goldup

You also need to move the earth cable from the left battery hole to the right one of course. You should also arrange a battery clamp or tie-down, bouncing around is not good for the battery, and it could short out on the underside of the cover. I've read of people putting a foam block on top of the battery, but this can get damp and trickle-discharge the battery, a suitably-sized wood block away from the terminals should be the absolute minimum.

Be aware that cutting the ribs off the battery could invalidate its guarantee.

A V8 (or any other engine) should require *less* amps when hot, and there should in fact be *more* amps available anyway when hot as the engine is easier to turn. If it's not cranking as fast when hot as cold then there is something wrong with battery, starter, or connections.
PaulH Solihull

And don't forget, due to the tight fit, your new 12V should have a lifting strap.
John Zajac

Alex, I have carried out a 12v conversion very recently on my 69B Roadster. It was the simplest thing to do. I also fitted a battery isolator while I was at it. The battery I used was a 063 as previously mentioned in another post. The footprint was just right for my MGB and it didn’t require cutting the ribs off. I was advised to use and 063 battery by the MGOC Technical advice line. They also ran through the procedure with me. It may require you to remove the tray rubbers and replace with a slightly thinner rubber. These rubbers will probably be perished anyway. As PaulH mentioned the battery needs to be clamped down its not looked on favourably at an MOT if it’s not.(7” Clamp eBay £5.00) I also got reassurance on how to complete the conversion from Rick Astley’s MGB Electrical Systems manual. I sold my two old 6v batteries on eBay for £40.00 and the new 12v 063 battery was £40.00. I love it when that happens! See attached photo. Good luck

Dave MGB

"I sold my two old 6v batteries on eBay for £40.00 and the new 12v 063 battery was £40.00. I love it when that happens! "

Wish I'd thought of that! Mine just went to the recycling centre/tip.

Best thing I've ever done on my 72 BTW (apart from getting it in the first place!) You'll make a small weight saving too!

I never seem to have any trouble with the twin 6 volt batteries in my '67. The last pair were 13 years old before one cell failed. The set, before that, lasted only 8 years. RAY
rjm RAY

Ditto, I'm still on my second set in 22 years.
PaulH Solihull

Paul, We must be doing something right, or we're just plain lucky. RAY
rjm RAY

If you are converting it and doing work there anyway you might want to also add a battery disconnect switch. Like this kind of thing:

It is very handy when you want to work on the car since you can totally cut off the battery easily. It also works well as a anti-theft device.

If you have a radio or alarm that needs power all the time you can use a smaller wire (with a fuse) to power them directly off the battery. Or else put a low value fuse across the cut off switch terminals. If someone tries to start the car (or anything else that pulls too much current) the fuse will blow.

I fitted mine to the heel board behind the drivers seat so I can easily reach around to pull it out when I park the car.
Simon Jansen

Ditto. Cut-off switches also prevent the battery draining - significant with modern alarm systems, something which caused the single 12v battery on the V8 to fail three times less than 2 years old when I stopped using it everyday. Also guarding against problems in the electrics, I've known of two case where vehicles parked in gear started cranking themselves along while unattended. There are quite a lot unfused always powered circuits in the MGB, a short on any of those could total the car, if not the garage and the house it is attached to. A bypass fuse won't stop the drains, but it will stop the other effects. I got fed up keep putting the clock right so fed that direct off the battery with its own in-line fuse. There's always a bit of discussion about which 'leg' to put the switch in, electrically it makes no difference, I opted to put mine in the 12v cable which runs down the heelboard on the drivers side, although that does mean cutting it and fitting new lugs. It's a bit easier in the MGB earth cable though as you can disconnect the existing one from the body and swing it round to the switch, then add a new one from switch to body. It would be more of a fiddle keeping clock or radio powered though.
PaulH Solihull

I keep my batteries, on the B as well as my motorcycle, hooked up to a Battery Tender whenever the vehicles are not being used. This maintains the batteries at full charge and shuts itself off when they reach 12 volts. RAY
rjm RAY

Ray, I do the same. Unfortunately I have the socket for that wired in AFTER my cut out switch so I need to leave that in place when the battery tender is connected. I need to change that. I have remote operated door locks so they are wired to be always connected (through a fuse) so I can hook into that wire. I also have a little solar panel for those times the car isn't at home (weekends away and so on).
Simon Jansen

Never used a battery tender, still starts with only a slight loss of cranking speed after 2 or 3 months. However when I do use the car, it very rarely runs for less than an hour, and frequently for several hours.
PaulH Solihull

I took my B out yesterday, after a hibernation of 3 months, and it fired right up. The beauty of using a Battery Tender is that you are starting off, each time, with fully charged batteries. I put over a 125 miles on the car going down, and then back up, the winding roads of California's Route 1. It was the last day where the weather is supposed to be good, before the rains return. The road, that I was driving on, was under 3 feet of water only a month ago. RAY
rjm RAY

This thread was discussed between 27/12/2010 and 28/01/2011

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