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MG MGB Technical - battery life

79 mgb
just got off the archives. did not find the answer.

it's winter and i am about to get a new battery, need to use my battery charger to start.
the current battery is a few months over 6 years old.

so, is there a consensus as to how long a battery is expected to last?

just curious
john sutter

I am a firm believer in Interstate batteries. My last set of 6 volt batteries lasted 13 years! The set before that lasted only 8 years. RAY '67 Tourer

6 years is not a bad life time, unless it is an Interstate battery, which as Ray states can go for twice that and more. If it is an Excide battery, 6 years is really pushing it. Cheers - Dave
David DuBois

John. In Arizona, three years out of a battery is considered about average. Interstate batteries last about a year longer. The Optima gel batteries seem to hold up quite well in our extreme conditions with six to seven years being common. We are using them in all of our rubber bumper cars with the oldest being three years old now. Also using them in two trucks.

For MG use, the Optima is excellent because you do not leak acid/acrid fumes into the battery box area. I would like to see how long the three batteries will last before making a firm decision on them. But, they currently have lasted longer than any battery other than the Interstate brand.

Les Bengtson

I'd expect to get six years at least, I'm still on my 2nd set of twin 6v in 18 years. I did have a spate of battery failures within the guarantee period when I stopped using the V8 every day, but that was down to the alarm system draining the battery over 2 or 3 weeks. Since fitting a battery cut-off switch (always a good idea for security and to protect the yards of unfused wiring) and using it when garaged I've had no more problems.

I've been reading up about batteries. Apparently gel types aren't recommended for automotive applications as they have a lower cranking capacity for a given size, and can be overcharged much more easily than lead-acid or calcium. Their main use seems to be for constant float in a sealed environment e.g. burglar alarm backup, or charge/discharge e.g. golf carts as they can take multiple heavy discharge without damage. Conventional chargers shouldn't be used, unless they have a special low-output switch position.

Advanced Glass Mat is a newer alternative, but they have the opposite problem in that they have to be charged at a *higher* rate than lead-acid, which may mean problems in MGBs with standard alternators and particularly dynamos. Superb cranking ability, but at 5 times the price of lead-acid are hardly suitable for conventional automobile use. They also seem to be used for charge/discharge e.g. boat 'house' loads.

I've also read that both gel and AGM are sensitive to high ripple currents and voltage when being charged. Given the very high and peaky voltages that come out of an alternator, which are only smoothed to a usable level by the battery absorbing the peaks and troughs, it seems to me they are both unsuited to conventional automotive use.

Even nominally sealed i.e. you can't inspect the electrolyte or top it up lead-acid and calcium still gas under charging and discharging, albeit less than unsealed types. They have vents which should be conencted to a tube going to outside if fitted in the passenger compartment or boots of modern cars. This is why I always argue that those plastic 'battery bins' should never be used for batteries, but only for the empty space if you have changed from twin 6v to single 12v.
Paul Hunt

I have had service lives between 2 years and 11+ years, generally you seem to get what you pay for. The current set are the ones tha have been on for 11 years so far. Next time I'm up there I will read the brand name. This set have had help from the intelligent charger of course.
The 2 year life caused me a lot of grief as I couldnt believe they had died so soon and I checked everything else first!
Stan Best

I seem to have to change one every 3years, but I don't change them both at the same time. The failure mode seems to be that an individual cell goes down ie fails to charge. They are over 50 each now. The plates in the last one I bought did look heavier duty, lets see how it lasts. Apparently there is only one firm in the UK making the 6v type now, but I can't remember what they were called.
Paul Hollingworth

I think that most batteries now days carry at least a 2 year guarantee. I have found these are good for 3-5 years. The three year guarantee ones 4-6 years.
I did not replace the pair of 6 volts because of the price. I was quoted between 75 and 120 for a pair.
I fitted a 12 volt (063) 2 years ago and it has been fine, cost was 20
Trevor Harvey

Both my Roadster and GT are fitted with 063 12v batteries and both are at least four years old so seem to be doing reasonably well. Four years is about the life that I would expect from one of these.
Iain MacKintosh

The battery in my 79 is the same one that came with the car when I bought it 7-1/2 years ago. It's an Exide. I'm sure it ready for a dive before too much longer.


79 mgb

I replaced the Diehard from Sears in my GT about two years ago which had been in that car since the late 90s. Maybe not exceptional, but perhaps what it really reflects is the condition of the charging circuit, more than the battery's innate wonderfulness?????
Bob Muenchausen

I had a Auto-Zone battery last ove 10 years in the 74 B I used to own.

Clifton Gordon

Ray,what did they cost you?Where did you get them at?
rich osterhout

Rich, They cost $125 for the pair. I purchased mine at the local bike shop where I was the mechanic for 6 years. You should be able to get them from any major auto chain. They can special order them. RAY

Bob,you have a 12 volt?
rich osterhout

I have owned two MGBs ('79 and '81) for 28 and 22 years respectively and battery life has been:

Original Unipart batteries: average 7 years
Lucas 089 (4 off): average 7.5 years
Halfords 072: first one lasted 4 years, second one 2.5 years (replaced under the 3-year guarantee), third one still OK after 5.5 years

Brian Shaw

A 12V? yes. I use just a single 12V in my 68 and have used 3 Diehards in it since the late 80s.

As I have said before, I don't know that they are the very best choice (brandwise) out there, but they have at least provided something more than ordinary service for the money.

Over the years in looking at my own experiences with several brands of batteries in various vehicles and those of others, what has become apparent to me is that a good charging circuit (good working order, good connections, good regulation of the generator/alternator, decent routine maintenance) appears to have quite a bit to do with the longevity of any brand of battery used. If any of these factors falls short, then it appears to me to have a direct connection to the life of any battery as well as the the battery's innate superiority due to materials, construction or other engineering/manufacturing considerations.

This doesn't mean that you will get a silk purse out of a sow's ear, but it does mean that you will at least get decent service out of whatever brand and price range you choose, and at times, better than you might have expected or even Consumer's Reports would have you believe. JMO
Bob Muenchausen


My battery just gave up, it's freezing here. What are the specifications for a new battery. A minimum of 330 CCA, but what about the ah (20), I have read 60ah. For the room I have for the battery (MGB 1974) only small sizes 12V will fit (max 20 cm?). With that size 45 to 50 ah (20) looks to be the maximum. Is that enough?

Please advice
Eric van den Berg

The Workshop Manual quotes 51Ah at the 10 hour rate and 58 at the 20 hour rate. But that is pretty irrelevant as the main use for the battery is for starting (unless you frequently park with lights on), and these days CA and CCA are more often used. I've seen 360CCA quoted for the original twin 6v so if correct this should be the absolute minimum for any replacement single 12v, and it would only be worth switching to that if the price of a pair of 6v are out of your range. My 73 roadster came to me with a single 12v, and when it came time to replace that I discovered the cradle had been enlarged to take a bigger version - too big to lift out of the hole. At first I thought they must have welded it in, but then I realised it was a sealed type, could be tipped over on end inside the hole, and lifted out that way.

Americans often quote group 26, which is sized at 208l x 173w x 197h here I've seen 475-575CCA quoted for these, which would be fine of course. I don't know how those group numbers relate to batteries in the UK but lists a Varta 544 402 044 as just about fitting in those dimensions with 44Ah at the 20 hour rate (quite a bit less than the twin 6v) and 440CCA which is quite a bit more. Given two 6 volt are just about twice the physical size of a 12v I'm not surprised at the lower Ah, the more modern technology *possibly* giving a higher CCA even with the smaller size. I say 'possibly' because whilst I can except that the original 'tar top' batteries (if you can still get them) may well be constructed exactly as they were originally I have seen at least two more modern designs and there is no reason why they could not contain modern technology. Indeed there is probably no reason why the original tar-tops could not contain original technology, unless concourse examination extends to Ah and CCA. Note that while the higher CCA will give better cranking the lower Ah will give a lower duration if you have to park with your lights on, for example, maybe only half as long.

The above site gives a link for 6v batteries which lists number 421 which looks much like the MGB 6v, at 170h x 175w x 225h. That has 57Ah i.e. virtually the same as the MGB at the 20 hour rate, but only 250CCA. Two in series will only give double the voltage of course, you will still only have 250CCA. If that is the CCA of the MGB 6v battery, and not 360, then a 12v of at least 250CCA should give the same starting performance, but even more of a limitation on parking with lights on as it is likely to be about half the Ah of the original 6v.
Paul Hunt

This thread was discussed between 08/12/2008 and 30/12/2008

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