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MG MGB Technical - Battery advice (how many ah?)


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?

Is the Odyssey PC1200T, with 44 amps per hour enough?

Please advice
Eric van den Berg

I've always measured the space where the battery fits, determined the position of the (+) & (-) connections and bought the battery with the most power that fits in the space. In the US it's referred to as either cranking amps or cold cranking amps. The more the merrier. While batteries with minimal reserve power (which tend to cost less) will work just fine you'll get longer battery life from a battery with more reserve power. Now, should you forget and leave your leadlamps on, you'll appreciate the extra power. As far as the alternator/dynamo is concerned, it only has to return to the battery what was removed upon starting. It could care less if it's a large or small capacity battery.
tom (member)

See 'battery life'. Note that modern 12v batteries with about half the physical size of twin 6v have significantly less Ah when it comes to leaving lights on, even though they have a significantly better CA and CCA, i.e. you will flatten a modern battery sooner.
Paul Hunt

My Haynes manual shows 55-61 a-h rating for the 2-6v systems. Doesn't indicate what for single 12v. The CCA is important for cold starts obviously. If you have a generator or idle a lot with heavier loads (headlamps, fans, stereo, etc) then the a-h rating is more important as the battery is able to draw down further while keeping voltage reasonable.

The a-h is far more important if you have a generator since there was essentially no charging at idle or low speeds.

By the way the "maintenance free" batteries don't like deep discharges. Each deep discharge will reduce life significantly. I try to get batteries with caps for refilling.

I use a single 12v in my 1965 with generator. I usually only drive in warmer weather but it will start it at -5 deg C okay. I probably get 3-4 years on a battery depending upon if I charge it up during storage or not.

Robert McCoy

Batteries recommended for rubber bumper cars by *suppliers* in the UK are typically 68-70Ah and from 440 to 630CCA for similarly priced products. By contrast a 12v that fits in a 6v 'hole' is only 44Ah and 390 to 440CCA. Some American sources quote 475-575CCA.
Paul Hunt


in my 1973 GT i replaced the two 6 Volt batteries for two 12 Volt units that I bought from a Fiat dealer. They were OEM parts for a Fiat Punto and excellent Exide quality products that did not cost anything more than the cheap ones offered by car service stores. The result of this 'upgrade' was amazing!

In the 1975 Roadster i installed a 66Ah Ford Mondeo battery 5 years ago and it is still fine, although you need to change the terminals if you go the 'Ford way', but they give a better connection than the original clamps.


Something else I meant to say is that running batteries in parallel the 'best' one will always tend to drain down to the level of the 'worst' while not being charged, and this will become more marked as they age.
Paul Hunt

What Paul says is absolutely right and should allways be considered when using batteries in a parallel setup.
When you go this way, be sure to buy two batteries of the same production badge. If your dealer can only sell you one and has to order the other one for you, explane him that they aught to be the same production date due to your plans of installation.

Within more than 30 jears of boating i learned, there is no problem with this setup as long as the batteries are of equal size and age.


This thread was discussed between 30/12/2008 and 31/12/2008

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