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MG MGA - Battery/Charging

My 1600 is standard. i.e. positive earth, 2 x 6 volt batteries ( new two years ago ), normal type dynamo.
Never used at night and only in wet conditions when avoidable.
However this week I had to make a trip in pouring rain and very poor light.
I found that the wipers went like the clappers until I put the head lights on and then they slowed down a lot.
It wasnt dark enough to be able to see any changes in headlight brightness but I have found in the garage that the lights brighten up as soon as I rev the engine and the intelligent battery conditioner that I hook up to each time I garage the car, took several hours to come back up to fully charged.
Have I got a charging or dynamo or regulator problem and
would the battery have eventually gone flat on me or is this quite normal for a "standard" set up ?

David, the std MGA charging system is a bit anemic. At a low idle, it may not keep the battery charged. That is with no accessories running. The charging light is usually just beginning to glow under 98-900 rpm. The generator needs higher RPM's to do it's job. Now, turn on the lights, wipers, and add a defroster blower and a few more things and you are approaching the limits of the maximum output of the generator even under optimum conditions.

Even a fully charged battery will draw charge from a charger.

If you want to verify this, place a voltmeter at the battery terminals and go for a spin. If you start out with a fully charged battery, the battery should be at around 12.6-12.8 volts. Start the car withn idle around 100-1200 rpm, the battery voltage should then rise to say around 13.8-14.6 volts. As you turn on different devices, the voltage will drop a slight amount. Voltage will be RPM dependent also. Lower RPM (idle speed) will lower the charge from the generator. Monitor your voltmeter while driving. As the voltage varies while you drive, items such as lights an wipers will respond according to the voltage applied. the giher the voltage, the faster the wipers, blower motor and the brighter the lights. This is not abnormal to the MGA or any other vehicle with a generator.

As long as you do not drop below about 12.8 volts for a significant amount of time, the battery will remain fully charged, even if does take a bit of time on your battery conditioner to top it off.

Of course diagnosing a possible problem from 1,000's of miles away is not 100%,
C Schaefer

CS - thanks for your comments. Not sure whether I actually have a problem and as you say diagnosis of a possible problem from 1000's miles away is not the best.
Cheers anyway.

Control boxes in general, and especially replacement units of the past 10 years or so, have a habit of slowly drifting out of calibration, usually toward a slightly lower control voltage. After a couple years time the controlled generator output voltage may be too low, and the resulting generator output current too low to keep up with full system demands when all accessories are switched on at the same time. Night driving in foul weather is the most likely time for this issue to appear.

The solution is a standard adjustment of the regulator relay in the control box by the book to bring the generator output up to normal working voltage.
Barney Gaylord

This thread was discussed between 01/06/2008 and 03/06/2008

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