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MG Midget and Sprite Technical - Overcharging Problem...

Hi everyone,

I'd really appreciate some advice; I've just had my midget back on the road and now she's off again.
The car is positive earth:

- Have just replaced the battery and the voltage control box, checked/changed the voltage cut out screw to cut out at 12.5V.
- However, the battery is currently pushing out around 15.5V, even when disconnected from the charging circuit.
- Also have misfire...not sure if it's related.

Any ideas on how to stop the overcharging? Is my new battery already fried do you think?

Thanks a million in advance,
Josh
Josh Spooner

Switch to an alternator?
Onno K

Josh. Your standard car battery is not capable of what you are claiming--"the battery is currently pushing out around 15.5V, even when disconnected from the charging circuit." Are you using some form of special purpose battery? Normally, the common lead-acid and gel type automotive batteries will put out 12.8 to 13.0 volts when new and fully charged.

If your description is accurate--you are measuring 15.5 volts at the battery terminals with the leads disconnected, I suspect your volt meter is inaccurate. And, if you used an inaccurate volt meter to set up your control box (voltage regulator), it too would be operating out of factory specifications.

Les
Les Bengtson

Yes possibly Les may be correct and the very first test is your volt meter.

the best test of the volt meter is to leave the battery standing disconnected for an hour or 2 and using your voltmeter see what voltage you read across the terminals If your voltmeter reads above 13 volts then it is INACCURATE and as Les stated it will have also been inaccurate when setting up the cut out.

Is you battery fried???

No!!

When the charging circuit is operating on a motor vehicle the voltage from the dynamo/alternator has to be above that of the battery otherwise it will NOT charge!! The charge rate of the battery is dependent upon a couple of things one of which is the charging voltage. !5 volts is a bit on the high side of course but it is probably around the kind of charge when using a Boost charger. The battery generally survives this kind of charge rate but the Dynamo does not!! To high a charge voltage/rate overheats the dynamo and eventually melts the solder on the wining which results in total failure.

So dont worry at the moment but first check your meter for accuracy as described.

I would not expect this to cause a misfire?
Robert (Bob) Midget Turbo

Yes possibly Les may be correct and the very first test is your volt meter.

the best test of the volt meter is to leave the battery standing disconnected for an hour or 2 and using your voltmeter see what voltage you read across the terminals If your voltmeter reads above 13 volts then it is INACCURATE and as Les stated it will have also been inaccurate when setting up the cut out.

Is you battery fried???

No!!

When the charging circuit is operating on a motor vehicle the voltage from the dynamo/alternator has to be above that of the battery otherwise it will NOT charge!! The charge rate of the battery is dependent upon a couple of things one of which is the charging voltage. !5 volts is a bit on the high side of course but it is probably around the kind of charge when using a Boost charger. The battery generally survives this kind of charge rate but the Dynamo does not!! To high a charge voltage/rate overheats the dynamo and eventually melts the solder on the winding which results in total dynamo failure.

So dont worry at the moment but first check your meter for accuracy as described.

I would not expect this to cause a misfire?
Robert (Bob) Midget Turbo

Just reading through - logically if your meter is reading between 2 to 3 volts high (based on 15.5 really being 12-13 actual) then when you set your voltage regulator you are actually setting it at 10 or less. It doesn't make sense in that if you were set at 10 you would pretty quickly flatten your battery as it wouldn't charge...

Anyway the point of my comment was that this may well explain your misfire if you were running low on electrons as it were ?

New voltmeter - first step ?

I assume you weren't running around in the dark as you would probably notice the lights getting dim...
John Barber

This thread was discussed between 31/01/2012 and 01/02/2012

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