Question about 2004 Mini Cooper

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I have a 2004 mini cooper. About a month ago the alternator failed and I had it replaced. Everything was working ok until I went on trip for a month and left my mini at home. The battery went completely dead so I had it recharged and again everything seemed ok. Now today the idiot light came on so tested the voltage across the battery and it was, while runnnig and it was11-12 volts, shouldn't it be 13-14 volts if the alternator is working correctly? I can't imagine an alternator failing in one month and if you know where it is in a mini its about $700 to get someone to change it. Could it be a bad battery? Even if it is a bad battery shouldn't I see 14v across the battery while the car is running?

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Yes, a good alternator on a good battery should deliver between 14 and 14.5 volts with the engine running. The first thing you should do is to check the contacts on the battery - make sure they are not oxidized, clean then thoroughly inside and out with wire wool.
If the voltage remains the same then you must test the battery - take it out of the car and put it on a charger ; if after 6 hours, without the charger connected, the voltage is below 11 volts then the battery (or more precisely one of its cells) has failed, you will need a new one.
Finally borrow a battery from someone - even an old one will do as long it has 12 volts unconnected) - and put it in and measure the voltage again - if it's below 13.8 then you will need a new alternator and to find out what is causing them to fail so fast. Most likely it's a bad connection that overstresses the regulator .

Posted on Oct 16, 2010


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