Tis the time of year for roiuting out the weak batteries. For future reference, when a low battery starts causing starting issues I have found the following checks useful;
Note values are from memory so could be +/- 0.5 volt.
1. As mentioned before check the voltage at the battery with the engine running, should be about 14 volts. Anything test 2 result to 13.5 volts suspect wiring/earths (see tests 3 and 4).
2. With the engine off check the battery voltage under load. This means switch on head lights, main beam, heated screens, heated seats. The voltage will drop but should hold around 11-12 voltsish. If the voltage continues drop and goes south of 9.5 volts, chances are your battery is dead. Take it to a motor factors and they should be able to test it properly for you with a proper load probe. Before doing this check be prepared to not be able to start the car after, so be somewhere you can jump start.
3. Measure the voltage between the -ve battery terminal and chassis and -ve battery terminal and engine block. Should be less than 0.1 volts. Anything more start looking at battery terminal connections and earth points as described above.
4. Measure voltage between +ve battery terminal and alternator +ve, and +ve battery terminal and +ve in main fusebox. Should be less than 0.1 volts again, wiring and connections likely causes of any high readings.
As I say all from memory so happy to take a hit on the exact values. When I get chance I'll do the readings and update, unless anyone with a good battery beats me to it.
Another import thing to remember with starting issues is just because you have a new or recent battery do not rule out it could still be duff. Have seen this many times.
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31st October 2010, 04:35 PM
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