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Guest Message by DevFuse

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Bad battery or bad alternator test

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30 replies to this topic

#26 uniberp



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Posted 28 September 2012 - 01:08 PM

It starts fine. It was NOT starting consistently with the remote starter. The volume knob on the radio and the cruise was working intermittantly. I think it was the alternator putting noise on the bus, (someone poiinted out this possibility above) combined with a weakened/partly discharged battery, made the electronics go wonky.

It remote-started and radio-cruise worked all week, so it was prolly either the alt or the grounds.

#27 forester2002s


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Posted 29 September 2012 - 01:03 PM

but I would suspect 300 amps for 15 seconds is not unusual

15 seconds?
If it takes that long to fire, then the car has other problems.

#28 edrach


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Posted 29 September 2012 - 05:22 PM

15 seconds?
If it takes that long to fire, then the car has other problems.

This was an example to demonstrate that there's quite a bit of energy taken out of the battery when starting; usually more than can be put back into the battery in a 15 minute drive. I picked numbers that were reasonable and that I could calculate in my head. Sorry.

#29 windrider354



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Posted 25 July 2015 - 11:27 AM

Hi Ed, well I would have agree with you there if the battery is severely discharged, so good call. I'm not sure what the peak to peak voltage would be but the average AC voltage level still should be pretty low.

Beware trying to measure AC voltage across the battery when charging.  My digital multimeter consistently gives a reading of about 30 volts AC!  Both with the engine running (alternator charging) and with my CTEK battery charger hooked up.  This is a bogus reading, which I confirmed by getting about the same reading with the engine off and no charger hooked up.  Apparently some meters don't like trying to measure AC voltage across a DC source like a car battery.

#30 Fairtax4me



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Posted 25 July 2015 - 05:05 PM

The meter has to be capable of blocking DC voltage while measuring AC.
Many commercial meters don't do this because there's no need to block DC in household or commercial wiring.

#31 Cougar


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Posted 25 July 2015 - 07:01 PM

Like FAItax4me stated, not all meters are designed the same way. When taking AC measurements you need to be aware of what the meter can do. It doesn't hurt the meter to have DC on the line when checking for AC voltage, it just means you have to be aware of that in the reading you get. To see if the meter blocks DC while in the AC voltage mode just set the meter for AC volts and then touch the probes to a battery. If the reading shows voltage then you know it doesn't block DC. You can block the DC component by placing a capacitor in series with one of the probe leads. A non-polarized 2 microfarad capacitor should work.  

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