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  #1  
Old 11-13-2012, 06:49 PM
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Weird Volt Readings with Engine Off

So, I have been having strange discharge problems with my '75 w115 300d with manual glow system. I have been checking battery often as it will intermittently drop volts and not start -for example 11.7 this morning so could not glow/start. Charging system appears to be working but just checked volts again (with engine off) and was getting rising readings of 13.8 to 14.29??? Huh??? Is this an example of a bad alternator (diodes) feeding volts back to the battery? Weird! Whats up with this?

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Old 11-13-2012, 07:30 PM
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Sounds like a battery on it's last legs. If a battery has increased internal resistance, this is not unusual. If possible, try disconnecting the battery immediately after shut-down and see if you still see this "rise".
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Old 11-13-2012, 07:42 PM
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Originally Posted by SD Blue View Post
Sounds like a battery on it's last legs. If a battery has increased internal resistance, this is not unusual. If possible, try disconnecting the battery immediately after shut-down and see if you still see this "rise".
Interesting, so a dying battery could see fluctuations like this? I did add mag sulfate to this battery some time ago - weird!
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Old 11-13-2012, 07:56 PM
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Sometimes they develop internal resistance from corrosion and the "higher" voltage is caused by a difference in potential. Starting batteries are designed for amps.
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Old 11-13-2012, 08:00 PM
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Originally Posted by SD Blue View Post
Sometimes they develop internal resistance from corrosion and the "higher" voltage is caused by a difference in potential. Starting batteries are designed for amps.
Load test is ok, not great . . . .strange internal battery short perhaps?
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Old 11-13-2012, 08:07 PM
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I have a new group 49 battery - perhaps I should put this in and see what happens for a few days -
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Old 11-14-2012, 08:29 PM
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When a battery is charging in a running engine the spread.of voltage is 13.6 to 14.6 volts. Once the car is shut off it takes awhile to trickle down to 12.6 volts. This is where a healthy battery will stay. If you check the battery a few hours after shutoff, and it reads lower, you have a drain on it or it is on its way out. To check for a good battery , charge it full without it connected to the car and let it sit a few hours. If it reads 12.6 the battery is ok and you need to look for the draining culprit.
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Old 11-14-2012, 09:07 PM
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Internal resistance will only be able to be an issue if current is flowing. With engine off and battery disconnected, you should get a little more than 12VDC. Any lower voltage would indicate either a bad battery or a battery that needs service. Any higher voltage than 12.75 or so and I'd say you have a very strange battery or a broken meter.
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Old 11-14-2012, 10:50 PM
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Originally Posted by Codifex Maximus View Post
Any higher voltage than 12.75 or so and I'd say you have a very strange battery or a broken meter.
It is common to see 13+ volts dues to what is known as a surface charge. The issue can be eliminated by loading the battery for a few seconds before taking the voltage reading.
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Old 11-15-2012, 05:41 AM
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yeah, with zero load, and a recent charge say, from a long drive and instant volt check, odd things can show up on the meter. check again with the headlights on. that'll show the true volts at the battery. with the headlights on, less than 12v or so and the battery is not any good.
most surface charge should be 13.8v
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Old 11-15-2012, 12:20 PM
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Originally Posted by warmblood58 View Post
...for example 11.7 this morning so could not glow/start.
A reading less than 12.7 volts means the battery is not fully charged. 11.7 volts is close to the lower limit, or fully discharged.

Quote:
...Charging system appears to be working but just checked volts again (with engine off) and was getting rising readings of 13.8 to 14.29???
13.8 to 14.29 would be expected with the engine ON, not off. If you get 13.8 or so with the engine running, the battery voltage should taper off rather quickly to a steady 12.7 or so within minutes after you shut it off.

Quote:
...Is this an example of a bad alternator (diodes) feeding volts back to the battery?
Bad diodes prevent the alternator from charging the battery.

Try measuring both DC and AC voltage right at the battery terminals. There should only be a few, if any AC volts when the diodes are rectifying properly. Shorted diode(s) will let AC through, maybe 15-20 volts or so?
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Old 11-15-2012, 04:25 PM
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Point of reference on batterys. Just replaced my group 49 with a blem from Interstate battery distribution house. Was very happy to pay $40 for mine.
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Old 11-15-2012, 06:59 PM
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I would expect a shorted cell from your general description. Still would do not too bad on a load check right off the charger perhaps.

This situation may be working your alternator too hard remember if so. To test, charge battery for a reasonable time. Let sit for a few hours. If voltage is less than 12volts then to me a shorted cell is suspect.

Right off the charger you want to see about 12.75volts. This is just the reflexion of five cells overcharged or surface charge and will drop back substantially with one dead or shorted cell. All cells good will not drop more than .25 volt.
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Old 11-16-2012, 07:27 AM
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Quote:
Originally Posted by qwerty View Post
It is common to see 13+ volts dues to what is known as a surface charge. The issue can be eliminated by loading the battery for a few seconds before taking the voltage reading.
Learn something new every day.

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