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Old 10-08-2014, 12:53 PM   #1
rwlc5
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Default battery voltage reading

What should the voltage readings be at the battery with the engine off and then with the engine running as tested with a volt meter?
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Old 10-08-2014, 03:41 PM   #2
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What should the voltage readings be at the battery with the engine off and then with the engine running as tested with a volt meter?
Ten different people will likely give you 10 different answers and that's because "It Depends". And those are not adult diaper Depends either.

My car has a battery that was new 3 months ago. I just measured the voltages you asked about on my car. With the key OFF the voltage measures 12.91. With the engine running it measured 14.53 at the battery and 14.57 at the alternator immediately after starting the car.. Battery lives on a battery tender when the car is not being driven and the battery voltage when the tender is removed is typically 13.15 or so but a hour after disconnecting the tender it is about 12.9 volts. Same after 24 hours if the car is not started.. If I start the car let it run a minute or so and then shut it off the voltage measured 12.83.

Condition of the battery along with the load being experienced at the time of measurement (Under hood light, interior lights, BCM draw, recent loads such as start and shut off, etc) will cause the voltage you measure to vary somewhat. With the engine running, again the condition of the battery and load on the battery (accessories you have running etc) along with the condition of the alternator will cause the voltage you measure to vary somewhat.

A good battery at full charge typically measures 12.7 to 12.9 volts or more with the key OFF.

Sorry for the long answer but as I said it depends.

Will be interesting for others to measure and post their results.
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Old 10-09-2014, 01:12 AM   #3
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Well said - "it depends"
A lead acid battery has 6 cells, each with a nominal voltage of 2.1 VDC. That leads one to believe that an unloaded (disconnected) fully charged battery should read 12.6 VDC. Not always the case depending on the individual battery and it's state of charge. As Mr. 8Vette7 said, when you connect any load to the battery, i.e. under hood light, cabin lights, or especially when you turn on your headlights you will see a significant voltage drop. Read the voltage when you are engaging the starter - it is likely to drop to 9 or 10 VDC.

With the car running, the battery voltage reads higher, because the alternator output is set to about 14.5VDC. So you are really measuring the voltage that the regulator inside the alternator is set to. The voltage at the alternator may be slightly higher due to a voltage drop along the length of the cable. When charging, the voltage is set higher than the nominal battery voltage to ensure the battery charges. If your regulator is set too low your battery may never fully charge. Again turn on your headlights with the car running and you may see a voltage drop. Rev your engine and watch it rise.

Batteries start to fail when they are no longer able to deep cycle charge - become severely discharged and lose the ability to be recharged to provide enough amperage to run your system, i.e. work under load. They may read 12+VDC - but measure it when your starter is engaged. If it drops below 9VDC then it is likely on it's last legs.
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Old 10-10-2014, 03:24 AM   #4
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Off, my battery reads 12.9. (Optima red bought last year). On dash it reads 14+ but I have read that reading might not be very accurate.
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Old 10-10-2014, 04:28 AM   #5
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Off, my battery reads 12.9. (Optima red bought last year). On dash it reads 14+ but I have read that reading might not be very accurate.
Guess I am unclear what you are trying to say. Is OFF relative to the engine but the key still on? ie engine off but key in the on position you read 12.9volts???? Is that 12.9 volts on the dash gauges or is it testing across the battery with a digital meter?? And is the 14+ volts with the engine running???
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Old 10-10-2014, 04:35 AM   #6
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Guess I am unclear what you are trying to say. Is OFF relative to the engine but the key still on? ie engine off but key in the on position you read 12.9volts???? Is that 12.9 volts on the dash gauges or is it testing across the battery with a digital meter?? And is the 14+ volts with the engine running???
Off=key off. Testing battery with volt meter
14+= on dash gauge with motor running.
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Old 10-10-2014, 05:02 AM   #7
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Off=key off. Testing battery with volt meter
14+= on dash gauge with motor running.
My experience is that the dash voltage gauges are not very accurate. That is because they are AFTER the ignition switch and we all know GM doesn't know how to make a reliable ignition switch. Just CHEAP ones.
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Old 10-10-2014, 09:08 AM   #8
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Quote:
Originally Posted by laurent_zo6 View Post
Off=key off. Testing battery with volt meter
14+= on dash gauge with motor running.
One thing to do when checking the battery voltage is to make sure that the various timers have shut everything off such as interior lights and the underhood lights. Wait for a minute before checking the battery voltage. 12.9 indicates a fully charged battery but 12.7 to 12.9 is a good range.

A reading above 14 volts for a short time after engine start is fine. Once the alternator brings the battery charge back up, you should see voltage reading around 13.5 to 14 depending on what things are running (lights on, A/C on, radio, wipers, etc.)

You should also check voltage at the alternator to see how close the digital volt display is compared to your VOM.
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Old 10-10-2014, 02:37 PM   #9
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This is an interesting thread as I had the exact same question. With the the ignition off and using a multimeter, I measured 12.86 volts. There was no load that I was aware of with the exception of the security system. The battery is a 5 year old Delco that is usually kept on a maintainer.
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