battery voltage reading
#1
Burning Brakes
Thread Starter
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?
#2
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.
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.
#4
Melting Slicks
Member Since: Aug 2013
Posts: 2,491
Received 312 Likes
on
227 Posts
St. Jude Donor '15-'16-'17-'18-'19-'20-'21-'22
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???
14+= on dash gauge with motor running.
#5
Team Owner
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.
#6
Drifting
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.
The following users liked this post:
04zjimc (09-15-2019)