Over the last 50 ears I have dealt with batteries either in cars, boats , airplanes and Farmall tractors. I would like to discuss newer 12 volt Batteries.I was under the impression that 12 volt batteries fully charged rested between 12.8 and 13.4 volts. In 1986 I installed 8 gel cell group 30's in my 48 ft sailboat. I had the charger and alternator set at 13.8 , the manufacturer recommendation. They lasted through tough use for 12 years. Then I decided to replace them with AGM's, set charging at 14.4 (again manufactures recommendation) and used them for another 8 years, Now in a car I just bought the alternator goes from 14-8 to 12.8 under way and drops to 12.1 at shutdown. Once I SAW 11.8 which I always thought was a dead battery Car starts fine after sitting four days. What say you
- Thread starter HenryO
- Start date
Henry, a conventional lead acid battery !!!!!!!! AT REST AND STABILIZED !!!!!!!! at 77 Degrees should read as follows per the chart I found (NO Warranty)
100% Charged = 12.63 Volts
75% Charged = 12.30 Volts
50% Charged = 12.00 Volts
25% Charged = 11.76 Volts
0% Charged = 11.64 Volts
I have no specs or data whatsoever regarding YOUR particular Alternator and its certain Voltage regulation circuitry orrrrrrr the accuracy of your meter but perhaps ITS NOT ACCURATE ????
Others may or may not know YOUR particular Alternators specs or its voltage regulation figures and if so ??????????? may be able to provide assistance. In broad general terms that 14.8 SOUNDS HIGH while the 12.8 SOUNDS LOW but again I don't have YOUR ALTERNATORS specs or data.
Its an electrical question so you should get a ton of responses, this represents ONLY my opinion, others may agree or disagree
John T
100% Charged = 12.63 Volts
75% Charged = 12.30 Volts
50% Charged = 12.00 Volts
25% Charged = 11.76 Volts
0% Charged = 11.64 Volts
I have no specs or data whatsoever regarding YOUR particular Alternator and its certain Voltage regulation circuitry orrrrrrr the accuracy of your meter but perhaps ITS NOT ACCURATE ????
Others may or may not know YOUR particular Alternators specs or its voltage regulation figures and if so ??????????? may be able to provide assistance. In broad general terms that 14.8 SOUNDS HIGH while the 12.8 SOUNDS LOW but again I don't have YOUR ALTERNATORS specs or data.
Its an electrical question so you should get a ton of responses, this represents ONLY my opinion, others may agree or disagree
John T
guido
Well-known Member
Hello HenryO,
Battery voltage at rest is 12.6 volts @ 80* F. Ambient temperature is seldom mentioned, but it is relevant and should be considered at all times and temps. AS you stated AGM batteries require a higher voltage setting 14-15 volts
If underway you mean with the engine running, May be you your car has that kind of battery?
If the engine starts fine then the battery is O.K. Are you taking voltage readings at the alternator + terminal? With the engine off that voltage reading has no meaning. If that is the case, it is not a volt testing lug, that is output voltage.
Battery voltage is what is important, and the output voltage will vary with load and engine R.P.M.'s anyway. Some time it is thee same voltage on the alternator lug and the battery though,
Guido.
Battery voltage at rest is 12.6 volts @ 80* F. Ambient temperature is seldom mentioned, but it is relevant and should be considered at all times and temps. AS you stated AGM batteries require a higher voltage setting 14-15 volts
If underway you mean with the engine running, May be you your car has that kind of battery?
If the engine starts fine then the battery is O.K. Are you taking voltage readings at the alternator + terminal? With the engine off that voltage reading has no meaning. If that is the case, it is not a volt testing lug, that is output voltage.
Battery voltage is what is important, and the output voltage will vary with load and engine R.P.M.'s anyway. Some time it is thee same voltage on the alternator lug and the battery though,
Guido.
Hobo,NC
Well-known Member
- Location
- Sanford, NC
New cars are designed to charge that way to conserve fuel...
Bob Bancroft
Well-known Member
- Location
- Aurora NY
I bought a new 2019 Silverado just over a year ago. I noticed the voltage running a little low occasionally. I was going to mention it to the dealer. Then, I happened to read in the manual that some computer brain somewhere [my words, not theirs] might change the voltage from time to time. Makes no sense to me, but that's the way it is.
Steve@Advance
Well-known Member
It's the computer dropping the charge voltage. evidently it completely stops charging and the load on the system draws the voltage down. As long as the charge light or info readout doesn't indicate a problem it is doing what it should.
Hobo,NC
Well-known Member
- Location
- Sanford, NC
Yep, and the starters are so efficient they will crank somewhat ok on a weak battery...
whereswaldo
Member
Our 2015 Escape got to where the computer would shut the system off to 'save the battery' while listening to the radio after 5 minutes. It had let it go for over a half hour. Tested the battery and it read
weak' under load. Got a new battery and that one did much the same thing, shutting off after only 15 minutes. Discovered I had to 'Reset' the computer, as it 'thinks' I still had the old battery in there. Wasn't fully charging it. Look it up, resetting or recalibrating your vehicle computer.
weak' under load. Got a new battery and that one did much the same thing, shutting off after only 15 minutes. Discovered I had to 'Reset' the computer, as it 'thinks' I still had the old battery in there. Wasn't fully charging it. Look it up, resetting or recalibrating your vehicle computer.
MARK ROBKE
Well-known Member
Sometimes dash gauges are not as accurate as a test meter. Mark
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