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| | | | Joined: Apr 2008 Posts: 899 'Bolter | | 'Bolter Joined: Apr 2008 Posts: 899 | 1949 3100, 216, 6v Optima battery with alternator from Randy Rundle's shop many years ago. With engine at idle what voltage should I see at battery terminals and what would be the max as rpms increase?
Last edited by Forty9; 04/29/2026 2:44 AM. Reason: correct spelling
Ed
| | | | | Joined: Oct 2021 Posts: 6,775 'Bolter | | 'Bolter Joined: Oct 2021 Posts: 6,775 | 6.8v - 7.2v with engine running
6.3v - 6.4v with engine off
1950 Chevrolet 3100 (Ol' Roy)
1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)
| | | | | Joined: Jan 2022 Posts: 2,450 'Bolter | | 'Bolter Joined: Jan 2022 Posts: 2,450 | Don't be surprised if the voltage at idle isn't much (or any) above the engine off voltage. Our engines turn so slow at idle that an alternator just can't put out much power.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
| | | | | Joined: Feb 2004 Posts: 29,792 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,792 | Any charging system- - - - -generator, alternator, or even a hamster running in a wheel, regardless of system voltage- - - - -will only produce enough voltage to push a charge into the battery. Against a very low battery, the charging voltage might only be 3/4 to 1 volt higher than the "engine off" voltage measured at the battery posts, but the charging AMPERAGE will be quite high. As the battery builds up a charge and its internal resistance rises, amps go down, and terminal voltage goes up.
You MUST know the state of charge of a battery before passing judgement on charging system voltage. Back in the dark ages when batteries had removable cell caps, we could stick a hydrometer in a battery cell, measure the specific gravity of the acid, and know for sure the state of charge of the battery. Now all we can do is check the open circuit voltage at the posts of a sealed-top battery and make an educated guesstimate about a battery's state of charge.
IF:
The battery is fully charged
The charging system and voltage regulator (internal or external) is functioning properly
AND
The engine is running at a fast idle speed or above and minimal accessories such as headlights, heater blower, etc. are turned off- - - - -
THEN
Battery terminal voltage should be around 7.5V on a 6 volt system OR 14.5V on a 12V system.
Under any other circumstances, you're making a SWAG ("Scientific Wild-Donkey Guess") at the very best!
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!
| | | | | Joined: Apr 2008 Posts: 899 'Bolter | | 'Bolter Joined: Apr 2008 Posts: 899 | Appreciate everyone's input! Ed
Ed
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