Correct Charging Voltage per RPM?

[Mucci]

I'm doing some diagnostics. Is there a chart for charging voltages at specific RPM?

 

Currently this is what I'm seeing by putting leads on the battery terminals:

 

1000 RPM = 13.48

1500 RPM = 13.61

2000 RPM = 13.61

3000 RPM = 13.59

 

Almost no change from idle to 3K. Most other cars I've tested have been 14V+ past 2500rpm but they've all been a decade newer. Is this normal or do I have a charging issue?

Battery, battery cables, alternator belt and bushings are new within the past month.

The car is still starting very sluggishly so I'm trying to make sure the battery is getting charged back up before moving on.

 

[TobyB]

Looks like your alternator is working as designed.

 

With heavy loads, it will have lower voltages at lower RPM, as it is 

current- limited when it's not spinning fast.

 

That looks very normal to me.  

2 things to check:  Battery voltage after it sits for 12 hours,

and battery voltage after it sits for 1/2 hour.

 

It's common for the bushings in the big starters to wear,

letting the armature hit the field coils.

It then cranks slowly.

After all, it's only 40- something years old!

t

 

 

[John76]

I checked the voltage on my '76 (after driving at highway speeds for 30 minutes). New, fully charged battery, rebuilt stock alternator and stock mechanical voltage regulator, with no loads (lights, wipers, radio, etc.).

 

900 rpm (idle)  = 13.30V

1000 rpm         = 13.31V

1500 rpm         = 13.31V

2000 rpm         = 13.30V

2500 rpm         = 13.29V

3000 rpm         = 13.28V

 

If the battery voltage is low (from sitting, just after a cold start, or extra loads) the voltage reading is about 13.50 - 13.60 at all rpms.

Once the battery has "recharged",  the voltage settles down to about 13.30V at all rpms.

 

John

 

PS: I agree with Toby....replace or rebuild that 40+ year-old starter!  Big difference for < $100.

 

 

 

[Mucci]

29 minutes ago, John76 said:

I checked the voltage on my '76 (after driving at highway speeds for 30 minutes). New, fully charged battery, rebuilt stock alternator and stock mechanical voltage regulator, with no loads (lights, wipers, radio, etc.).

 

900 rpm (idle)  = 13.30V

1000 rpm         = 13.31V

1500 rpm         = 13.31V

2000 rpm         = 13.30V

2500 rpm         = 13.29V

3000 rpm         = 13.28V

 

If the battery voltage is low (from sitting, just after a cold start, or extra loads) the voltage reading is about 13.50 - 13.60 at all rpms.

Once the battery has "recharged",  the voltage settles down to about 13.30V at all rpms.

 

John

 

PS: I agree with Toby....replace or rebuild that 40+ year-old starter!  Big difference for < $100.

 

 

 

 

Thanks for testing that for me! Glad to know my alternator is a-ok. I'll move on to looking at the starter as the slow cranking culprit. 

[Chris_B]

Your alternator is weak at best. It should be able to maintain +14V except under extreme load (with an 02 that would be with headlights and heater blower on). Regardless, the important point is that your charging system is keeping your battery charged (>+12.6V), so this your situation seems to be good enough.

[Chris_B]

BTW, for an accurate reading, you should be checking at the battery. Measuring at the "cigarette" lighter will subject the reading to a number of insertion losses between the battery and the cig plug.