Anyone know how close the battery/system voltage should be to alternator output voltage when running? I can’t seem to find a spec.

So if you think about it, it is not a hard number. Several aspects need to be evaluated.

The first question is the battery getting the proper charging voltage. If it is and the cables are not getting warm then you are probably ok. The variable that defines the voltage at the battery is primarily where the voltage regulator sensing wire is. If close to the battery then the battery voltage will be fine. But if you are pulling off the alternator and have a high drop then you might be feeding your stuff with a higher voltage than you may want.

If the regulator is in the alternator and you can't sense at the battery, this gets more tricky. Now you need much more robust wiring. I would say absolutely no more than 0.5 volt drop with closer to .2 to .3 volts, but that is just my opinion. Other opinions may vary.

Considerations are is the battery getting proper charging voltage, is the stuff being fed from the battery and alternator at a good voltage and are the cables getting warm.

My 2 cents..........

if the alternator is putting out 14.2v at the output stud, i would like to see no less than 14.0 at the battery +, with no warm cables.

zero difference is ideal. Fluke says

0.00V across a connection
0.20V across a wire or cable
0.30V across a switch
0.10V at a ground

https://www.fluke.com/en-us/learn/blog/automotive/electrical-automotive-troubleshooting

If the battery is 100% charged & taking no more current, the difference will be zero voltage.
A balanced charging system.

Thanks guys, I assumed it should be practically the same but wanted to make sure I wasn’t missing something.

I had no electrical issues, then my alternator stopped charging. Swapped regulators, no change. Installed an OEM single field round back alternator (should be 35 or 46 amp) and it charges about 20 amps at idle. Swapped VR back and still 20 amps at idle. Tried another old oem 35 or 46 amp and still 20 amps charging at idle. I cleaned all connections and looked for obvious issues and high resistance among any wire runs, but I have;

14.55V at alternator
13.94V at ammeter
13.82V at sensing wire (blue VR wire)
13.55V at battery

Obviously have an issue here somewhere, but its odd that it started with the alternator apparently going bad. Tried another battery, and bypassing the ammeter too, same deal.

you have quite the drop and it is possibly the bulkhead connectors that are corroded.
but eliminate one other thing, where did you measure the ground from?

the vr sensing line should be using the vr case for ground.
same for the alt, you should be using the alt case for ground and not the battery ground.

you can also test again, by bypassing the sensing line and using power from the battery and see if your voltages change again.
if it is the bulkhead, you will see drops inside the car still, but your battery and alt voltages will be the same.

Check both ammeter pins or jump the ammeter as a test to see what happens.

When troubleshooting a situation like this, measure the voltage drops across connections not to ground. For example measure from the alternator terminal to the battery.

I would get the factory schematic and start measuring voltage drops starting at the alternator to first point past that, then next point, etc. This will give you some clarity.

Does certainly appear that you have a bad connection. If you have the factory bulkhead that is probably where it is. In my cars that I don't care 100% about originality, I take out the bulkhead connectors for the 2 high current feeds, drill them out slightly and run the wire through the connector eliminating the connector. Works great but you are cutting the wires if you need to take it apart. You can also successfully replace the connectors.

So I dug out my 67 Coronet schematic for reference. Yours should be the same.

You want to measure from the alternator side on the ammeter (black wire) to the alternator stud. Then you want to measure from the battery side of the ammeter (red wire) to the battery. That will give you the voltage drops across the 2 bulkhead connections.

Missed this or is it another functionally useless bit of "help"?

And your point is Richard.

You're an arrogant donkey

You are the one thats picking the fight. You seem to enjoy jabbing at others as well as my self. If you have issues with me take it up with a PM.

Electonics 101A teaches on to avoid connections when possible as each one is a potential problem.
As stated, ideally 0 V drop. In these cars with their multiple charging system connections and 60 years old technology I do not believe that is possible even with a brand new everything including harnesses especially at idle.
IMO a. 2 to.5 v drop is acceptable depending on the harness condition etc.

You also should be checking at 1500- 2000 RPM as most of the earlier and current (no pun intended) alternators do not reach full output under idle conditions. But that goes back to system load, pulley ratios and how fast the Alternator is turning.

With the engine running and the meter ground on the battery negative check the voltage at the alternator post, the battery and the starter relay. Check the voltage inside the car at the fuse box as well using the dash frame as the ground. They should be the same but maybe not..

At least 5 grounds should installed along with the ammeter bypass wire with a fusible link, but no need to bypass the ammeter. Not required. 1972 c bodies and other cars had the bypass wire from the factory....see the fsm.

The large red and black wires that pass thru on the bulkhead should be properly routed thru the firewall and hard connected by soldering and covering with 2 layers of shrink sleeve.
One of the grounds from the battery needs to come into the passenger compartment and attached to the dash frame. Use this ground connection for all aftermarket grounds.

Grounds
battery to engine
battery to core support
battery to interior - use one of the bolts on the large bolt that holds the dash at the bottom on the side
VR to engine
engine to frame rail - use a large cable
ignition ground mopar box case to engine
msd box gets a battery ground
I ground the alternator case as well, who knows...
If the battery is in the trunk multiple large cable grounds work very well

Use an ohmeter to check for resistance from the battery negative to the other ends of the grounds. It should be less than 1 ohm if not zero

Considering the load. That's not terrible.
To answer the first question.
There is an acceptable voltage drop spec but it is tied to the specific tests. It's in some of the master technichian's booklets, and IIRC its in the shop manuals, but I won't swear to it.

Next. I'm going to strongly recommend charging the battery on a battery charger. If its a manually adjusted charger, set it for 5 amps or even better 2 amps. While the alternator can charger a battery at 20 amps, its hard on all of the equipment including the battery. If you put a voltmeter across the terminals while charging, you'll see for yourself how the charging voltage relates to charging rate.

Now lets diagram what you measured and it should make some more sense.
The diagram will assume that ground has no resistance.


We can estimate the alternator's rotor and the ignition use about 6 amps. So there's about 26 amps flowing out of the alternator to the main splice.
Between the alternator output and the ammeter, there was 0.7 volt dropped.
(We'll assume the welded main splice junction is perfect)
From the ammeter to the battery the voltage drops is 0.3 Volts when 20 amps is flowing through that line. Again can claculate the Resistance in ohms, but that not terrible for 20amps through several connections, including a push on type, and a 16 gage fusible link.
From the main splice to the VR is a 0.1 Volt drop, with a guestimated 6 amps flowing through most of the line.

The biggest drop was in the alternator output line (R6 in the FSM) .
If you want to really know which line had the most resistance, assuming the there grounds are perfect, calculate the resistance using the relationship change in V = I x R. Solve for R.

The field circuit test, with a rotor draw of 3 to 3.5 amps (typical roundback) allows as much as .35 to .55 V difference between battery and the pre-70 style regulator depending on which side of the regulator measured.

Charging circuit test, with the alternator output regulated to 10 amp, (so roughly 2 amps to ignition and 8 amps to the battery) allows no more than .3 volt drop between alternator output and battery.

This is from the '69 MTSC.

Equating the tests to your in use measurements, it would seem that with 26 amps output, rather than 10 amps, the 0.7 V drop could be close to the limit.
I'd check the R6 terminal at the bulkhead and also do some direct votlage drop measurements to be sure the drop is in the positive wiring and not a loss in the grounding.

Maybe I should explain this.
The voltage drop increases when the current through resistance increases.
Current is electrons flowing. The low battery increased the current.

Normally after starting battery will not need much recharge. With the alternator providing power at 14 Volts, the battery will draw 10 to 20 amps only for a few moments, then as it recharges it draws less and less. After 10 or 15 minutes it essentially draws nothing.

A battery that has been run down will draw much more current for recharging given a recharging source at same 14 Volts.
That's why there is a noticible voltage drop now. Instead of 5 to 10 amps going through the wires and connections, there's now 26 amps going through those connections.
It only gets worse as the weak junctions heat up under that high load, because when they heat up, resistance goes up.