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How much voltage is too much?

11 January 2021
Ontario, Canada
Search on alternator and regulator turned up empty, with most complaints of low voltage. My car starts and drives fine, so I'm not sure there's a problem. All I could find was:
Driving around & noticed that my volt gauge is reading ~15.0V (I know, hard to be sure with that gauge, but it pointed pretty much to the 15 notch.) I had never noticed that before, but I've only had the car a few months & I'm not sure I looked at it before. Seemed high compared to other Hondas I've had. I was wondering if the voltage is too high, or if the voltage is good but the NSX voltmeter is reading high. I assume too high voltage would damage something.
NSX voltmeter compared to my Fluke multimeter:
Ignition on, but not running: NSX 12.8V, Fluke 12.2V
Ignition on, at idle: NSX 15.0V, Fluke 14.6V. Revved to 3k: no obvious change
So my NSX voltmeter seems to read ~0.5V high. If this is just the way it is, I can live with that. It's a blunt needle with only 90 degree sweep.
Alternator voltage?
Reading the service manual (23-101) is a flowchart for if the charging system light is on, but my charging system light comes on and then goes off as it's supposed to. The Regulator flowchart (23-103) seems to suggest that voltage up to 15.0 volts is fine, so I assume my 14.6 V measured with my Fluke means it's just a dash gauge that reads a wee bit high.
Anyone else notice that their voltmeter reads 15V when driving routinely?
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The quick and dirty answer - I wouldn't sweat it :) .

The slow and cleaner answer.

The dash voltmeter measures the switched voltage off the ignition switch which is the instrument cluster supply and a few other things. Due to that other stuff that may or may not be running that voltage would normally be fractionally lower than the alternator / battery voltage so it would not surprise me if Honda put a bias into the gauge to try and make it read something closer to the alternator voltage. The dash voltmeter is good for monitoring changes - if the needle moves significantly from where it was last week that suggests a problem, otherwise don't try to read too much accuracy into that measurement.

The easiest place to get a good voltage measurement for the charging system is the jump start terminal in the engine compartment. As long as the wiring is in good condition that point is going to be within millivolts of the alternator terminal. Depending on how much current is being returned to the battery at any particular time the battery terminals will be fractionally less. My Fluke 21 (which was factory calibrated in the previous century and has been dropped on the floor several times) says the voltage at idle at the jump start terminal with all accessories off is a nudge over 14.9 volts. I think I switched on the AC or the headlights (can't remember) and some other stuff and it dropped to around 14.7 volts.

The ideal voltage will depend on the battery temperature (academic since the NSX does not correct for battery temperature) and the battery type. Search around and you will find that AGM batteries need a higher charge voltage to achieve full charge. I think the AGM needs about 0.2 - 0.3 volts higher than a conventional flooded cell battery. I think 14.9 volts is on the mark for keeping an AGM battery happy around 0 - 10 C. However, my previous battery was flooded cell and it lasted from 2012 to 2020 so it did not appear to materially suffer from my slightly high voltages. If I lived some place where it was eternally hot that voltage might have shortened the life of a flooded cell battery.

Without knowing exactly where you made that voltage measurement, 14.6 volts on your Fluke is probably just fine and should keep a flooded cell battery happy. If you turned on the AC and headlights and it dropped below 14.6 volts then you might have a low rather than high voltage problem In cold weather 14.6 volts might be suboptimal for an AGM battery; but, not to the point that it is going to kill it quickly.

Battery manufacturers may provide different guidance on their recommended charging voltages. Assuming you can find published data for your battery see what the manufacturer says about optimum charging voltage.
Thanks @Old Guy . I was contemplating hooking up a big inverter to draw 60-70 A to test it but I think I'll let sleeping dogs lie. Sounds like my alternator and regulator are right where they should be. Voltages were measured at the battery, which shouldn't be too far off the jump start terminal with the low current scenario I was measuring at.

My Fluke 73 is also older than my NSX & dropped countless times. Too late for a protective case? It's sort of protected by +++ hot glue from when my boss tried to help me by gluing it to the side of my experimental apparatus. 🤔
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I noticed something funny this morning too. The car has been sitting for about 2 weeks in a very cold garage. Fired it up this morning an the charging system light came on. It ran perfectly fine and it showed 14.5 volts on the dash gauge. I wonder if my battery drained too low and is not being charged by the alternator? Light never went off during the drive to the office. I'm going to put it on the tender when I get home and see if it fixes the issue. My understanding is that the light comes on if there is a voltage discrepancy between the battery and the alternator.
The charging system light operates whenever there is a voltage difference between the output of the alternator (there is a special diode isolated terminal for this on the regulator) and the 12 volt vehicle system (in this case the dash fuse box).

Charging system ligth.jpg

The charge light is normally on after you key on; but, have not started the car. There is 12 volts on the fuse box side and 0 volts on the regulator terminal side. The voltage difference powers the light. Once you start the engine the alternator bootstraps itself and starts producing voltage. When this happens the isolated terminal on the regulator rises to the alternator voltage (which ends up matching the voltage in the fuse box). With no voltage difference across the light it goes out.

The fact that your voltage gauge rises to 14.5 volts suggests that the alternator is probably working and that the light should go out. It would be good to confirm the voltage measurement with a voltage test at the jump start terminal with the engine running. Assuming your voltage is good, I suspect two possible failures
1) the internal circuit in the regulator that supplies the regulator L terminal (wht/blu) wire has failed and is not registering the alternator output voltage. That wht / blu wire should rise to the alternator output voltage after the alternator starts working. Unfortunately this is not that easy to check because of the possibility of the second failure.
2) there is a short to ground someplace on that wht/blu wire. The 1991 service manual describes the procedure for identifying this particular problem starting at the bottom of page 23-103. If you go through the test procedure in the service manual and confirm that there is no short to ground on the wht / blu wire, by elimination you are left with a problem at the regulator terminal L. I would check for a bad connection at the plug before I did anything else.

The above presumes that the charge light comes on with the key in the run position. If it is off in the run position; but, illuminates once the engine starts you have the opposite problem - a short somewhere on the Blk / yel wire. Check the #2 fuse. I don't expect this problem because the alternator will have trouble bootstrapping itself the voltage would typically not rise above 12.2 volts.
Funny thing is the light went out while driving to Honda to check the battery. By the time I got there, battery was good. Light hasn't come on since the car has had a chance to fully charge. I wonder if the system also detects low voltage at the battery and leaves the light on to alert you that the battery should be checked/changed? It's only been 1,000 miles since the alternator was rebuilt, so I'm suspicious of a fault there.

I had to look it up in the owner's manual- I've never seen the light on in "run" before.
What you describe is theoretically possible; but, unlikely. The light bulb will illuminate whenever there is lower voltage on one side and higher voltage on the other side. The normal failure which causes illumination is low / no voltage at the alternator and 12 volts at the dash fuse box. Given your circumstances, it is pretty hard to have good voltage at the alternator terminal and really low / no voltage at the dash fuse box.

Once the alternator is running it pretty much drags the whole vehicle 12v electric system up to its operating voltage. The dash voltmeter reads the voltage off of the IG1 Bat A ignition switch terminal which is the same terminal as supplies the alternator circuit (through a different fuse). If your dash gauge was reading 14.5 volts, even if your battery is dodgy you should have had 14.5 volts on both side of the light and it should not illuminate. If you had a ground somewhere on the blk/yel wire side of the light that would cause the light to illuminate when the alternator is running; but, then the light would not illuminate with the key in run position; but, engine not running.

I expect that it is more likely that you might have had a temporary ground somewhere on that blue / white wire. It is possible that if during the two weeks of non use the car had gone through some freeze thaw cycles which caused condensation to form on the alternator voltage regulator plug or on the regulator itself and might have created a high resistance path to ground that caused the light to illuminate. Heat from operating the car for an hour or two might have burned off the condensation making the problem go away. That is my guess. If the problem goes away and stays away that is good. If it returns you might have to do some of that fault tracing described on page 23-103 of the service manual.

Have you by any chance put a LED into that charging check circuit? If so, that can cause erratic behaviour in this circuit. A good old fashioned filament bulb works best for this and based upon the number of minutes per year that it is normally on will typically outlast the car.
Light hasn't come on since. Like you, I was wary of using LEDs in the gauge cluster, so I replaced them with all OEM bulbs. It must have been a temporary short to ground somewhere (loose plug at the alt?). The garage never gets below freezing, but it was in the 40s in there for a while. The car barely cranked- I should have put it on the tender but forgot. It's so dry here in Colorado that I'm not sure there was any condensation, however the engine bay got real good and fogged up in the car wash prior to going in the garage. Maybe there was some trapped condensation?

By the time I got to Honda, the car fired right up with no issues. The volt gauge has been rock steady at 14.5v the entire time. It's totally back to normal.

I'll check the alternator plug and add some DC-4 grease to help seal the connection. I guess it will remain a mystery...