Battery Voltage drops under higher rpm load?

I am noticing while looking at my datalog that while my battery voltage jumps around a lot 11.8-13.8 while cruising that under higher rpm load the voltage drops down to 10.8 at points.

Is this a sign of an alternator that is just undersized and unable to keep up with the load or maybe an alternator that is going out?

OR maybe as simple as the belt slipping?

Or maybe a battery that is having issues?

Thanks

I'd be looking at slip, but dropping under 11 says the batt is not much good or not getting charged.
WOT electric demand is way less than lights fans and heat, so easy testing.

Or supply wiring to your sensor....

When I first did my TPI conversion, I got my ECU ppower from the '60s wiring in the cab, and due to voltage drop, I'd rythmicaly drop about 50 RPM while the blinkers were on.

Verify verify verify all connections...

I suspect a weak battery and dying alternator.

Well the belt feels tight, I put it a little tighter so we will see what the next catalog shows. For sure the battery is a little weak. Biggest thing I noticed is that my solid alternator mount has some play so that could be the cause of the big fluxuation so I will get it swapped out for a poly when I find time.


Battery is in the trunk and has 0/1 cables to starter and block. And from there a 4ga to the alternator. My grounds are a bit all over the place so I am already planning some time to clean those up. While the car did have everything on last night ; 90/100 headlights run off relay blower fan on max with heater radio going along with feeding power to the 255 intank and 044 inline. But for sure sounds like something is up. Alternator is a high mile used den so unit so I will see about getting it replaced soon with another new / rebuilt unit and go from there.


Thanks, just trying to get some ideas on where to start checking.

If your belts and pulleys are shiny they may slip a little under WOT. Some 80 grit sandpaper can fix that.

Do parts store alternator testers spin fast enough to simulate your high RPM?
Perhaps it is not working well at RPM it is not designed for?

I would suspect both battery and alternator. Alternator should keep up unless the belt is slipping but the battery shouldn't drop that low either. Check cables first of course. Buy an amp clamp if you don't have one...

Gate leakage from a diode

Look to see if the timing mark on the harmonic balancer matches the crank timing mark. It could be the balancer is spinning and you just aren't able to hear it at high rpms.