Low battery voltage under load--battery or alternator at fault?

[TheFleshRocket]

I am in the process of datalogging my Cobra for remote tuning, following a TVS install. My tuner noted that battery voltage was low in my WOT pull datalog. Sure enough, it is.

That graph shows battery voltage during a second-to-third-gear pull. When I was in-it-to-win-it, battery voltage hovered around 12.7V, and dipped as low as 12.25V. As soon as I let off, even at high RPM, battery voltage jumped back up to the mid 13s.

My Cobra is an '04 with 33K miles on it. The battery is a Motocraft from 2013, and it's weak. After driving the car, or after charging the battery overnight, resting voltage is in the mid 12V range. I'm pretty sure it has a bad cell.

The current alternator is a PA Performance 130A model that I bought in May 2016, and it has only about 1500 miles on it. (As you can guess from the low miles, my Mustang only sees 1500-2000 miles per year. The previous alternator was a Motocraft reman that died after only 3000ish miles, and the OEM factory alternator lasted about 28K miles before dying.)

Since the battery is weak, I'm inclined to point a finger at it, but I feel like the alternator should put out enough juice to keep the voltage in at least the mid-13V range at all times regardless of the condition of the battery. Thoughts?

 

[01yellercobra]

You could have the battery tested. But I wouldn't consider mid 12's with the engine off weak.

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[TheFleshRocket]

Technically speaking, 12.6V at rest is considered a fully charged battery. When a battery has just finished charging (or when you just shut off your car after driving for a bit), the battery will typically have a "surface charge" so its voltage will be in the upper 12s or possibly even low 13s, and it takes the battery a while (couple hours, perhaps) for its voltage to drop down to its true resting voltage. The battery in my Cobra shows mid-12s immediately after being charged, and its resting voltage is usually in the low-12V range, so it's definitely not performing optimally.

This thread explains surface charge (and quite a bit of other battery-related stuff) in pretty good detail: https://mechanics.stackexchange.com...charge-and-how-does-it-affect-battery-testing

My primary question is whether a weak battery can cause low voltage under load, or whether the alternator should put out enough juice to compensate for a weak battery.

 

[Soap]

When you are WOT, the electronics are pretty much being driven off the alternator.

--Joe

 

[TheFleshRocket]

When you are WOT, the electronics are pretty much being driven off the alternator.
--Joe

So, basically you're saying it's the alternator's fault if I have low voltage under load.

Crap. Good luck to me getting the alternator RMAed if it seems to produce sufficient voltage under "regular" conditions.

 

[94slowbra1]

I'm having the same issue, WOT I see as low as 12.0 at 6700 rpm. Brand new battery. I'm guessing my alt is getting weak, still does fine while cruising around. I also have a lot of things taxing my electrical system: twin 465 pumps, hid's, big amp for stereo, ect... im going to look into an upgraded 200 amp unit soon so I know I'm good.
But to answer you original question, electronics are mainly run off the alt when the car is on. So my money is a bad alt, not the battery.

 

[TheFleshRocket]

When datalogging today, battery voltage didn't go any lower than 13.2ish. So it's definitely not consistent.

 

[procharger]

Something to think about, how is your aftermarket amp wired for power, have you physical checked voltage at battery terminal/alternator or are you taking reading from handheld?
I'm a Raymond Level 2 Technician, Battery/Electrical issues is what I deal with 5 days a week, ideally equipment that runs on a battery when load testing you don't want to loose anything more 2.5 volts. As for cars everything is 12v maybe a couple things that require more like boost pumps @ 13v. Upgrading your alternator with a Good/Tested/Reviewed brand is the best thing you could do for your car. Adding an capacitor in-line for your amp is a big help, as well as getting a battery(AGM) to store all the amps. High power alternators on a regular batteries will cook the cells.

Sent from my LG-H918 using the svtperformance.com mobile app

 

[procharger]

Sorry, got wrapped up... Sounds like alternator.

Sent from my LG-H918 using the svtperformance.com mobile app

 

[lx347cid]

I just went thru same issue. Replaced alternator. Its will fix it for a while.

 

[olympic]

Check actual voltage at the battery vs what you see in the data log. Just a simple comparison at idle will suffice. I know my car logs 0.5v lower than what I see at the battery with a DMM. If that's the case then you're fine. Even if the logs are accurate those very brief dips down to 12.4 aren't the end of the world.

 

[MalcolmV8]

Voltage drop under WOT is almost always alternator on these cars.

 

[TheFleshRocket]

I sent the alternator off to PA Performance (more than a month ago) and they said it tested good. (I also had Autozone have a go at it, and they concurred.) When reinstalling the alternator, I noticed some corrosion on the harness eyelet that bolts to the alternator. I cleaned it up with some sand paper, and voila--voltage stays around 13.8-14.2 at all times.

 

[MalcolmV8]

I sent the alternator off to PA Performance (more than a month ago) and they said it tested good. (I also had Autozone have a go at it, and they concurred.) When reinstalling the alternator, I noticed some corrosion on the harness eyelet that bolts to the alternator. I cleaned it up with some sand paper, and voila--voltage stays around 13.8-14.2 at all times.

Just FYI when autoparts stores and even real alternator shops test alternators it's at a much lower RPM and doesn't simulate WOT conditions. It's very frustrating as I've been down that road with low voltage at WOT but everyone keeps insisting the alternator is good. Swap out the bad unit that everyone claims is just fine and boom problem fixed.
While probably rare alternators can put out good voltage and current at low RPMs but fall on their face when spun hard.

 

[TheFleshRocket]

Just FYI when autoparts stores and even real alternator shops test alternators it's at a much lower RPM and doesn't simulate WOT conditions. It's very frustrating as I've been down that road with low voltage at WOT but everyone keeps insisting the alternator is good. Swap out the bad unit that everyone claims is just fine and boom problem fixed.
While probably rare alternators can put out good voltage and current at low RPMs but fall on their face when spun hard.

Yeah, I figured that the test conditions at PA Performance and Autozone probably wouldn't mimic the alternator being spun to 6000 RPM engine speed (not sure how many RPM that would be at the alternator--15K or so?), so I wasn't very hopeful when I reinstalled it--I figured the problem would persist. Fortunately cleaning the eyelet fixed it, but I don't doubt that there are certain people whose alternators might test good but not be.

Actually, that reminds me of my recent alternator issues with my '94 Caprice. It was a new Delco Remy 130-amp unit. The original alternator stopped charging altogether, and the new one was just fine... up until 4000 RPM, that is. Then the headlights would go dim and the dashboard voltmeter would drop into the yellow zone. It was consistent, too. Seemed to be based on RPM, though, not load--it would do it even under part throttle. Of course, I never took it to Autozone to have it tested, I just RMAed it to Rock Auto, but I bet Autozone might have said that it was good.

 

[01yellercobra]

Just FYI when autoparts stores and even real alternator shops test alternators it's at a much lower RPM and doesn't simulate WOT conditions. It's very frustrating as I've been down that road with low voltage at WOT but everyone keeps insisting the alternator is good. Swap out the bad unit that everyone claims is just fine and boom problem fixed.
While probably rare alternators can put out good voltage and current at low RPMs but fall on their face when spun hard.

Another issue is heat. I had an alternator that was good until it got hot. It bench tested good every time. Swapped it out and all good.

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