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Anybody logged voltage? When viewing data, how do you determine what is erratic and what isn't? Lots of peaks & valleys in my data, most of which seems to follow the RPM, but I'm concerned my alternator may be kicking in too often. Also, the voltage the my MXL2 is indicating is at least .5v higher than what I read at the battery (& cut-off switch) with my Fluke DDM and I've had to tweak my warning light values accordingly. Anybody had that problem?
I don't drive a P* car, so YMMV, but I'm mostly looking for a voltage that indicates the alternator isn't charging at all. That can mean a failed alternator or a missing belt.
If your battery resting voltage is 12.5v, set a warning for RPM > 2000 and voltage <12.5 or 12.0.
I had an alternator that was failing, and at the start of a session it was OK, but as the session progressed the voltage dropped enough to trigger my warning. Put a new alternator in and fixed the problems.
What is the difference between the peaks and valleys? It can be deceiving when you see what looks like large variations and they are really .01 volts!
In my experience, the difference in voltages can be from a number of things. Depending on the car and wiring setup, it can vary based on where the components are connected, where in them it measures voltage, and then measuring methods (I think). It's frequent that an ECU reads a different voltage than the dash. What I do is look at what the car runs when systems are ok and use that to set my alarm values. I set them similar to what Jeff said above.
Anybody logged voltage? When viewing data, how do you determine what is erratic and what isn't? Lots of peaks & valleys in my data, most of which seems to follow the RPM, but I'm concerned my alternator may be kicking in too often. Also, the voltage the my MXL2 is indicating is at least .5v higher than what I read at the battery (& cut-off switch) with my Fluke DDM and I've had to tweak my warning light values accordingly. Anybody had that problem?
Agreed on the earlier question of what the variance in the peaks and valleys was? Are you by chance able to share the exported data? Odd that the unit is showing a variance of .5v, is there a calibration that can be performed? We offer a bit of a simpler module that connects to an app on your phone to view data so when we're looking at a profile like in the image below it's give's us a better idea if the charging profile itself might be doing some odd things. Definitely not a full feature module like MXL2 but you get the idea of what I'm referring to viewing the profile in regards to what I'm asking that it is you are seeing (disregard the % by the way as this is the lithium option)
I took a look at the data I have and voltage range stays consistent from start to finish of the session. Also, the traces are relatively similar from lap to lap. The car is a Spec Miata and the ECU controls the Alt. charging. I haven't had the AiM system long, so the first couple events were basically checking system functionality, adjusting alarm parameters and learning the new dash. I have the upper alarm set at 15.5v and low at 12.7v (I think). Factory Alt output should be 14.7v. So my first concern was I was over charging. Assuming my multi-meter is correct, it is getting old, than the AiM is reading high. If there is a way to calibrate the unit, that would be great. If not, I can live with the alarm adjustment. Second, concern was the Alt. is ON too much and robbing Hp and we need all we can get.
Here's a not very clean rpm/volt lap trace. SM volt data
I think a 0.3 volt difference is probably fine. It is odd that it's reading that high though. I have an email into the hardware person and will report back what they say.
If you look at your data, the voltage spike is at the RPM drop. My guess is this is how the charging system works. Maybe ask some other Miata folks if they see the same. I don't know SM rules, but there are some neat things you can do with alternators to save some power...
Agreed that the voltage is getting up there and a bit higher than they should..but if the readings are inaccurate by .5V then it's still in an okay area and threshold and from I see here it doesn't look too erratic compared to others we've seen..but like you said it could be weakening the HP performance output a little if it's going on frequently. Honestly not too familiar with the Miata's either but wondering if something can be altered in the programming in the ECU to adjust..for example in the PIWIS in a Porsche a change to an AGM profile for the battery will drop the voltage output and make it more efficient for track use...I have to imagine as Matt mentioned some other Miata folks might have a good idea if the behavior is normal and if anything can be altered or changed like the profile from the ECU.
Here's a look at a failing alternator in our NA Miata enduro car (data from Autosports labs Podium). This is the kind of thing you want to watch out for:
And here is a look at a pair of good alternators. The top car is an NB with a standalone ECU, and the bottom car is my NA:
I asked my alternator rebuilder why i was seeing 15.x V and he said that voltage and amperage are linked, inversely. When there is a light load, voltage will rise. When the alternator is under heavy load, the voltage will drop (all other things being equal).
To really understand your alternator load, you'd need to have an ammeter, or a sensor that measured amperage (current flow).
Here's a look at a failing alternator in our NA Miata enduro car (data from Autosports labs Podium). This is the kind of thing you want to watch out for:
And here is a look at a pair of good alternators. The top car is an NB with a standalone ECU, and the bottom car is my NA:
I asked my alternator rebuilder why i was seeing 15.x V and he said that voltage and amperage are linked, inversely. When there is a light load, voltage will rise. When the alternator is under heavy load, the voltage will drop (all other things being equal).
To really understand your alternator load, you'd need to have an ammeter, or a sensor that measured amperage (current flow).
There is a good ammeter that Aircraft Spruce sells.
I think a 0.3 volt difference is probably fine. It is odd that it's reading that high though. I have an email into the hardware person and will report back what they say.
If you look at your data, the voltage spike is at the RPM drop. My guess is this is how the charging system works. Maybe ask some other Miata folks if they see the same. I don't know SM rules, but there are some neat things you can do with alternators to save some power...
Thanks Matt, interested to hear what they say regarding the voltage difference. I'd rather not have to make the 0.3-0.6v adjustment in my head when glancing at the gauge, but with time and adjusting the warning light one will adapt.
In general, the volt swing does not seem extreme at only 0.7v but, I would have thought it would have been a little more dampened than it appears. I also agree the the upward spikes make sense, as the demand on the coils would drop quickly with rpm loss on up-shifts. I've reached out to some SM folks to see if any have similar findings. So far, no apples to apples data.
Regarding SM rules and the Miata ECU, the rules don't permit us to manipulate the Alt. or ECU. And, the ECU is pretty much a blunt instrument, locked down from the factory.
__________________ -Peter Krause www.peterkrause.net www.gofasternow.com
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01-09-2020, 01:10 PM
Voltage Data
