Ok, so this will be a long post because i want people to have the whole story and all the details. Also sorry grammar is not the best with me as well thats a different story but a legit excuse lol.

So about 4 weeks ago i had safelite come out and replace my windshield. Durring this the dumb*** left the car on with the radio playing and power on. after this i went to start the car and it just clicked, thought ok he killed my battery, jumped it with the honda element we have and went on my marry way. car was acting funny but figured ok just weak battery. So about a week later i bought a new battery. the Following day on my way to work the cooling system shut down and i lost power steering, resulting in the rad cap failing and coolent went everywhere. I towed the car to mini, they did a leak down test and a comp check aswell as tested the new battery. came back and say well your battery is dead and your leak down on 3&4 didn't meet standards. Said ok, went to autozone and got a replacement battery. Later that night i was going to sell the car and i noticed every electrical system was failing like an alternator wasn't charging and was running off my battery. had to pull off the highway and tow the car back. app-on testing found that was in fact the alternator. So picked up a re-man and thought all was good. Now here is where it gets fun. so after a new alt was put on the car, i test drove it and got in to it, hitting about 6200 rpm, noticed when it got above 5k rpm the TCS, BAT, Tire light, Check engine light all came on then went out and i lost the alternator and battery light stayed on. when i limped it home and tested it, it showed that the alt had failed. Went and bought a brand new alternator from autozone and not a re-man this time, got the money back for the re-man as well. waited the 2 days for it to come in and then installed it.... after installing it i let the car warm up, and took the car in to high rpm for a brief second and noticed the same issues where happening on the dash. Having my father hold and watch the voltage out put we noticed that under 5K the alt was putting out 14.2-14.5v which is in spec but when taking it above 5k rpm the voltage output was 16.5-18 volts. Now i have another new alternator coming but wanted to reach out and see if anyone has had this weird issue. Ive even talked to previous colleges at mini from when i use to work at a mini dealer and they are kinda lost as well...

some weird things we picked up,

with the power wire off the alt but the 3 pin still plug in, No issue threw the rev range.
No change with the radio being out.
car will run with battery unhooked but still in high rpm has this weird power serge.
wire from alt to DME has .002 ohms
wire from alt to fuse block .003 ohms
engine ground check out and is making proper connection.
battery ground in good shape
positive side battery is making proper connection, as well as no issues with the trunk/battery box fuses.
No open shorts that can be found on the power wire from battery to starter and alternator
no open shorts in aftermarket radio harness adaptor
no change with removing the amplifier out the radio system with removing fuse and de-pinning it from the harness

The only things that are done to the car are the following
coilovers, adjustable control arms in the rear, 17% pully, aftermarket bypass valve, aftermarket seats and steering wheel with proper resisters in place to keep the airbag/seatbelt light out, JVC single din radio, and a couple other suspension mod's

so as you can see not a whole lot of stuff done to the car. It is a 02 S w/ sport pack.

Has anyone had an issue with over volting in the top end of the rpm range but not in the low end? there is no burnt smell from either fuse block, nor the DME. never have had any electrical issues either until safelite killed the bat.

Any input would help on what to look at because as i said before past colleges and my self aswell as my father whos works with electronics all his life is kinda stumped by this damn can-bus or what could be causing this, and replacing alternators every other day is getting old. Autozone cant be sending out **** alternators every single time(lest i would like to think.)

 

oh, forgot to mention, while it was at mini, they did the power steering pump and fan recall.

 

FWIW, and probably not a lot, my 2018 JCW battery voltage pretty much stays flat line at around 14.8V. While I'm not logging RPMs this is during almost all trips and I can assure you that at least once I take the RPMs up to 5K.

Pretty much....

However, I did notice last Dec. the voltage was rather low for some days, for example starting out at 12.4V then after not even a minute climbing to ~14.9V, then after 5 seconds falling to 12.7V then in about 5 seconds going back up and repeating this with longer times spent at the lower voltage levels: 12.4V, 12.7V, 13.3V, 13.9V. It did something similar over a nearly 38 minute drive. There are couple of days (in the middle of Dec. 2018) where the battery voltage looked like a square wave of varying high and low times.

I'm sure I would have recalled had the car acted up or in anyway behaved oddly. I was using the car every day or so. I did this on the 9th, the 11th, the 12th -- 4 trips on the 12th out of 9 trips total that day ranging 45 and 37 minutes (my work commute I'm sure) with the rest in the 5 to 7 minute duration).