I've recently just purchased a 2008 G35x. After driving the vehicle for about 20 minutes the battery (and brake light?) come on.

I've cleaned the battery posts, and the alternator is spinning. It works intermittently as when I test the battery on cold start it puts out over 14V, but when the light comes on it stops working completely.

Could this maybe be a bad ground? Does the vehicle being warm have something to do with it? If anyone has any ideas or diagrams of alternator wiring I'd appreciate it. I'm not sure where the alternator grounds to but I'd like to check that. Thanks!

Sounds like the voltage regulator (part of the alternator) is failing due to heat. Probably just needs a new alternator.

The alternator grounds to the block directly. It could be the regulator like cleric mentions. You can rebuild them if you can find the correct parts, but it's easier to buy one. Check the wires and connections also make sure they aren't corroded or anything.

How old is your battery also? It might not be able to hold a charge.

Was this a flood/salvage vehicle?

Telcoman

It's was a salvage, but I bought it from the previous owner as a rebud

Knowingly purchasing a salvage vehicle you need to be prepared for electrical problems such as these.
Flood waters mixed with salt, toxic chemicals, sewerage, and other unknown items getting into a vehicle cause many future problems
Corrosion, poor electrical connections, sensor and wiring harness problems are why insurance companies are quick to declare these vehicles salvage. They know it is not economical to repair.
If you are unable to do the necessary electrical troubleshooting yourself and spending the time as well as the money you probably should do what the previous owner did.
Get rid of it and find another vehicle.
These vehicles can be unsafe in case of an accident.

Just my $.02

Telcoman

It was not a flood loss. I have the damage estimate and have seen pictures of it pre-repair. It was a front end collision to the drivers side. Though I am not ruling it out, the chance of the charging system failing due to this is quite low I'd say. I'm pretty sure it's just a failing voltage regulator/alternator, just wanted to see if anyone had any input.

Well in that case find an electric motor rebuilding shop and let them first bench test the alternator and repair it.
Better to rebuild your existing alternator than take a chance on some cheap replacement

Telcoman

The problem with a bench test is it won't experience the same heat as it would while installed. Since it DOES charge when the engine is cold it would probably pass the test, doesn't hurt to try though, explain the symptoms to the shop doing the test because they can always just shoot a heat gun at it for a while to simulate the ~150F engine bay temps.

Did you ever find a solution to this? Im having a similar but different issue. I also find that on startup the voltage goes up to 14, but then quickly drops to ~12.9V where it stays. Car runs fine like this, no lights on the dash etc. However when it rains, or more recently its been quite cold hovering around the freezing mark, the voltage doesn't drop, it stays at 14 or 13.8-13.9 for the entire day. Im really not sure which way round is supposed to be normal.

P.S. I use something like this to monitor. Also has a low voltage alarm when it drop to 11.8 or so

https://m.gearbest.com/car-charger/pp_598833.html

Sure glad I installed voltage and oil pressure gauges so I can monitor what's going on under SLICK'S hood! Voltage s/b between 14.5 and 15 volts, that's what the alternator on both my cars puts out. Anything below that I know I have an issue!
Gary

Yeah I need to put in an oil pressure gauge, did you use an oil filter sandwich for yours Gary?

How old is your battery?
Even a fully charged battery after a long drive is going to lose a small charge overnight.
When starting the vehicle the next morning the alternator/voltage regulator is going to initially charge the battery at 14.4 volts and then drop as the battery reaches a full charge.
Test the individual battery cells for their state of charge level with a battery hydrometer.



Telcoman

Battery is brand new. And up until we got into a serious cold spell it would run at 14.1V or so on startup, then drop to 12.9 after 3-4mins.

It seems some are running at 14+V all the time, the last few days it shows the 14.1 on startup, but only drops to 13.9 and stays there.

Yes!