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Discussion Starter #1
I have a 2008 Frontier. Car wouldn't start this morning due to low battery (lights worked, tried to turn over, but wouldn't). I cleaned the terminals of corrosion, jumped it, let it charge for half hour while I was busy doing something, then went to O'Reilly's to have the charging system checked. They said the battery was fine, but there was a voltage regulator error and I needed a new alternator.

Can the voltage regulator fail intermittently? Because the alternator appears to be charging the battery now.

There have been no other indications of electrical problems.

My multimeter wasn't working so well (maybe needs new battery) and it was hard to get a reading to stay for long, but it did appear the battery had about 13.5 volts with engine running (and A/C on low I believe) and 12.5 volts with engine off.

I do have 230,000 miles on the original alternator, so I wouldn't doubt if it's going bad - I just want to make sure I understand what's going on.
 

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I don't know if the 2008 alternator or charging system is any different than my 2012 but I will give some of my experience over the last while.

Last spring I was noticing flickering of the headlights on cold mornings so I picked up a code reader that could give me realtime run data. I didn't want to start changing parts if not needed so I used this as my excuse to finally pick up a code reader. On my 2012 the charging system usually runs at 12.9 volts or at 14.1 volts. I think on alot of the older systems most alternators put out 14.5volts almost all the time. Newer cars have much different setups and the alternator output varies greatly depending on system load etc. My code reader has live data with a graphing capability so I would hook up mine and go for a drive. It was nice to see how the voltage changed and after awhile I started to understand how it was controlled.
12.5 volts is good or truck not running. Here is a link to a nice chart on battery state of charge.
http://www.mmbalmainauto.com.au/PDF/State_of_charge_12_volt_batteries.pdf

My battery was 12.2 truck off,(50%charged) but I still never had an issue during starting. I would suggest monitoring voltage during startup. Hook up your multimeter and set it up so you can see it through the windshield. You will see a drop of voltage during starting but you want to make sure it doesn't drop too low. Here is a video describing the process.
If you have access to a load tester I would do this as well to give you another idea on how it is performing before I started to swap parts.

If I went to a shop and they saw 12.9 running volts I am sure they would say I need an alternator, but I know now 12.9 is normal on my truck. If you watch your voltage gauge you may see it usually sits in two different positions. Mine does and after getting used to it I can quickly tell if I am at 12.9 or 14.1volts.
 

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Discussion Starter #3
Thanks for the reply jrussell. I used a multimeter at work yesterday afternoon and checked - 12.4 volts when off, 14.2 volts when engine turned on. Turning on lights and A/C it dropped into low 12.9-13.2. I stopped at Advanced Auto on the way home and put it on their tester - no errors noted, but we took out the battery and charged it for 35 minutes and re-tested. This time it ran around 14 with lights/ radio/ A/C off (engine running). After turning everything on, it would dip to 13.5 when A/C compressor kicked in, but then go back up to 14 when it kicked off. I'll watch it more over the next couple days, but I'm thinking the corrosion was just preventing the battery from getting a decent charge.
 
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