Hey guys, Monday Morning I went out to my duramax to start it after sitting all weekend. It turned over just enough to start. I drove to work hoping it would charge and wondering why my 3 year old batteries were weak. After work it struggled to start, then I started to smell rotten eggs. When I got home I followed the smell to the passenger battery. Sulfur dioxide... Gross!
After a google search, other forums had several thread about the passenger battery boiling. Weird thing is that when I load tested the batteries they showed strong. Hmm, re-read and found someone that said their alternator regulater had stuck open and over charged/boiled both batteries.
Next morning the truck started fine. While on my way to work the gauge started creeping down from 14 volts, great... Got parked nose out then shut'r down. Of course, batt dead. After work I pulled the alternator and walked about a mile to the local parts house. Tested fine, WTF? Walked back then pulled the batteries. Pissed off walk back to parts house! Sure nuff, passenger batt had a weak cell. Bought new batteries, all good now. Hope this will help someone one day!

 

Never heard of an an alternator boiling a battery!

Glad you got it fixed!

 

What happens is, when the regulator fails it sticks open or closed (usually closed) but, on occasion it can stick open. This means that the alternator's 105amps are on continuously. This in turn overcharges the battery. The battery when over charged gets hot and if left unchecked starts bubbling out sulfuric acid (what I called boiling). When the lead plates inside get hot like that, they can warp and short out internally. About ten years ago I saw a guy loose his boat bcause of this. His regulator quit on the water, he wired it "on" and by the time he got to the dock it got the battery so hot it caught fire. Goodbye boat, fiberglass burns FAST.

 

damn, good to know!!

 

It over charged the battery.

 

@ the OP so the regulator sticking in the open position is just an intermittent thing? So this can happen again and kill another battery?

 

seams to me that it was more likely to be a shorted cell in the battery that was the problem.

a battery with a shorted cell will be a 10 volt battery. 14 volts on a 10 volt battery will cause it to draw a whole lot of current and boil. it will also discharge the good battery to its voltage. the more cells shorted the worse the problem. when a battery worms up its internal resistance goes down and more currant will flow, this in turn will heat it up more causing it to get hotter and draw more until it boils all of the water out or shorts out complacently (assuming it doesn't blow a fuse first)

once a regulator, transistor, mosfet, etc goes bad it wont start working correctly again. if you have a intermittent broken wire in the alternator then it would stop all together or work but not stay on all of the time. if it was a broken control wire to the alternator then the regulator would default to 12 volts to keep everything working

if the alternator checked out at the auto store then i wouldn't worry about it, instead i would be looking at the batteries

my 2 cents

 

The post above is right on the money. Newer alternators are designed to fail safe, meaning that when something goes wrong it "should" just stop putting out power.

For the record, by the time this was all over both batteries had shorted out dead cells. left battery 10 volts (assume) one bad cell, right battery 2volts... They were under warranty and battery guy said that in 17 years this was the first time he ever saw two go bad at the same time (a week apart). Right battery lost a cell went low on power and caused the over charging and rotten egg smell. 2004lbz is right on, the system saw low power and just kept crankin power to the dead cell. I'm still puzzled why the other batt got killed too but, not worried because both warranties were up next month and now I have two brand new crappy batteries!!!