I'm posting this for one of my best Buds.He drives the '03 with now 350,000+ miles on that I often refer to in some of my posts.Anyhow; he called up today with a question……………………..He told me while driving yesterday that the caps on the passenger side battery blew off. He found this out after getting home of course. He pulled out his tester, and that one registered 18+ volts while running and the drivers side registered 14.1 or so. Batteries are less than 2 years old.He picked up a new alternator and installed it and at the same time cleaned all battery terminals and cable ends…………………….he didn't clean the battery ground……………………I asked!!! Same results when testing the alternator.Battery cables is what I'm thinking now…………or battery ground.:2cents:My question to y'all is this………………………why only one battery and not both since they're connected in series???

I have seen this many times and its the opposite battery that is bad. The ecm read the voltage off the drivers side battery. I dont understand it either but everytime the drivers side battery is changed, the over charging goes away

So correct me if'n I'm wrong here……………………the voltage is taken from the drivers side battery but the alternator charges from the passenger side battery………………….so if'n the drivers side battery is bad, all the "voltage/amps" go to the passenger side battery irregardless of them being connected in series????

Ya dont ask me why but thats how it works. My buddies truck did the same exact thing about a year ago. Drivers side battery fixed it. I would test them both

Did not know this! I have no personal experience with this problem but it is getting logged away for future reference if needed.

Hot battery is a sign of a internal short and the battery should be replaced. To verify disconnect both battery in the evening and leave stand over night and measure both batteries in the morning the hot battery will most likely be near dead.

The batteries are wired weird, positive connection off one battery , negative connection off the other battery... but the batteries are NOT series, they are wired in parallel. + to +, - to -. Series would give you 24 volts! I think disconnecting both batteries & letting them sit overnight will help in sorting out the issues. Since the batteries are in parallel, both should be the same voltage. The fact they are not, tells a story. I've seen a shorted battery... it was too hot to touch & we cut the cables to get it out of the car. The battery sat & smoked for 3 days outside the shop. (It was my brother's car.) When it cooled it was dead as a brick.

Hot battery is a sign of a internal short and the battery should be replaced. To verify disconnect both battery in the evening and leave stand over night and measure both batteries in the morning the hot battery will most likely be near dead.

That's what I thought too but on my buddies truck it was overcharging the good battery. I would of assumed it was the hot battery but it tested good.

Since the batteries are in parallel... the alternator could be sensing the shorted battery & be trying to charge it up... But both batteries are connected so both are getting the charge but the shorted battery isn't taking it. Batteries should be replaced in pairs anyway.

Like Flagmanruss is saying.Say that the battery on the driver side is staying excessive cold then battery temp sensors reports back to the PCM cold battery and increases charging voltage. Then the other battery which is most likely boiling and hot is on the passenger side. Because a hot battery would cause the charge voltage to fall and be lower.

So correct me if'n I'm wrong here……………………the voltage is taken from the drivers side battery but the alternator charges from the passenger side battery………………….so if'n the drivers side battery is bad, all the "voltage/amps" go to the passenger side battery irregardless of them being connected in series????

This is a common sign of the crossover cable being loose or failed. ECM sees normal voltage, and doesn't realize the pass side is being over charged. The temp sensor is also under the drivers side battery, so a hot pass battery won't trip a code.

One semi I drove had one battery fail by getting real hot and stinking like rotten eggs. With one bad one the other three (yes, there were four in parallel) went dead. I disconnected the one bad one and ran the truck on the other three and when I got back all that was done was put a new battery in and they were still the same 4 years later when I left there. As long as the one battery checked out good I would just replace the hot battery, clean the connections and call it good.