I agree that 4 years in Las Vegas heat is pushing it. So, now I have two new batteries and the voltage while driving is 13.0 to 13.3. I will see how this goes. Since I have a digital voltage readout I can keep a close eye on it all the time.

 

ok here is why your meter dropped to 10V when you started it. when you initially start your truck it draws all it's power from your battery. as soon as the engine is running it switches over to the alternators power. the alternator will replenish the battery power that was lost at start up and then continue to run everything electrical in your truck. even though your voltage is a little lower than normal it still isn't low enough to really worry about. once it starts dropping below 12.8 is when you have an issue. also when your voltage didn't change after you turned the a/c on that is a good thing and means that your alternator is still able to hold a load. if it was really bad then it would have dropped considerably.

i am with you on changing the batteries now though. i really hate to be stranded. i am actually going to be changing mine out here very shortly even though they are still working good.

the duralast gold tops at autozone are an excellent battery plus they come with an 8 year warranty. if anything goes wrong with them you take them back to autozone and replace them with no questions asked.

 

mxer0022, thanks for the reply. I am not worried about the drop to 10 during start. Before I replaced the batteries my running voltage was low 12 to high 12. Since replacing the batteries it is staying at 13 to 13.3. About 6 months ago it would be around 14.

Can an alternator slowly go bad? If so then I will just upgrade when I get some more cash.

 

I've never had one slowly go bad, but I'm sure its possible. There is a couple good threads on here about getting the belt off. I personally paid someone to do it because after reading them I didn't even want to touch it I went ahead and had them replace the belt while it was off and got a alternator with a lifetime warranty and a couple more amps. Ended up being like 150 in labor. Kind of nice to have the peace of mind.

 

You do know that the air conditioning compressor is driven by the engine right?

Also, if you are down into the 12s, be it 12.8-12.9, you will slowly kill your batteries. Ideally you'd want at least 13.4-13.6, with 14.4 being perfect.

As far as the Duralast batteries, I heard they are made by Johnson Controls - the same people who make Optima batteries.

 

this is very true but it pulls a rather heavy load on the alternator to engage the a/c compressor clutch. it isn't purely mechanical.

this is also true. i didn't mean to sound as if i was saying that voltages to 12.8 were completely normal just that i don't think that you can deem an alternator "bad" until you get to this point. but i could be wrong.

i work at autozone part time, military pay isn't the greatest...haha, and i have never had someone bring in a battery and had the warranty denied. 8 years is a long time and knowing that i wouldn't have to buy any batteries is a nice piece of mind. especially two.

 

if you test both batteries together essentially you are testing 1 REALLY big battery. therefore the load for the test should be twice as large.yes you replace them in pairs even if only 1 has failed. however they must be tested separately to get get an accurate test. you just replaced both batteries and yet your voltage is still down, take a good multimeter and check voltage at the battery posts, not on the clamps.(every connection has a small voltage drop)if a diode is weak the alternator will be able to produce full vlotage as long as the amp load is low but as load increases its ability to maintain full volts is hindered. with the use of a good battery/alternator tester you can load test the alt and see if it can produce spec amperage. Lost wages huh lots of grit in the air to grind up alternator brushes and commutators,lots of heat to stress connections inside the alt,which can cause high resistance which reduce output. my opinion is that even with a dead or no batt at all you will show full volts when running, so if volts are down look at alt first. not a master but i have worked around 12/24 truck for about 20 years now. i have had lots of guys tell me they HAD to have all new batts, abut they passed load test perfectly, cleaned posts a couple new cable ends soldiered on and it started like a million bucks