Explain battery & alternator voltages please

[Mgicm]

I feel like im not fully up to speed so going to ask some basic questions on a 73 gmc with 350 engine.

1. Brand new battery reads 12.65v with truck off. When truck is running, i see 14.4v being sent to battery. Come back next morning, battery at 12.5 volts. Is this as expected? Or should battery be holding 14.4v? Or might something be draining overnight. I dont think it is draining overnight as right after i shut off it is about 12.6 and about the same the next morning but have to test more.

2. Had a few incidents where truck would not start after driving for 30+ minutes. I thought it was a hot start issue but might not be. Truck started fine, drove 30 mins and shut off. Came out 15 mins later and nothing. Sat for 3 hours and still nothing. Battery showed 12.5v. Got a jump off another car and started the truck like it was not even an issue. But it starts in the am cold with 12.5v. Totally puzzled.

Battery was brought back to store and checked to be good. no issues with that. Alternator is 2 months old and seems to be ok as i see the 14.4v when running.

[wfojon]

That is how they work, the battery is 12 volts and while running and alt puts out 14+ and that powers the truck not the battery.
You probably need a new starter, I just replaced mine for just that reason. When it got hot the starter wouldn't work but when cooled off it worked just fine now the heat doesn't bother the new one.
Jon

[jjzepplin]

Check grounds on cables. Etc. Cable clamps are also notorious. Spend a few extra bucks and get some good ones. Stuck at the grocery store with melting goods is worth a few bucks eh.

[Wgesnerjr]

Originally Posted by jjzepplin

Check grounds on cables. Etc. Cable clamps are also notorious. Spend a few extra bucks and get some good ones. Stuck at the grocery store with melting goods is worth a few bucks eh.

I once had this problem on an old Catalina. Turned out the positive cable on the starter was loose. didn't realize it until I was watching it start up with the hood open and saw SPARKS! Really scary.

Also, clean the ground point on the engine. May have a little rust preventing a good connection.

[Mgicm]

Originally Posted by wfojon

That is how they work, the battery is 12 volts and while running and alt puts out 14+ and that powers the truck not the battery.
You probably need a new starter, I just replaced mine for just that reason. When it got hot the starter wouldn't work but when cooled off it worked just fine now the heat doesn't bother the new one.
Jon

That happened 2x. But now the last time it would not start after 3 hours without a jump. Could it still be a starter issue. It is old.. i will say that.

Are ministarters better in some way than the direct replacement?

thanks

[wfojon]

I bought the life time warranty Napa starter for 49.99 and my old one as a core. You can't hardly beat that for piece of mind.

['76 Big Block]

You might check wiring going to the starter solenoid for loose wires/terminals, etc.
Sounds to me as if you may have a bad solenoid, but then, several things can cause the type problem you're describing, ( such as a worn neutral switch).
I'm running headers on my 454, and once they heat up, when I go to restart it, the starter will hesitate/turn slow, but usually comes up to speed after that----fortunately, mine's tuned pretty good, so starting is never a case of having to turn over more than once or twice....

[hatzie]

Easy enough to do static voltage drop tests on both the ground and hot main power cables in the charging circuit with a decent Digital Volt Meter.

Set your meter on VDC
Measure from the - on the battery to bare metal on the block. If you read more than .2v you have a loose connection.
Measure from the + battery terminal to the Alternator BAT terminal. If you read more than .3v you have a loose connection.
Follow the wires and test at each available connection to the battery till the drop normalizes at or below .2v.

[wilkin250r]

So you've already got one answer, but I'll give a little more detail. A 12V car battery is actually made up of 6 cells, each one just a hair over 2.1V per cell, giving you a full voltage of about 12.7V when fully charged. They'll actually hold a small overcharge, it's not uncommon to read as high as 13.5V fresh off a charger, but it will settle to 12.7 almost immediately when you put a load on, or after 24hrs sitting all by itself.

However, lead acid batteries like a higher voltage when charging, which is why your alternator is pushing 14.4V.


As to your other problem, it's very perplexing. When your engine does not start, are you getting literally "nothing"? No cranking at all, no solenoid noise, nothing?

I can think of several reasons you would measure 12.5V but not be able to start. But half of those reasons wouldn't disappear with a jump-start, and the other half wouldn't disappear the next day. NONE of those reasons would exhibit both of those symptoms, I'm totally confused, and would like more information.

[Nick_R_23]

Are you missing the heat shield on the starter solenoid? People are notorious for leaving those off when the original starter got changed many years ago. Likely you'll need a new one but also make sure the heat shield gets put back on as well. As everyone else has stated, check all the wires for looseness, bad connections, or bad crimp/solder joints.