Alternator charging

30 mins probably enough to start it again warm, a few hours to have enough to start cold. 13.1 not enough, alternator should be closer to 14.2 volts. Likely has blown diodes.

 

The alternator is probably 150 amps.
Someone else might know the exact answer.

The batteries are close to 1000 amps. X4 and that's close to 4000 amps.

So how long would it take 150 amps to charge 4000 amps?

When the truck is off..there's a parasitic draw for the electronics. It's not much. But if the truck is dead. Runs for 15 minutes. It'll probably be dead again after 24 hours.

It usually takes a full day to charge em back up.

But you have a problem. Low voltage alarm and truck shut down. If the alternator were working. You wouldn't have that problem.

A good alternator will be able to run all accessories and still have .5 volts left to charge the batteries. That's 13.1 volts minimum with everything turned on. You had 13.1 volts with everything off.

Dead batteries usually start with 13.5 and climbs as the batteries charge up. To 14ish.

Doesn't sound like your alternator is working.

 

I jumped a dead truck the other day. 10volts and zero % charged on battery monitoring. Took 15 hours at idle with nothing on to charge 4 group 31’s to 80% state of charge.

 

If you want to charge the batteries a little faster, don't idle, bump the engine up to 1100 rpm.

 

No, 14 volts just means the alternator is now working at normal output. The only way to tell from the dash would be if the truck had an amp meter, I’ve only ever had one truck that had one. It would show 100+ amps while charging and then it would slowly lower to 10 or 20 amps once the batteries caught up.

 

Why did you change batteries and alternator 4 months ago? Was it the same reason as now?
Check all your battery cables including those to your alternator.
If your truck is shutting down while it is running either your alternator isn't charging, your batteries won't accept a charge or a short in the system. Could be 1 of your batteries is bad. My alternator sits at 14.1 all the time. Low idle or running at 70 mph.

Get a different source to charge your batteries. When fully charged disconnect all the cables. Check each battery for a full charge. IF all four aren't fully charged you have a bad cable or bad battery. IF all 4 batteries are charged, leave disconnected, and check for charge again an hour or so later. See if any lost power. IF they check out ok. IT is your alternator not charging. Or you have a cable issue. Could be a short in the system.
Get one of these.
good luck

How long has your charging system been acting like this?