01 Expedition - Battery Dead in 45 Minutes

New problem that just started a couple weeks ago. We live in FL and the past few weeks have been colder usual. In the morning, the truck would take a while to turn over. Now over the past 3 days, it's dead in the morning. Today, went to leave in the morning to take the kids to school - dead. Jumped it, turned right over. Wife got back in the truck after dropping the kids off - dead. I meet her, jump it, turns right over, now we are back home.

Once we are home, I leave the truck run and use a DMM to check the alternator voltage. It averaged 14.4 so I'm guessing the alt is OK? Go to leave two hours later - dead. Ok, jump it again, turns over, check voltage, measures around 14.4v again. I left the truck run for 10 minutes, then leave to pick up kids (15 minute drive), leave the truck running at school (5 minutes inside), go back home (15 minutes) and park and let it idle. So for roughly 45 minutes, the alternator has been recharging the battery. While I know this isn't the best way to charge a battery, my charger quit and I didn't have another solution.

Turn the truck off, immediately measure current voltage (engine off), reads around 12.8.

I then gave it 5 minutes, now read 12.6.

I gave it 10 more minutes, read 12.2.

Waited 30 minutes (now 45 minutes from key-off), read 10.6.

Battery is 3 years old. I was going to replace the battery when something else crossed my mind from the past... can a diode be defective? I was going to take the alternator out and have it tested @ Autozone but I read somewhere else that their benchtest may not pickup a faulty diode.

I dont have a problem replacing the battery and/or alternator but I'm trying to narrow down by testing which it is. I have a non-autoranging DMM but my capacity with it is limited.

It's probably time for a new battery, because it's not holding a charge like it once did.

Have both the battery + alternator load tested 1st.
Fix ASAP, because your creating too much of a demand on your alternator.
I hate to see it go bad too.

Thanks for the reply. I was going to replace the battery anyway because I'm sure it's been weakened by several recent jumpstarts and figured I would have AZ test the alt for me. My concern is how accurate their testing is. I have had them test alternators in the past but I still question the results. I assume that their test would be able to tell if a diode is shorted?

Update today... battery and alt came off last night. Took the alt to AZ, they benchtested and all three test passed. I noticed the third test was specifically for diode testing. So I got back home wanting to open up the alt and test the diodes myself. That stopped being I couldn't get the 1" nut off. So put the alt back on and will be taking the batt back for replacement. I'm leaning back towards the batt again being last night when I took it off I read 10.25v, today when I put it back in and before reconnecting the cables, I'm now reading 8.6v.

1) Disconnnect one side of the the battery and connect a amp meter to see how many amps are being used with the ignition switch in the off pos.

If Amps are being used, focus on Batt fuses in the engine compartment.
Isolate fuses, by pulling one fuse out at a time until no amps are being used.

What ever fuse is burning amps is your problem, now you need to know the fuse # , so you can troubleshoot bad circuit, or control module.

So far, so good. Replaced battery on Tuesday and able to start without problems. Its been 3 days and no problems (knock on wood). There is still a slight parasitic draw but that will be tackled over the next few days.

It sounds like your in good shape.
I'am glad you caught the problem, because a bad battery can create a big demand on your alternator, causing it to go bad.
There will always be a little amps being used for security reasons.