Battery/alternator problem

[MDmounty]

I went to start my truck up this afternoon and it started to crank the engine good but by the 3rd crank it gave up and just started clicking. The same thing happened about 4 or 5 months ago too. So I jumped the truck and metered at the battery with the truck running and was getting 9~10 volts. I threw the charger on my battery and let it sit for about 30 mins and when I checked on it the charger says it was full. So I tried to start it up but no luck. My battery gauge has also been a bit lower than it should for the past few months.
I metered my wife's battery with her car running and I got the same voltage but I also read if your battery is fully charged then the car will reduce power going to your battery. I did that to compare the readings but her battery is fine.
Thanks for any help

 

[SupaSwope]

Sounds like the battery. Check the alternator to see if its send 12.5+V to the battery. Also theres a little test that works on some cars never tried it on the explorer where you can take the positive terminal off while the cars running and if it still runs its probably the battery. Also it could be as simple as bad connections somewhere in the cables.

 

[Turdle]

You did not mention cleaning the battery terminals and posts. Even if they look clean, get one of these and see if it helps.

 

[koda2000]

12.5 volts is just battery voltage. You should see something around 13.5+/14+ volts measuring from the alternator with the engine running with lights and accessories off.

 

[MDmounty]

So I've cleaned off my battery posts and threw some petroleum jelly on them and I'm charging it right now I'll see where the day takes me

 

[MDmounty]

Update:
So I charged my battery all day, put new terminal clamps on (the others were badly corroded), and removed all corrosion from the battery. I started the truck and it was very weak turning over still. It barely starts the truck up. All voltages are good, 12v when it's off and ~14v when it's running.

My battery was installed in July of 2012. How long do batteries usually last?
I'm leaning towards a battery now

 

[koda2000]

Generally speaking, I find batteries are good for around 5 years. Some last a few years longer, some don't make it to 5 years. IMO anything over 5 years old and your living on borrowed time. Sooner or later they'll leave you stuck. I had replaced the battery in my '01 5.0 EB in Aug '12 and it crapped out on me last winter (Dec '16). Today's batteries don't give you much warning before they go. They seem to work fine one minute and won't start your vehicle the next.

Most auto parts stores have a device that they can use to determine if your battery is failing by doing a "load test". They will do this test for free.

If your battery tests as GOOD it could be your cables need to be replaced. You said the terminals were very corroded, are the cables lumpy looking? You also need to check the terminals on the starter and ground ends. If the cables are good it could be your starter is failing. Is your only current symptom slow cranking while starting?