Wife's '05 Durango was dead as a doornail this morning. Hooked up the battery charger, battery was drained. Ran the charger for a few hours and got it up to 50%, at which point I started it.

Let it idle for 20 minutes or so, shut it down, restarted fine.

Fast forward about 5 hours. Battery is flat again, won't turn over. Back on the charger, battery is showing 25%. Whaddya think, battery or alternator? And is there an easy way to check?

Recharge the battery and leave it disconnected. If possible load test the battery. If it stays charged there is a draw on the car.

Put a voltmeter on the battery and start the car. If the alternator is charging, the voltage will increase.

I'm leaning toward a battery - the charger has been on it all night (about 12 hours) and it is less than 75% charged. This is after starting at 25%.

In contrast, I can charge my boat batteries fully in about 4-6 hours. Unfortunately, all my tools are down at the boat (long, stupid story...) so I've got access to nothing. Think I'll take it up to Sears and have them test it, as I've got a feeling I'll be buying a new Diehard at the same time.

'05...Isn't it still under warrantee? Let the dealer worry about it

With an alternator it takes voltage to make voltage. A dead battery will not "excite" an alternator enough to make it charge. Disconnect the battery, charge it overnight and load test in the morning. If it falls flat, you have found the problem.

A simple test to check if an alternator is working is to put a screwdriver on the back bearing while the engine is running. If you feel a magnetic pull, the alternator is working. Back to basics: Make sure all connections are clean and secure. Good Luck.
If it's a Sears DieHard, it's probably a defective battery. Exide is the way to go.

05, simple solution. Dealer's problem.

My truck started doing the same thing. I know the battery is 5 years old now. I tried charging it last week but it didn't last.
Went out on errands yesterday and out of four starts I had to be jumped twice. For emergency use I pull one of the batteries from the boat and put it in the truck to keep it running till Friday.

do a draw test on the battery,anything over 25 milli amps
with kill the battery.
make sure nothing is left on in the truck like the sun visor lights,
and make sure the under hood light goes off when you close the hood.

To do a proper test on both the charging system and the battery.... You MUST have a tester with a carbon pile that is capable of puting a ten second load on the battery equal to 1/2 the cold cranking amps... That means if your battery has been fully charged and you put a 425 amp load on it for ten seconds it should not drop below 9.5 volts during the test. After the test has been done you can then determine how much amperage is coming out of the alternator. You Must have a tester that has an inductive lead that surrounds the ground cable for this test. This isn't some cheap "chrome toaster" with heating coils from a Trak Auto for 19.95 that clamps to your battery terminals.. My snap-on AVR tester was several thousand new. Good luck.