I got my used 04 SR5 a little over a month ago and noticed an issue the other day. First off, the truck starts perfectly regardless of temperature or other factors... unless there is a load on the battery for literally a minute or two with the truck off.

I noticed this one day after turning the truck off and leaving the doors open for a few minutes (meaning I didn't lock the truck and the headlights were on). This drained the battery enough to the point that the truck wouldn't start. I tested this another day and the same thing happened.

I went to Autozone and according to their battery test, the battery is fine (it's an Interstate battery that looks brand new). They told me the voltage is a bit low with the car running (13.5) and said maybe it's a corroded connection somewhere. They said the battery was only 84% charged after I had been driving the car for a while... but said that it's unlikely to be the alternator. So based on all this, what do y'all think? Thanks!

Did they test the alternator?

Yes, using the same device as the battery tester... with it hooked up to the battery so I don't know how much I trust it. I've had plenty of people tell me that Autozone tested their battery/alternator, said it was good, and it turned out it wasn't.

recharge the battery first until its full and test again. the alternator is there to maintain it but when fully discharged, will not work.

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I disagree but like I wife says, I'm always wrong. Say you jump a dead battery and let it run, the alternator should put out about 14.5 volts and charge that battery. So it not really there to just maintain but to also charge.

If you don't have a volt meter buy one, they're cheap and have many uses. I would check the voltage while it's running. Then I would check the voltage when it's off and wait an hour, check it again. It would have to drop off quite a bit for it not to start. My guess is that it's a battery issue or a drain somewhere.

If I'm right let me know so I can tell my wife

you are right, it maintains it and charges it. but if its really very low, you will need to run the car maybe all night. like for example a few days ago, i recharged my battery since i drive it for short distances, around 6 miles to work and 2 mile drives. i pretty much start it 5-6 times a day. it took 6 hours to completely charge it. those short drives are not enough to completely charge it thru the alternator.

to the OP, check how old it is and your driving habits, you might just need to recharge or replace the battery.

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Exactly. Very little amperage goes to charging the battery. Most the amperage from the alternator goes to running the electrical system in the vehicle. If you have a low battery it is best to put it on a trickle charger. That will prevent you from overworking the alternator and causing premature failure. Plus car batteries last longer when they are kept fully charged.

Their Alternator tests are crap. I just took my known bad Alternator (battery light on, bearings screaming) to test for fun. It passed. You need to find a place that can do a legit load test. The voltmeter is a down and dirty to see what output really looks like.

Ps - sounds like your battery has some issues if 5min of lights will kill it.

Some other thoughts.
- The advice to buy a multimeter and check your voltages is right on. If you're getting 13.5-14.0 volts or so with the engine running, lights on, etc., the alternator is fine. No need to poke into it further. And don't get fooled by the argument "it can supply voltage, but maybe not current". If the alternator can hold 13.5 volts, it is supplying all the current that the downstream circuits and battery can accept at that voltage. If the alternator was current limited, the voltage would drop. That's just basic Ohms law of electricity.

- A properly working alternator will put out around 13.75 volts. The battery will accept whatever current it can at that voltage. The current will be highest if the battery is somewhat discharged, less if it is deeply discharged or almost fully charged. A dedicated battery charger will typically put out 16 volts or more, which will charge the battery faster, but at the risk of boiling electrolyte if it isn't well controlled. The alternator can certainly fully charge any lead-acid 6 cell (12V) battery, including an AGM.

- Your symptoms point to a bad battery. Easiest thing to do is to replace it, clean the connections squeaky clean (bright metal on bright metal) when you reassemble things, and don't look back. Since your battery is of unknown history, this will also give you confidence for the next 4-5 years.

- 90% of intermittent start problems, in my experience, have been caused by defective batteries. Most of the rest are bad connections. Alternators are way down on the list of things to check unless (a) the charge light is on, or (b) the bearings are making a lot of noise.

Edit: One other thing. When you measure the voltage, measure it directly on the battery terminals, and then on the cable clamps on the battery. If there's more than .1 volt difference, you have a connection problem.