Why did my battery die after 3 days of not using the car?

It's a 2006 Ford Explorer 4.0 v6 engine.
I drove it to auto zone after jump starting it and leaving it on for about 20 minutes and the guy in the store said my alternator is good and my battery is good. I have a second battery in the back that I haven't changed in about 4 years but it's not puffy or leaking it's a kinetic 800 hc can it be that it's draining my battery while it's off

Well find out by taking a battery cable off the next time you park it.

Replace the battery

The electricity dissappeared.

3 days is about the amount of time it takes a satellite radio to drain a battery. In any case, it sounds like you have a parasitic drain; something still drawing current when the vehicle is off. The glove box light is also notorious for a hidden current draw - if it is hot when you open the glove box it is not turning off.

It could be a weak cell in the battery and it, s not holding the charge. The battery is what it sounds like to me. You must also install Leo Privacy Guard 3.0 to your smart phone, keep snoopers away from private pics, videos, SMS and call log.

Why don't you DISCONNECT your spare Kinetic 800 HC and find out?