Prius Aux battery dead?

I am wondering if anyone can help me. The exact same thing happened to my 2006 Prius. I got home from a trip and the car had not been driven in about 3 weeks. On Sunday, it was very cold and the door did open with the key fob and as soon as I opened the door, there was beeping. When I tried to start it, all of the lights on the panel would light up, the interior lights looked dim, but it wouldn't start. The manual group lock would not work. We had a lot of snow, so I didn't do anything about it. Now when I try to start it, there is absolutely nothing lighting up.

The 12V battery was replaced in May 2020. I would like to think it's still ok? I do not have a voltmeter. What is the best course of action here? I do not know anything about cars. Can you explain like I am five? Is it hard to jump start--I have never done that. Is there anything special to be concerned about with jump starting a Prius? It is safer to use a portable charger? Should I take the battery out and have it tested somewhere? (How do I get it out?) Should I just call someone and pay them since I am so incompetent? Thank you!

 

Making assumptions is one of the biggest time wasters when trying to diagnose any (car) problem.

Get one and watch some YouTube videos to learn how to use it.

Maybe. Some have reverse polarity protection which can save you from something stupid.

The biggest screw-up is a reverse-polarity jump. Don't do that.

Definitely get it tested but is not necessary to remove it from the car, although it can be easier (testing-wise) if you intend to take it to an auto parts store (rather than the whole car) for a free check. As for the second part, I'm sure there are good enough YouTube tutorials out there, but really if you have watched any and you don't feel confident, see below.

Given these admissions, if you're not the curious, willing-to-take-a-risk type, then yes, find yourself a competent professional. Not everyone is good at everything.

 

Yes, call AAA or any other major roadside service and have them:

1. first test the the 12V using the jump point in the fuse box under the hood. (To open fusebox: pop the hood, see a large black plastic box near the front fender on the driver's side, reach down on the front side of it and press the piece you feel to release, lift from the front. DO NOT lift from the back, it will snap off the two hooks there.) Look on the passenger's side of the open fuse box for a little door that flips up - like the destruct button cover in a movie, flip it up, that is the jump point. Note the voltage because you may ask other questions here later and that is the first piece of information we would need. They will probably use an electronic battery tester rather than a voltmeter. (When done flip the little door back down and put the lid on the fusebox by engaging the hooks at the back first and then lowering the front until it clicks.)

2. If the service person thinks it is possible, which they might not if the 12V is reading less than 10 volts, have them jump the car. The positive (red) lead goes on the jump point, the negative (black) anywhere on the frame. If they get it backwards it will screw up the car badly, so watch them.

Now, assuming the car starts, you will want to run it for quite a while because if the 12V battery was that low it will take a long time before it is charged enough to start the car again. The service guy may say something about running at high RPM, but that is advice for a car with an alternator, which this car doesn't have. It charges the 12V from an inverter, and it does so at a pretty low rate which is essentially independent of anything else you do to the car.

If the HV battery has run down there will be other issues, but for now let's assume it is just a highly discharged 12V.

Be aware that a deep discharge like that will probably damage the 12V battery. So even if you get it working for now, it is likely to be on borrowed time.

 

Given your comfort factor with things mechanical an AAA membership might be the best option. They are experienced, have the right equipment and training. In this weather working on a car outside is no fun. Check the levels of membership for AAA, years ago we upgraded to premier and got a free battery with warranty. Not sure if the free battery is still a thing but regardless AAA is worth the money with an older car.