What the heck is wrong with my Prius??

For the past week, 80% of the time I turn on my car, my overhead light flickers and I get a warning light with the red triangle and "!" Today, when I got in from my car after work, there was a constant interior beep (with car off) and when I pressed Power, I heard a single honk + the red triangle (!). The final symptom is that if the overhead light is on, and the water coolant sound goes off, the light dims about 50% and flickers (FYI, I switched the overhead lights to LED a couple years ago).

I think this all started when I left my interior light on maybe 2 weeks ago. But since then I haven't left my lights on, and as I said, the problem is intermmitent. I've referred to the Prius manual as to what the red triangle (!) means and all it said is this (page 427): Master Warning light: A buzzer sounds and the warning light comes on and flashes to indicate that the master warning system has detected a malfunction. .... Take appropriate action in accordance with the message displayed on the touch screen. The problem is I don't see a message on my screen. Plus this is only a problem that is symptomatic when I turn my car on or off, and interior lights are on/off. There's nothing wrong with the driving or mileage, as far as I can tell.

What is wrong with my Prius? I'm worried one day it's not going to turn on... Thanks.

 

How do I check? Is this free at a dealership?

Edit: working on it..........

 

Thanks for the link I checked the 12V and I'm just as confused as before:

Vehicle Signal Check: was at 9.8 (12.4-12.8 is normal). Not looking good.
Pressed power to put a load on the battery: dropped to 9.4. Again, not looking good.
Braked, turned car completely on: Went up to 14.0. Isn't that normal?

So do I have a bad battery, or not? Does it absolutely need to be replaced? If it does, how much longer do I have until my car won't turn on? I feel like the 12V battery will recover over time, like a person's immune system, or a hybrid battery that gets recharged, haha. But unfortunately that's probably not how it works. I'm not sure what to make of those results...

 

Before I plop down $200, is this true??

If the 12V battery is low you can try using the Prius itself to recharge it by turning the car fully ON ("Ready", as though you were going to drive it) and letting it sit with the shift in "Park" for nine hours. It can take that long for a full charge because the battery is rated at 36 Amp-hours, and the system is limited to 4A. Of course be certain that the car is physically secure, and, if indoors, has an adequate air supply.​

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Read more: http://priuschat.com/threads/weird-stuff-happening-mpgs-dropping-test-the-battery.73400/#ixzz2LVImVs6F​

Also, is the one sold on eLearnAid the one that doesn't require the converter kit?

I also found this on Amazon, is it the same one (that is a direct replacement/no kit needed)?
Optima Batteries 8171-767 Yellow Grp S46b24r 450cca : Amazon.com : Automotive

Basically, where do you recommend I purchase from?

Also, for purchasing and informational purposes, I live in Los Angeles. It's 54 degrees now.

 

And you guys are absolutely sure that these warning lights are due to a low 12V battery? I'm only at 52,000 miles.

 

Thanks Andrew. I'm gonna call them tomorrow to see if I could pick it up.

 

Wait - I'm confused with what this achieves. Are you saying if I leave my car on overnight (8 hrs), then my battery unloaded will be back at 12V, no more warning lights, no need to buy a new battery, and problem solved??

 

Hah. Nope. I haven't learned nothin'. Most of those posts were about mods or like "hey I like your mod."

 

It sounds crazy. And you're SURE it won't ruin my car tomorrow morning? I gotta go to school.