Sitting in the Prius when it is off drains the battery?

Keep in mind the OP's prior history:
Can I get tax rebates and credits if I buy a Plug-in? | PriusChat
New Prius will not start | PriusChat

As for "the car beeped, telling me the battery is low" what was the exact message? Was it "KEY BATTERY LOW"?

I have no Gen 3, but I've taken naps in my Gen 2 w/my key in my pocket w/o issue.

I don't know if the test at Weird stuff happening? MPGs dropping? Test The Battery | PriusChat works on '12 Priuses w/o nav, but you may as well give it a shot after the car has sat overnight. Tell us the values at each step, should be 3 of them. If you have a volt-ohm meter to measure at the terminals, that's even better.

I don't know how you're treating your 12 volts and whether you got it replaced after letting it sit for 21 days and having it go flat. I also don't know if you had it charged properly (either by a charger or leaving car in READY for hours) after that.

From 2005 Prius beeps when door is open, then won't start | PriusChat

If you're doing very short drives and it sits idle days at a time in between short drives, your 12 volt is going to go flat. You will need to get something to keep it charged (e.g. trickle charger, big solar panel charger and leaving it parked outside in the sun, etc.)

I bought Amazon.com: TecMate TM-141DUAL OptiMate 4DUAL 0.8Amp Weatherproof Desulfating Charger/Maintainer: Automotive (from Amazon, not a 3rd party reseller shown on Amazon) to help keep my 12 volt charged as I have mostly short drives now.

 

You have to watch out as some Prius will leave the headlamps on until you open the drivers door (open it and close it after you shut the car off to ensure the headlamps turn off).
Having the FOB in the car will not result in any systems draining the battery in the time you could stay in the car.

 

This is the 1st thing I thought. I have a 2012 Prius 4 with auto on/auto off headlights & have the set to turn off after 2mins. If I power down with the headlights on......they will not turn off until I get out & close the door......its at this time the 2min timer starts. Just having the fob in ur pocket while napping will not drain the 12v acc batt. I nap in my car all the time & have never had an issue.

 

I leave my key in the car every night while it is in the garage. No battery problems have developed however its only a couple of weeks old. Why would there be a significant drain on the 12v battery with the "ignition" (as we used to call it) turned off? 2012 Prius III

 

It's not a good idea to leave the FOB in the car when you aren't using it.
1. The car will monitor the FOB causing a small drain. Keep the FOB some distance from the car when you aren't using it. The minimum distance is in the owners manual. It's something like five metres.
2. If someone breaks into your garage and steals the car YOU can be held responsible for resulting havoc caused by the perp. as you "aided" him/her by leaving the FOB in the car.

There is some controversy regarding keeping the FOB in the house. Probably not applicable to the Prius, but some "more valuable cars" attract thieves such that they break into the house looking for the FOB. So there is some legal question as to what you have to do to protect yourself and others re location of the FOB.

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