starter battery goes flat

Normal. Better put the 12V battery on a battery minder or smart charger. Prius likes to be driven rather than sitting.

 

Yep, there's your problem. See the TSB at 2010 II 12 Volt Battery Dead After 17 Days of Parking | PriusChat. And, if you've killed 12 the volt a few times, you're going to need a new one.

Either follow the directions or get a good battery minder/slow charger for the 12 volt. I personally bought Amazon.com: TecMate TM-141DUAL OptiMate 4DUAL 0.8Amp Weatherproof Desulfating Charger/Maintainer: Automotive straight from Amazon (not from a 3rd party reseller listed on Amazon).

 

That's pretty unique, when my car was only a few hundred miles old, I went oversea for vacation for a month and come back it start up just fine.

 

It all depends of the stand-by power requirements. In the best scenario car will switch off SKS after few days and only alarm sensor will consume power. If you leave your keys near car SKS will drain battery very fast.

Typical Lead Acid batteries used in cars don't like deep discharge. They are constructed as a buffer. You discharge them only a little in between rides (for few days or so) and then during (a bit longer - 15-30 minutes) drive car will recharge and condition it. When charged it works as a buffer for 12V line for fast power surges, where inverter wouldn't be fast enough (during light switch, vacuum pump and so on).

If you discharge it below 50% for few times it may be dead and need replacement.
This is why HV battery only gets charged to 80%, maximum discharge level is 40% and it is mechanically switched off (beside obvious high voltage security reasons). If it would discharge deeper or stay fully charged for longer period of time it would be destroyed very fast.
And if anyone observed process of charging cell phones or computers (I know - other chemistry, but the same applies) - fast charging is often possible only up to about 80% SoC. Regen braking would be problematic after this level.

 

How fast is "very fast"? What is the power drain on the battery to continually run the outside SKS sensors and the computer to operate that part of the SKS when the Smart Key is not near the vehicle? How much more is the power drain if the Smart Key is kept inside the car when not in use? Is there a warning in the manual regarding this matter?

 

my car beeps if I leave the key inside it for too long. Noticed it when I had the car off and was cleaning the interior

 

I've never heard any beeps and the key is in the car all the time its in the garage. Perhaps it was because you had a door open.

I am going to experiment with battery voltage with the key in the car over night or not.

 

Maybe it was because the door was open..but it was sensing the key regardless. I know this is an issue with quite a few smart key vehicles across the $$ spectrum. Most manuals specify thattheykey must be quite a distance away. Don't know if Prius manual says anything though. I automaticaly keeo it away from my car .

 

The manual has quite a few warnings about the Smart Key. Don't drop it, don't expose to high temperatures, keep away from electronic equipment, magnetic fields and (my favorite) keep away from metal. It says nothing about leaving it in the car.

 

It always beeps when I leave my key in the car, but I have a tendency to always lock my car so the Prius won't lock it and will beep to indicate that either my key is in the car or my car hasn't shut off. Its good practice to make a habit of locking your door when ever you leave the car.