Battery Question for '06 Prius

Just switched from an '04 to an '06 Prius and have had a few curious incidences.

About three times in the past three weeks I would pull the car into a driveway,
put it in park, and leave it running (still powered up). I would get out, and when
I returned to it just 2 minutes later (literally 120 seconds), I would find that
the main battery had discharged from 6 blue bars down to just 2 purple bars at the
bottom.

It would charge back up again as I drove it after that, a few minutes later I'd be
back up around the 6 bar level again.

This behaviour is something I had never seen before in my '04.

Is this normal? Can anybody tell me what is happening here?

2006 Gen II Prius, 102,000 miles

 

No, the engine never started to recharge anything.

Two days ago, I tried an experiment I read about online -

After a drive, I parked the car in my driveway, with the SOC meter showing 7 green
bars. I then engaged the parking break, and put the car into drive, and then began
timing with a stopwatch as I watched the SOC meter drain while the car pulled idley
against the parking break.

5 & 1/2 minutes later, it went from the 7 green bars to 6 blue bars.

9 & 1/4 minutes later it went to 5 blue bars.

2 minutes later I switched it out of Drive gear and put it in Park. There it stayed at 5 blue
bars for about 1 minute, and then plummeted all the way down to just 2 purple bars in
just 1 more minute. The engine then kicked on and it charged to 3 blue bars in a minute
or so, then the engine switched off again.

I then put it back into Drive and let it pull idley against the parking break again and timed how
long it took to drain from the 3 blue bars down to 2 purple bars - answer; about 5 minutes.

I put it back into Park, and it sat at the 2 purple bars for about three more minutes without the
engine ever kicking on to recharge it again.

I then drove it exactly 1 mile, wherein the battery recharged itself to 5 blue bars.

For the next two days, it seemed to work very well, even getting better milage than it did
in previous weeks (it had been doing rather poorly before then), until today.

Today I drove it and it appeared to be doing really well. But then the battery meter began
indicating all 8 green bars after I made a long expressway ramp exit. I then drove about 1
mile on a 35 mph road all on the electric motor and battery, a stretch which always took two
bars drain on my old 2004. But today, it started out with 8 full green bars, and plummeted
all the way down to just 2 purple bars. Afterwards, the gas engine felt rather lame as it
tried to pull the car. The battery recharged itself back up to 6 blue bars in the last 1
mile to my home.

 

While I have no Direct comparison, because I just switched from the '04 to this '06, it does seem to be getting 4-5 mpg less than the other car under similar temperature conditions. Also, the MFD numbers I was used to seeing on any particular road were much lower on this car than on my other. After two complete tank fills and a miles driven/gallons filled calculation, I've noticed about 5 mpg average less than my other car got.