Traction battery acting up... impending failure?

Edit: vehicle is a 2008 Prius base model.

Hey folks...

So, I'm in the middle of a cross-country road trip, from Nebraska to LA. About 500 miles into my drive today, I notice the car is laboring a lot trying to pass/go uphill, so I switch over to the battery display, and see that the battery is at 2 bars. It was windy, I had my air conditioning on, and it was starting to get hilly in the mostly uphill direction (just after crossing into Wyoming from Nebraska), so I figured that was the issue.

I needed a break anyway, so at the next town, I stopped and filled up the tank, and drove around town a bit to get a little stop-and-go going and get the traction battery charged, which I did, up to 6 bars (top of the blue range). I headed back toward the interstate, and was at 5 bars when I hit the onramp. By the time I was back on the interstate going full-speed, I was back down to two red bars again.

There were no indicator lights, so I kept going with it, being careful to try and not let it sit in the red range (slowing down to cut the wind resistance). My top speed was 75mph during this stretch, but I would slow to ~60 to get the traction battery out of the red when necessary. At one point I had several minutes of steep (5%) downward grade, during which I took my foot off the accelerator and let the engine charge the battery to full... however, once I started in the uphill direction (not nearly as steep), it only took a couple of minutes for it to get back down to 3 bars.

After that point, I never broke 4 bars again at all when I was on the interstate.

I thought the heat of the day + possibly working the car hard and long may have caused the battery to overheat a bit, and to the credit of that hypothesis, near the end of the drive when the temperature had dropped from mid-80s to mid-40s, the traction battery seemed to do much better in-town than it had an previous in-town stops.

The last disturbing thing was, right at the very end of our trip when we were driving slowly trying to find our hotel in town, whenever I would hit the brake pedal, there would be a short squeal right at the beginning of braking. I didn't try and pin it down much further than knowing that at that point the friction brake should not be engaging... honestly, I just wanted to give the poor guy a rest.

The car JUST had it's 80000 mile service done, and I asked them to do a careful inspection in anticipation of this trip; they said everything checked out. The 12v battery is <6 months old. So, my question is... does this sound like a possible battery overheat? If that was the case, would I not get a check engine light or something of that nature? Could there be an issue with my transaxle? (Batt seemed to charge fine when I wasn't fighting wind and hills)? Or, am I staring face-first into a full-on traction battery failure right in the middle of my long road trip?

 

Yeah, I noticed it right around Cheyenne when I was trying to pass some trucks, going uphill, with the wind against me.

For the speed I was going, the trip MPG is pretty close to normal (42MPG after 900 miles today, normally 42-45@75-80MPH, especially considering how hard the wind was blowing almost the entire time, which always kills my mileage.

I figured as much about the mountain driving being the culprit; the only thing that doesn't really fit that for me is that the battery just seemed to be discharging really fast even when the conditions didn't seem to be terribly taxing; I've certainly never seen a 3-bar drop just from an on-ramp before.

 

I'm only finding a reference to a recall for 04-07 Priuses, mine is '08. Perhaps I'm missing it...

Also... am I correct in thinking that an indicator light would appear any time a code is thrown? I.E. if I'm not seeing check engine or triangle of doom, the car hasn't thrown any codes for HV battery overheat or failure?