Increasing Main Battery Errors

I've been receiving an increasing number of Main Battery errors on my display recently, usually just after I've started the car and switched into reverse, though it also happened once when I was going up a steep road into the mountains.

I am the original owner of this 2002 Prius, with just under 70,000 miles.

Typically, I turn off the ignition for 5 to 10 seconds, try again, and everything is fine, though today it let me drive but ran entirely on the battery; the gasoline engine wouldn't start. So I pulled over, waited the 10 seconds, and again everything was fine. When it gave me the error today, I checked the battery level in the Info screen, and it was at 80%, though I notice it typically runs about 50% these days while it always used to show 80%.

I'm naturally getting a little nervous at this point, and I don't trust any of my local dealers, who would like nothing more than to sell me something I don't need at an overinflated price (they wanted $430 to replace the 12V battery, ha!; I bought it at the parts counter for $170 and paid someone a few bucks to install it).

Any armchair analysis would be welcome. Bad inverter pump? One bad cell in the big battery? Need to replace the whole shebang?

Thanks!

 

No. The ICE starts immediately upon key turn. Then I shift to R. But maybe I shifted a little too quickly?