The battery died (or at least got weak enough to trip the error codes). We got the big red triangle and several other trouble lights on the dash, which is exactly the indication that is expected. It happens all the time on older Prius's. The PriusChat forum, which is a huge Prius owners forum, is full of stories. I suggest anyone who is considering a Prius go look there. The batteries will last longer even much longer in cool/cold climates like the Northwest US (Washington, Oregon, along the CA coast). Hot places like Texas (and Arizona, etc) cause batteries to fail sooner. It is simple physics. The heat drives the degradation process. Regular 12v batteries don't make it past about 3 years either in Texas (I have a couple of Honda Accords to attest to that fact). If you are lucky, and you call Toyota directly, you can get sort of a rebate check from Toyota for part of the replacement cost of the battery. I did.