Good Ok, that's actually the "accessories mode" reading. This is basically the minimum power draw that you can have while still having the display powered on so that you can take the readings. This is the real telling reading, this one is called "Ig-On mode". Here most of the prius systems are powered on, so there is a fair bit of current drain on the battery. This is the test that sorts the wheat from the chaff. A battery in good condition will remain above 12 volts during this test and a battery which is marginal will generally be around 11.5 to 11.9 volts. At 10.4 volts your battery is undoubtedly toast. This reading shows that the DC-DC converter is working hard to recharge your sick battery. The battery has a higher than normal internal resistance (bad) which can actually cause a weak battery to sometimes have a higher voltage than normal here. Most people with a healthy battery only report about 13.9 to 14.2 at this test. In summary, it definitely sounds like your battery is sick. However you might want to test the connections to the battery terminals (as well as the pig-tail from the negative terminal to the car body) just to rule out those as a possibility. If no problem is found there, then it's almost 100% certain that your 12V battery is the issue.