Will Prius show signs of weak battery (12V & Traction)?

The 12V battery will start showing signs of random warning lights, diagnostic trouble codes (DTC), etc. There is no warning light just for this battery. The best thing to do is check the battery with a voltmeter on a regular basis. Most on here agree that it is best to change out in about 4-5 years.

As for the High Voltage battery, it will normally take care of itself. If you are having an issue with this battery you will be seeing DTC's and warning messages and need to take it to the dealer.

 

I think if my 12V battery lasts 5 years I will replace it, could be a real pain it goes dead at the wrong time (which it would). With a non-HSD car I can usually tell because the starter slows down, but not on a Prius.

Traction battery should last for a couple of hundred thousand miles (I hope) so I am not even going to worry about it.

 

The first time you have to jump the 12V, replace it. Failure generally occurs overnight or after days of sitting undriven. See the banner ads that have been running for the new Optima drop-on replacement.

The traction battery will not show signs of weakness until cresting a combination of age and mileage, usually 5 years and 100,000 miles at the very minimum. A drop in MPG is a common sign. My guess is that 95% of traction batteries will last the duration of 10 years/150000 miles, otherwise it would not be a viable product for Toyota if they were expecting to replace a high number of these under CARB warranty. I do not expect to see any reports of Gen III battery failures for a few more years, except for cases of true manufacturing defects.