6

The winter temperatures were very low overnight in the area recently, with a low of -8°F/-22°C. When I went to start my 2007 Toyota Prius that was outside that night, it wouldn't start on its own. I ended up jumping the Auxilary 12v battery sucessfully. I believe the battery is original, so around 7 years old at this time. I figured it was just simply time to replace it after this jump was needed. Then I found a video on YouTube that provided a way to check the battery condition. It suggested going into the diagnostic menu to do a vehicle signal check. I did this and my battery was within all measurements noted:

  • Under no load battery should be between 12.4v-12.8v = Success
  • Under accessory mode battery should be above 12v = Success
  • Under full powered on car battery should be between 13.6v-14.4v = Success

Based on these tests, is my battery in good condition and does not need to be replaced? Should I replace it anyways just because it did fail to start my car this once? I was surprised to find out that the Prius auxilery battery is made by Toyota OEM and only a single other manufacturer, and the price is quite high $170-220USD.

dpollitt
  • 233
  • 1
  • 2
  • 8

2 Answers2

5

The Auxiliary Battery in your Prius is a VRLA battery, either Lead Calcium Battery, or AGM (Absorbed Glass Matt). I believe 2007 used the AGM. They are similar to a lead acid (Flooded Cell) but it doesn't have any liquid in it. They are more resistant to vibration and don't off gas when charging which makes them safe to be in the passenger compartment. That's why they cost more.

The Auxiliary battery isn't used to start the car in the conventional way. It's used to power up the electrical systems and control modules, that control the high voltage battery. The HV battery is used to actually start the gasoline engine.

The test shown in the video you linked to isn't enough to determine the health of the battery. I have listed the steps here for a more comprehensive test. Ultimately most parts store will test you battery for free so I would definitely do that before replacing the battery.

With all that being said, unless it happened again or it tests bad at the parts store I wouldn't worry with it.

Community
  • 1
Move More Comments Link To Top
  • 18,788
  • 5
  • 54
  • 76
2

The very cold temperatures overnight you have described may have exposed a physical problem with the internal construction of your battery. The extreme cold will have caused a contraction internally in the battery causing it to expose a fault if there is one.

I imagine when you used the diagnostic test on your vehicle the ambient temperature was greater. This would make the diagnostic test inconclusive because it would not have been carried out under the same conditions.

One test you can have done is a high load discharge test. This will show any internal faults by a high drop off in battery voltage.

The aux battery on your vehicle is a straight forward lead acid battery. A battery with the same dimensions, battery post orientation and capacity could be used as a replacement.

Allan Osborne
  • 6,749
  • 13
  • 9