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.

  • The last check, with the engine (and thus alternator) running, doesn't tell you anything about the health of the battery, only that the alternator is working.
    – jscs
    Dec 10, 2013 at 1:34
  • @JoshCaswell What about the first two? Do those tell me much if anything?
    – dpollitt
    Dec 10, 2013 at 1:48
  • 1
    The first one tells you about the battery's charge, but not whether is actually storing enough energy to start the car (that is checked with a load test, the steps for which Larry has in his linked post). It's kind of confusing, but I think this is a pretty good explanatory analogy: batteryuniversity.com/learn/article/… I don't think the second test tells you anything about the battery's health.
    – jscs
    Dec 10, 2013 at 1:53

2 Answers 2


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.

  • Thanks! It is in fact an AGM type lead acid battery according to my Bentley repair manual. I took the old battery out and headed to the parts store. They tested it and determined it was faulty, due to it's significantly depleting test results on successive tries. At any rate I have a nice new yellow top deep cycle Optima installed now, no worries for years! I do find it odd that the built in diagnostics are rubbish, but oh well.
    – dpollitt
    Dec 10, 2013 at 1:10
  • AGM do have acid in them, but it is absorbed into the glass mat. They do gas during charging and recharging but most of the gas is reabsorbed by the glass matting. Some gas will be vented to atmosphere via a vent hose. When the battery is fully charged, then the battery will gas freely. The differance in pressure will force the vent valve to close and force the process of absorbtion back into the matting to save water loss. This extends the life of the battery. It also ensures that there is not a lot of gas in the vehicle interior should there be a fault in the battery vent hose. Dec 10, 2013 at 13:27
  • @dpollitt, those auto parts stores may have little idea on how to use their own testers; those battery tests are supposed to show degradation on subsequent tests -- the tests involve drawing a significant current for a prolonged time -- you can't do that to any battery and not expect degradation.
    – cnst
    Aug 17, 2016 at 22:39

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.

  • Thank you for your comments Allan. The diagnostic test was not done in the same ambient temps, it was done in an unheated garage at about 5°F, so a bit warmer but not much. I could easily retest it tomorrow morning when the temps are around -8°F once again though. I will also look into this discharge test. From my research on the aux battery, yes it is a standard type internally, but the posts and size apparently are very specific and thus require either the OEM unit or the Optima high performance unit linked to, and thus the high cost :(
    – dpollitt
    Dec 9, 2013 at 16:54

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