Aux Battery - How long will they go? Nine years here.

That’s a very good run, likely due to your high usage, kept it well charged. Still, I’d start shopping, at least by the fall. A lot of automotive retailers selling batteries these days have electronic load testers, can make an in-depth assessment of its health.

 

Yes. It is 9 year old. That is reason enough.
Unless you like unexpected adventures.

12.44 indicates that it is roughly 65% charged, which really isn't too good.

 

At 7 years I could tell the lights went dimmer when the car was off, so I preemptively changed mine.

 

After parking for 24 hours in cold weather still having 12.44V, the battery looks good to me. Sure, one can say it should be replaced purely because of its age, but where do we draw the line? 9, 7, 5, or 3 years?

Ultimately it is a personal choice.

 

I'd suggest you replace it. Be sure to take off the gas-vent tube on the battery as the replacement will not have that part. You will lose your presets, but it will give you peace of mind. I waited too long on my battery and my Prius went into "suicide mode". Here's a link on the thread:
Prius Won't Start | PriusChat

 

If you don’t use a memory saver. No big deal though.

 

I not sure I agree "12.44 indicates that it is roughly 65% charged, which really isn't too good."

I probably going to regret this... but I'll take a chance and share another view-point and probably way too much detail on this subject. I'll do this mainly because you should not be so fearful of an older 12v battery in a Prius (at least from standpoint of the car leaving you stranded).

A few years back I did a similar test and monitored the state of my battery in the same way you cited and the battery easily lasted another 2 years (at a lower reference of only 12.34v). Based on that you could expect another 2 years of service... so you should have no worries.

Using voltage alone as the reference (which is not a good idea but since it's the sole reference being used thus far in this discussion)... 12.44v is about 35% of the capacity needed to start the car so you should be good for a very very long time.

Digging a little deeper and from my practical experience, (As I recall) I've only seen real problems when the Aux battery drops below 10v (so 12.44v gives you lots of margin).

In cases where the battery is starting to show issues I have seen that the Prius is quite forgiving and she will give you lots of time to get a new replacement battery. In other words you will know when the battery needs attention and it will give you plenty of 2nd, 3rd, 4th, 5th etc. chances before you will need a little boost. And if you every need a little help you will find those ~$50 lithium battery jump boxes are more than sufficient to get you going several times in about 1-2 minutes.

Remember the Aux battery is not like a traditional car... all it has to do it boot up the control logic to engage the HV battery relay. Once that happens the car runs and gets charged by the larger HV battery. The main load that the 12v battery faces is the brake booster pump. Once that comes up to speed you are well on your way with minimal current requirements. While the booster pump can be a significant load, it's nothing compared to the current required to fire up and crank a traditional vehicle. The point is the demands on the 12v aux battery are minimal and predictable.

So from a practical viewpoint (with a weak battery), you will hear the brake booster pump struggling when you open the driver door and press the brake petal, but once that finishes you are good to go. If you have a really weak battery the booster pump will take a long time to finish it's task. You may have to pause and you may even have to press the Start Button a couple of times or hold it down, but once you get to the "Ready" state, again you are go to go. And generally once you get the Ready state, the car will bring up the surface charge of the battery in short order and you'll be good to go all day.

Having said all that… the real way to check the status of the battery is to measure it's performance with a load tester that is consistent with the needs of the car. Another way to gain insight is to monitor it's 12v strength and endurance after and while being taxed by the booster pump (before pressing the Power Button to gain the ready state).

Example: Get in the car, press the power button two times to gain IG status. With the power button released, press and hold the brake petal to engage the booster pump.

This sequence will also would give you the opportunity to turn off extra loads from the climate fans, heater, radio, etc. in situations where you have a low battery situation.

Hope this detail sheds some light on the subject and will help someone if they find themselves in a battery challenged position.

 

Relating your experience might help.......get someone INTO a "battery challenged" position.

Just because you were able to get another 2 years does NOT necessarily mean that most people in this situation will.
The older it gets, the more likely it is to suddenly fail at some point.

People just need to understand that it is a gamble, and the longer you go the worse the odds are.