Time to replace the aux battery?

The 12V aux battery has nothing to do with performance, except in the extreme case where it's drawing much current through an internal short. In such a case, your car certainly wouldn't start, and probably wouldn't run due to a fault-code.

Yellow-top would be an improvement, but you probably don't need it. Cranking amps aren't an issue with the Prius, and deep-cycling is only useful if you have a habit of draining the battery. These are the two main virtues of an AGM battery. There are better AGM batteries on the market.

 

I would start by putting the existing battery on a charger overnight, 4A current flow. After the battery has been fully charged, let it rest for 8 hours and then see what the IG-OFF voltage is using a digital multimeter. Then see what the IG-ON voltage is, and compare to your prior measurements.

Optima would not be my first choice for a replacement 12V battery, but that is your decision.

 

years
I agree. A good AGM battery should last for years. My Intimidator AGM is 6 years old and still going strong.

 

poor 12v will reduce your mpg's because the hybrid battery has to keep charging it and the engine has to run more to charge the hybrid battery. those voltage's are low, i would pick up a new yuasa from a dealer. i think they can be had for 25% off if you shop around.

 

That would be true, only if the battery were shorting inside, which is a catastrophic condition, not just a result of age. If significant amounts of power are being drained by the aux battery, the energy is dissipated as heat, which would quickly destroy the battery and create a serious risk of fire in the car.

We are conditioned to think of the word "battery" as something which steadily runs down, and we should replace often. This should not be the case with an automotive battery. It should work up to spec for at least several years before showing a loss of capacity. Even with reduced capacity, it shouldn't degrade performance of the vehicle in any way.

 

IMHO, 11.4 is low. As Patrick stated above, try charging the battery, use the vehicle for the next day, as you normally would. Let the vehicle sit over-night, and then, run the test again. If the voltage is below 11.9, replace the battery.

I had the same experience with my '07. When I went to Toyota, (I have a good dealership, and my rep discounts me on alot of stuff), they stated that they normally charge to test the battery before they replace it. I stated that I did not want the test, just replace it. They did. Afterward, the tech said he tested it after he removed it (no charge), and confirmed it had a dead cell.

After the new battery was installed, and the vehicle went through its learning curve, my mileage went back up to 50mpg.

 

i'm only going by the many reported experiences here, from people who had a loss of mpg's and no other symptoms. battery test showed low, and replacement restored mpg's. i no nothing about batteries.

 

I don't disbelieve or believe this claim. It doesn't make sense why it should affect mileage, but we never know what we don't know, therefore my mind is open to a possible cause. We might even learn something.

 

if you buy the yellow top, make sure it is 13 volts out of the box. if not, charge it before installing, or return it for a better one. check the date code too, some have reported getting 6 month old batteries. yes, the correct model will be a drop in replacement.

 

Last night when I came home I turned off my '09 (97.5K miles) Prius and it started beeping. Didn't think too much of it and went in the house. This morning I went to go to work and when I got in it was beeping. Pressed the start button and nothing. The display was on and the start button was orange instead of green. I did notice that the clock display was very dim. I suspect it is probably the 12v battery going out or low on charge. The last week we have had 9-10 degree over night lows and daytime temps in the high 20's. My car is garaged at night but sits out during the day at work.

I'll do some testing on the battery and if it turns out it is bad, is the Optima yellow top battery the best one to get for a replacement?

 

there is no 'best' unfortunately, they all seem to be a bit hit or miss. the best thing you can do is try to get a fresh one and check the voltage before leaving the place of purchase.

 

My opinion (take it leave it, if you wish) is that the aux battery is best left alone, as long as it's working. I don't believe the condition has any bearing on the car's performance. If you don't have any unnecessary power drain, the battery will do fine, even if it's capacity is low.

 

If it is the original battery, it is very likely that is the problem. My OEM in the '07 lasted 5.5 yrs. If your is OEM, that is the same amount of time. Once the 12V gets toward 11V and below, you will have difficulty staarting the vehicle. A Prius doesn't start like a normal vehicle. It boots up various CPU's and then switches to the HV Battery once "started". If the 12V is low, it will divert more power than usual to try and charge the 12V. This causes less charge to the HV and more engine time looking for power to charge the 12V. This is why people experience a drop in MPG.

Also, a tell-tale sign on the GenII that the 12V is low or getting old is the noise from the thermos pump when you shut off the vehicle. Because it happens slowly over time, you do not notice the sound change from higher pitch and quick thermos fill to a slower/lower pitch.

I would recommend the Toyota OEM battery. I thought about the Optima, but I didn't like some of the posts I read about dead/low charge out of the box.

 

Yes, battery is OEM. I also noticed the red triangle with the exclamation point coming on during startup and shutdown more often. The Thermos pump was making more noise than usual as well.

 

Can I only "sorta" agree?

The aux battery on The Prius isn't supported in the traditional manner with an alternator as in a regular ICE vehicle.

And I have read just too many threads where an "Alice In Wonderland" number of strange symptoms seem to manifest as the Aux Battery weakens or fails. Including often claims of decreasing gas mileage.

So I guess I would say to the OP?

Time to replace the Aux Battery?....I don't know.
Time to check it? ...Why not?

And as my Prius ages I plan to be more aggressive concerning Aux Battery replacement and maintenance, just because I've read so many threads that suggest it's failure can lead to so many possible problems.

With a regular vehicle? Battery failure seems easier to evaluate as it happens. Weaker headlights and the engine is slower to turn over.

The Prius "Boot Up" and use of the Aux Battery seems to mask or change the symptoms.

 

The 12 V system runs the computers which do EVERYTHING in the car except actually making the wheels turn.

When the main component of that system ( the battery) gets weak, it can and does cause all kinds of weird things to happen when random errors start to creep into the computers communications with each other.

 

The common sense way to test that theory is to hook up a volt-meter to your 12V system. If the voltage is low, the DC-DC converter is the problem, but it would have given you a warning light or a code. Even a poor battery will not affect the operation of of the car when the charging system is working properly.

 

This the crux of the matter. My opinion is a battery that does not hold a charge is not working.

I agree that the common sense way to test that theory is to hook up a volt-meter to your 12V system and if the voltage is low, the DC-DC converter is the problem.

However, I disagree with your next statement about a poor battery not affecting the operation of the car, as it depends on how you define the operation of the car.

I have had the experience of a battery in poor condition and struggled to get good MPG's until after the 12V battery was replaced.