love/hate my prius

It’s the other way around: the auxiliary (12-volt) battery is charged from the hybrid vehicle (HV) battery, through a DC-DC converter, when the car is in READY (on). The HV battery is not charged from the auxiliary battery, but only when the gasoline engine runs and with energy captured from braking. Indeed, when the car is off, the system main relay disconnects the HV battery for safety.

Do you know how the auxiliary battery was tested?

It’s a basic diagnostic step to check the no-load DC voltage between the positive (+) and negative (–) terminals of the auxiliary battery. This needs to be done with the car off; otherwise, you’ll probably be measuring the output voltage of the DC-DC converter, not the battery on its own.

If the auxiliary battery voltage is outside the range 11 to 14 V, then the battery needs to be recharged or replaced. If the voltage is in this expected range, though, it doesn’t mean the battery is completely fine, just that it’s reasonable to proceed with troubleshooting in other areas.

If this further troubleshooting—or in your case, the seller of the rebuilt HV battery—gives a reason to question the health of the auxiliary battery, then it should be tested more thoroughly. At Toyota dealers, this is usually done with a customized version of a Midtronics GR8 diagnostic battery charger, but many auto parts stores and better repair shops have similar instruments to measure the health of 12-volt lead-acid batteries.

I’m not aware of a diagnostic trouble code (DTC) on a Prius specifically for a faulty auxiliary battery. Depending on exactly what’s wrong, a defective auxiliary battery could cause other malfunctions, having their own DTCs.

You only mentioned one DTC, U0126, which is unlikely to have anything to do with the HV battery or the hybrid system at all. Do you know the other DTCs that were found by the dealer when they said the HV battery needed to be replaced?

 

We hear this all the time with used batteries, buyer beware and make sure the company that sells it is reputable so you won't have issues dealing with the warranty

 

hometown hybrids. 'nuff said.

 

12 volts is low. have it load tested for free at an auto parts store. a new battery, fully charged is close to 13 volts, and the prius computers expect a reasonable voltage after the losses getting from the battery through the wires, and other loads reducing the voltage.

 

+1

With the aux (12v) battery out of the car it should read at least 12.5 volts, but in reality should be closer to 13 volts.

Electronics weenies call this an open circuit voltage test, and it's about the most basic "go / no-go" that you can do for a lead acid battery.
If you yank the battery out of the car and take it someplace like the Auto Zone or the Walmart - they'll likely have a machine that will put the battery under a load briefly to give it a more real-world test.

How to Check the Voltage of a Car Battery | YourMechanic Advice

BUT.....

Remember that (a) place like the Auto Zone or the Walmart make a profit by SELLING batteries, and sometimes people that sell batteries do not know as much about them as you might think that they do.

Something else.....(b)

Your car's aux battery does not actually start the car, but rather it boots the car up.
This means that the ability to delivery cold cranking amps not nearly as important as it is to maintain proper voltage level, which is exactly the opposite of how a traditional car's starter motor works.
Usually it's not much of a factor, since a 12v battery needs to be healthy to deliver each or both, but sometimes batteries that are on the margins will cause a Prius to throw codes and illuminate instrument panel lamps, whereas they would (if just barely) start a Corolla.

The difference is.....with the Corolla you will sometimes get a few warnings that a visit to the battery section of a store would be a very smart thing to do, but with the Prius you will just about always either get a clean boot up or.......not.
So....
MY guess is that you might need to fully and finally test the aux battery, or if you're still within the 12 month warranty of the Traction (big) battery you might just want to replace the aux (little) battery and then wave the sales receipt at your mechanic if the traction battery continues to give you trouble.

Unfortunately, one of the rules of troubleshooting is to always look where you've been....so from several states away and just from what you've written it could still be either battery......or neither.

Good Luck!

 

Hello Brenda T,

Without knowing all the details about your 2005, one would hope that before you purchased the car, they had replaced the original hybrid battery with a new one. and THEN put it up for sale. When you finally discovered that your hybrid battery was no good, spending the $3,500 for a new battery would have saved you a lot of grief. Used batteries just are that.......used. There is a person in this forum who manufactures and sells new, round shaped cylinder module units for around $2,000 and you get to keep your core to re-sell to off-set your cost even more. He has spent years perfecting it, it fits perfectly in your model and he is a Priuschat bro. I cannot remember his name, but I am sure someone ready my post will remember and send it to you.

I think it was your Hybrid battery all along, and the dealer just recharged it when you took it back and replaced something else for the heck of it. I truly wish you luck. My 2015 I bought new and is running strong. Only thing I have done is oil changes and tire rotations. When treated correctly Prius' are extremely reliable.

Wishing you the best!

 

I wish you luck, @BrendaT
That's not much use to you right now, but here's hoping everything falls into place from now on.

 

Actually @2k1Toaster 's kit is $1600 shipped but you either need to assemble it using the casing, etc. from your car or have a local mechanic do it.
The link is in my signature but I think he tends to have trouble keeping the kits in stock.