How do I know which battery needs replacing?

It's a 2012, probably on the road since 2011, if it's the original 12v it's due anyway, batteries die.
Most any parts store will change the 12v, make sure the new one is fully charged, by process of elimination, if the traction battery is still funky....but there's a chance the weak readings are from a weak 12v.

If you have a trickle charger you could charge the 12v for two days and see how the display acts without spending a penny.

 

I'd trickle charge it to maximum and then see how the traction acts, but it seems like it's time to test the traction cells.
If you have the skill to replace a bad cell or two and stay ahead of the problem, great.

 

That doesn't sound good. Maybe iinvest in a dealership visit: they can connect to the On Board Diagnosic port with Toyota's proprietary software (Techstream) and do an assesment of the hybrid battery. I believe it's referred to as Hybrid Health Check? Should'nt be over $100, but check.

 

Ask Toyota about "goodwill".

 

For the HV battery, it depends on where it was first sold and where you now have it registered. At your mileage, it would still be covered in California and other full CARB states, but you don't list your state.

In all non-CARB states, you are out of the battery warranty.

 

Are there any warning lights at all, or are you just reacting to some things you notice while driving, and worrying about them?

-Chap

 

The battery indicator on the dash shows the state of charge (SOC) of the traction battery (the big hybrid battery). With only 2 bars available when you start driving, acceleration will be sluggish because (1) the internal combustion engine (ICE) will be cold and (2) the traction battery supplies most of the torque needed to get the car moving via the electric motors. The torque of the ICE alone is not designed to get the car moving. The ICE is designed to propel the car once it has achieved 15-20 MPH.

If the traction battery fills up within 2 miles of starting your drive, then it has to be that there is a lot of downhill coasting that allows regen charging combined with ICE charging to fill it up.

Since your 12V battery checks out good, you likely have a degraded traction battery; one that is no longer capable of holding a complete charge. This is indicated when your SOC battery meter rapidly (within 2 miles) moves from low charge to full charge, and vice versa.

If you would have your traction battery load tested, it would likely show several individual cells that are not holding a full charge. These are likely in the middle of the battery pack, due to increased heat in the middle because the cooling airflow is less effective in the middle of the pack.

You can continue to drive in this condition, your traction battery has not failed. Failure is indicated by warning lights (triangle with '!'), messages mentioning "hybrid system" and most severely significant loss of power and inability to reach the "Ready" state.

You're driving on borrowed time, eventually you'll have to re-condition the traction battery (replacing weaker cells within the pack) or replace it entirely usually with a refurbished one.

 

Hey Edward, the "traction cells" are the individual cells/modules in the HV "traction battery" under/behind the back seat. The HV battery pack is built from a series of 7.2v modules that themselves comprise many 1.2v cells inside of them.