Toyota bZ4X Auxiliary Battery Fault - Info to Share

[timtimmo]

Hello to all,

In the spirit of sharing to help others - here are some details FYI.

1. Started happening 4 weeks ago - Car is 10 months old.
2. This has happened on three separate occasions now with all the AA call out hassle and phone calls to the dealer 
3. After the "no fault found" diagnosis by the dealer, I decided to prove it for myself as shown in the attachment (I am a retired Electrical Engineer ).
   Dead Auxillary Battery Fault.pdf
4. The Fault symptoms are:-
   1. Car put on charge (usually overnight).
   2. Car reaches full charge and turns off automatically.
   3. Car left plugged in.
   4. Auxiliary Battery discharges whilst plugged in!
5. The dealer is going to fit a new Battery tomorrow so I will update the thread when this issue is fully resolved.

Regards - Mark

 

[philip42h]

Interesting ... something's not right - but I suspect that you knew that ... 😉

I don't know, so could be wrong, but I rather doubt that the car charges the auxiliary Battery while it is plugged-in to charge the traction Battery. Your analysis appears to show a voltage drop throughout. And even if it did, that would / must stop when the traction Battery is fully charged, and the charging turned off. I don't see any 'fault' at this point.

Your picture (4) 8.5 hours later shows a draw of 2.6A (and "unknown load") - this is definitely very wrong and indicative of something wrong with the car rather than the battery (?). I doubt that a new battery will fix that issue!

Is there a problem / recall in the DC-DC converter space on the bZ4X? There was on the RAV4 PHEV - this resulted in a similar, unwanted, parasitic drain. If not that, there's something 'switched on' and drawing current when it shouldn't be.

[timtimmo]

Thanks Philip,

Didn't mention that a conversation @ the dealer today put this theory forward.  When the "smart key" is close by, it will "wake up" the car in preparation for you walking towards the car to open it.  This "wake up" may create a small load on the Battery?

Our keys are 3-5m away so not sure this stacks up but to disprove this, keys will be living in the kitchen after tomorrow!

[Parts-King]

45 minutes ago, timtimmo said:

Thanks Philip,

Didn't mention that a conversation @ the dealer today put this theory forward.  When the "smart key" is close by, it will "wake up" the car in preparation for you walking towards the car to open it.  This "wake up" may create a small load on the battery?

Our keys are 3-5m away so not sure this stacks up but to disprove this, keys will be living in the kitchen after tomorrow!

Put the key into "sleep mode" Press and hold the lock button down, whilst holding it down, press the unlock button twice. The fob will flash four times and the remote will go to sleep. To wake it up, press any button 

[philip42h]

2 hours ago, timtimmo said:

Thanks Philip,

Didn't mention that a conversation @ the dealer today put this theory forward.  When the "smart key" is close by, it will "wake up" the car in preparation for you walking towards the car to open it.  This "wake up" may create a small load on the battery?

Our keys are 3-5m away so not sure this stacks up but to disprove this, keys will be living in the kitchen after tomorrow!

I thought that since about 2021, Toyota keys were equipped with motion sensors and would 'sleep' automatically after two or three minutes and only 'wake up' again when they are moved. We tested this over on the RAV4 forum - my 2020 keys don't sleep; folks with 2021 and later cars have keys that auto-sleep.

But now that the dealer has made this claim, you'll have to test it! 🙄

[Lasser Drakar]

The PHEV manual says that the 12v Battery is being used while the car is plugged in to charge. This is to operate the systems relevant to charging. Essentially while the traction Battery is being charged or more accurately while the car is plugged in the 12v Battery has a load on it. The manual might even say to unplug the car once it's finished charging for this reason. I guess the idea is that the car would be driven after having been charged and the 12v battery recharged then.

[Cyker]

The key proximity could be a factor - We suspected that was the reason loads of people were have the same issue with the first Auris hybrid way back when I was a new-comer to these forums!  

It does seem to be a weird blind spot with Toyota - Given how rabidly they strive to improve things I'm constantly surprised the whole 12v Battery drain issue keeps popping up randomly on newer models!

[Mister Mike]

On 10/31/2023 at 5:07 PM, philip42h said:

I thought that since about 2021, Toyota keys were equipped with motion sensors and would 'sleep' automatically after two or three minutes and only 'wake up' again when they are moved. We tested this over on the RAV4 forum - my 2020 keys don't sleep; folks with 2021 and later cars have keys that auto-sleep.

But now that the dealer has made this claim, you'll have to test it! 🙄

I tested this on my bz4x. The key does indeed go to sleep if it sits motionless for a couple of minutes. You can easily test this by placing the key somewhere near the locked car and waiting for a while. You will not be able to open the car until you lift up the key again.

[bZ4X]

On 10/31/2023 at 12:39 PM, timtimmo said:

Hello to all,

In the spirit of sharing to help others - here are some details FYI.

1. Started happening 4 weeks ago - Car is 10 months old.
2. This has happened on three separate occasions now with all the AA call out hassle and phone calls to the dealer 
3. After the "no fault found" diagnosis by the dealer, I decided to prove it for myself as shown in the attachment (I am a retired Electrical Engineer ).
   Dead Auxillary Battery Fault.pdf
4. The Fault symptoms are:-
   1. Car put on charge (usually overnight).
   2. Car reaches full charge and turns off automatically.
   3. Car left plugged in.
   4. Auxiliary battery discharges whilst plugged in!
5. The dealer is going to fit a new battery tomorrow so I will update the thread when this issue is fully resolved.

Regards - Mark

 

Please kindly share the new batteries performance after replacement so we can compare. Thank you for sharing this.