u/mrmao

This will be a long post but it documents, with a battery monitor, how a new 2026 BZ can get a dead battery. This is the second time I got a dead 12V battery after the dealer replaced it with a new battery less than a month ago. Skip the background section if you have been following all the updates and understand the DCM issue and just want new info from this incident.

Background:

It's been pretty well documented with multiple cases of dead 12V batteries in the BZ and variant Subaru models. I had my first dead 12 volt battery within the first month of purchase and I only left it parked for one day. I took it to the dealer and they said it was a faulty battery and replaced it. I bought an Ancel BM200 Bluetooth battery monitor to track any causes. I noticed that 3 hours after turning off the car, there would be a sawtooth pattern causing a battery drain that lasts for about 4.5 hours. This process would cause a battery drainage and lower the state of charge and voltage, shown in the 1 day graph included on 7/6. Eventually, I would also notice that this sawtooth drainage pattern can sometimes last 9 hours and drain the battery even further. I've done multiple attempts at isolating the cause of the drainage with trying to store the keys far away from the car, locking the car, unplugging the car, uninstalling the app, and none of it worked until another user pointed out that you can disable the DCM services which is the cellular connection to the car that sends data to navigation, maps, collision detection, car info details on the app, and emergency calls. I disabled the DCM on my phone app and it eliminated this sawtooth pattern discharge and kept the voltage pretty stable. I personally believe the DCM is the root cause of battery drainage. I think what is happening is the DCM is activating and causing repeated drain on the 12 volt battery to go to a low voltage beyond its capability and eventually will render the battery unusable causing dead batteries. So I turned the DCM back on to get more data.

This Incident:

Referring to the 3-day graph included in the photos, I parked my car on 7/4 and did not drive it again until 7/6. You can see in that period, multiple 9 hour episodes of the sawtooth pattern draining the battery and lowering the voltage. On 7/6, the voltage went as low as 11.7V (not at all good for the battery) and when I tried to start my car at 8:20AM, it would not engage in ready mode and would only turn on to accessory mode with some error codes and I noticed the voltage dropped down to 9.2V. I tried multiple times to push the brake and start button but it would not turn on to ready mode. I attached my battery jump starter pack and only then could I start the car into ready mode. This time there were no error icons or messages to go see the dealer like the first episode but the first time, the voltage went down to 7.4V which I think was not able to power any electronic systems at all.

What I learned:

1. The DCM is the issue

2. It seems like the 12V battery threshold is around 11.7V where the battery does not have enough power to start the car. I've previously started the card at 11.9V in the past.

3. DCM engaging and causing the sawtooth drainage pattern usually happens for 4.5 hours but can also happen for 9 hours. I have not seen it last longer than 9 hours.

4. It seems like a lot of the cases I've read about happen after trips of 7 days+ but I can get mine to happen only after one day of not driving it but this was likely due to the multiple 9 hour long drainage episodes.

Unanswered Questions

1. Why is the DCM engaging 4.5 hours versus 9 hours?

2. Is it a faulty DCM module or faulty software causing this to happen?

3. Will an AMG battery have enough resiliency to not be affected with this issue?

I'll be bringing my BZ into my dealer and requesting a TAS case be open but who knows if they will care enough. I'll also be requesting an AGM battery. At this point I'm so frustrated that I'm toying with the idea about doing a Lemon Law return. I know exactly how to cause the battery drain and can reliability reproduce it so I could get it into the dealer enough times to make a case for it. If a bunch of Lemon Law returns is what it takes to make Toyota finally fix it for everyone then it might be worth all the trouble. Am I crazy?! It's too bad since I actually really like the car otherwise and would settle for a simple fix.

Any info I'm missing would be much appreciated.

Background info: so we are getting multiple sources where DCM is activating around 3 hours after the car shuts off and has a sawtooth pattern for 4.5 hours draining the 12V battery. It can drain up to 50% if the 12V battery.

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Update Is that yesterday I noticed this drainage pattern going on for 9 hours! See attached screenshot. I have no idea why this happened and this hasn't happened before. Voltage went from 12.2V to 11.9V but the car still started fine.

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The first time my battery died, I measured around 7.5V after not driving for just one day so still have no idea what can drain the battery so quickly. I'm wondering if dead batteries are due to some runaway process where drainage just keeps going on without stopping.

This is going to be a long post but I think I figured out the cause of the dead 12V battery issue. There are lots of reports of dead 12V batteries on new 2026 BZs. Mine happened within a month and I did not drive it for just 1 day. Dealer blamed it on a faulty battery but I didn't really believe them since they initially said it passed and backtracked and said it failed. I've tried to test every variable I could think of from locked, unlocked, key distance, key put to sleep, Toyota app uninstalled, battery monitor unplugged at neg terminal, disabled remote key, etc and nothing worked until I tried disabling the DCM. DCM option is in the phone app under Account ->  Privacy Portal -> Vehicle Specific Settings. This is the wireless phone data which allows services like road side service, crash detection, GPS, remote status car info

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What is happening to the battery? I used my 12V battery monitor to track usage for a while. The pattern I've seen is at about 3 hours after turning off the car, there's a battery drainage shown as a sawtooth pattern(see graph with sawtooth pattern to right) that keeps lowering voltage and last about 4.5 hours. Others have confirmed this same drainage pattern. Draining reduces battery charge 30-50% or Voltage 0.3-0.5V. This process happens every time you turn off the car. I think this drainage process is the DCM.

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So far I've disabled DCM for 3 days and there was no sawtooth drainage pattern. My voltage was stable and drops very little (about 0.05 to 0.1 V) when it used to drop 0.5V regularly overnight. Then I reactivated the DCM which took around a day and the sawtooth drainage pattern returned the same day (see attached 3 day voltage graph showing sawtooth pattern at right when DCM was reactivated)

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I think what happens is the DCM engages 3 hours after parking and drains 12V battery 30-50% (goes from 12.6V to 12.1V often). This stresses the battery since it gets down to 12.0V frequently (which is not suppose to happen and reduces battery life and performance). Maybe this is why these batteries don't last very long and need to be replaced so quickly? Since the battery is so stressed by getting drained so low, it doesn't hold it's charge as well and dies quickly when parked for extended periods of time.

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Also for those who get dead battery who didn't drive for an extend period, I think what's happening is that their drives are too short and 12V battery doesn't get enough time to recharge during the drive. When you park with a low voltage battery like around 12.1V, that drainage happens again and voltage goes well below 12V and the car can't start. If you drive longer drives regularly, then you give enough time to get 12V battery to recharge. However the wide voltage swings to such low voltages are not really good for the battery life either way

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The question is if there are just a few faulty DCMs or this is the happening for everyone  and you're just a few short drives  away from a dead battery?  I feel like this issue should be an easy software fix from Toyota but who knows how long it'll take them or if they care enough to do it. 

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I also don't know if this sawtooth pattern happens with an AGM battery either.

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It's a little sad I need to disable a lot of features to protect the battery. Otherwise the BZ is great.

​ *Edited formatting

I got a 12V battery failure one month after getting a new 2026 BZ. It was only sitting without a drive for 1 day. Dealer blamed it on a bad battery and replaced it. I was skeptical so bought an Ancel Bluetooth battery monitor.

See attached graphs showing SOC (state of charge which is battery % charged) and voltage over 24 hours of the same day

This is what I noticed happening every night after monitoring it for a while. About 3 hours after shutting down the car, the voltage starts oscillating (see voltage graph from 1500 hours to 2300 hours) and the battery % or SOC starts dropping (see SOC graph from 1500 hours to 2300 hours) from 55% to 10% battery charge. Battery % drop each night can vary from 20-50% drop in battery charge. Also notice the voltage also slowly dropping closer to 12V.

To me, this suggests something is powering on to drain the battery even though the car is off. Most nights this drainage lasts about 4.5 hours and then stops. Then voltage and battery % remains the same the rest of the night. The peaks in the voltage graph are when I drove the car.

Things I've tried to do to see if they affect battery drainage overnight:

-locked vs unlocked

-Key turned off vs left on 15 ft away

-Toyota phone app installed vs uninstalled

-Remote key disabled vs enabled in car menu

None of the things I did above made any difference on this overnight battery draining. I did not do any aftermarket alterations like dashcam. I can see how this can drain my 12V battery if I don't drive it much.

Anybody have any idea what is going on? Is this normal? Is this happening to anyone else? Any other idea on what to test? Would an OBD plug in help show what is turning on and which one would you recommend? Thanks.