12 Volt Battery drain Gen5

just install a Noco genius!

 

i think there is more drain, a lot of people seem to be in your situation

 

I would be sceoptical. Any 12 volt battery drained to 4 volt will not last much longer, if at all.

What's your parking situation? Do you have secure, off-street parking with 120 volt AC outlets available? If so,l look it smart chargers, around 5 amp rating, either on-board or exterior to the car, depending on the situation: if you're parking outside an onboard charger is more secure, just a power cord protruding from grill (like a block heater cord) is the usual setup. If you're in a secure garage you could also use an exterior-style charger.

My setup:

 

The 12 volt battery discharge problem is common for the generation 5 prius.
The problem has several causes including:
The remote control too close to the vehicle for example when washing and/or waxing the vehicle or even in the garage during maintenance.
Those who do not close the Toyota application after use.
The fact that if the temperature is above zero Celsius, the DC-DC converter charges the 12 volt battery only 5 minutes after the vehicle leaves, after which it only maintains it.
When you recharge the high voltage battery, it's the same, you only recharge the 12 volt battery for 5 minutes.
In short, we discharge it more than we recharge it.
On this document:
Toyota/Lexus 12v Battery Failures - Analysis of Charging Issues - Deep Dive Analysis & Workaround by /u/andy_why - Google Docs
It is recommended to disconnect the connector near the negative terminal of the battery so that the DC-DC converter always charges at 14.4 volts (not just 5 minutes).
To find out the state of health of your 12 volt battery, test your SOH, if it is less than 75%, it should be replaced.
The SOH gives the remaining capacity of your battery, for a 45 AH battery at 75% SOH, this means that there is only 34 AH of capacity remaining.
It is easy to find and buy a tester that measures SOH, garages also have this tester.
Using a smart charger is a good idea, but avoid NOCO where you stay, because it barely charges the battery to 60% when the temperature is below zero Celsius (poorly temperature compensated).
The best smart chargers for very low temperatures are the BATTERY TENDER.

 

Didn't know that about the low temp performance of the NOCO. You live in a cold weather testing laboratory, so you should know!

 

You can charge the 12 volt by putting the car into “Ready” with a charged up Traction battery. Shutting off the heater will prevent the engine from running.

I believe that you can lock the car with the manual key.

Leave it for 4 to 6 hours.

Unplugging the battery monitor at the negative terminal, as suggested by Danny Dan, will also help, but in cold weather the charge voltage should remain above 14 volts without doing so.

 

This all sounds very familiar. I have experienced many dead batteries, which happens at unpredictable times. I alerted the dealer to the problem, but they informed me that the factory battery was testing up to spec. Another dealer actually replaced the battery, to no improvement at all.

I have my own theory; I believe the live updates are draining the battery at variable and unpredictable rates. Sometimes the 12V battery will hold a charge for a week, but other times, it drains in just a few hours.

I believe this to be a design flaw, but since no update or recall is available, I have resolved myself to using a battery maintainer and a booster pack.

This all could have been avoided if the system were designed to maintain the 12V battery off the hybrid battery.. We can only hope that Toyota will someday see the light and fix this problem.

 

I've had poor luck with "smart chargers."
They tend to shut down if the battery isn't in top condition.

 

Next, when I went to the dealership for service 2 (oil change and inspection), I plugged in my 3 amp smart charger to recharge my 12 volt battery when I returned.
To my surprise it took 5 hours to recharge it, that means they drained 15 AH from my 45 AH battery.
They therefore discharged the 12 volt battery by 33%.
If the temperature is above zero Celsius and the DC-DC converter only works for 5 minutes, it will take a long start to recharge it and in the meantime it will have time to sulfate.
This is why some people disconnect the connector near the negative terminal of the 12 volt battery to force the recharging of the 12 volt battery with the DC-DC converter.
Don't forget that if the temperature is below zero Celsius, the DC-DC converter always charges at 14.4 volts.

Note that the last update of the Toyota application, Toyota corrected the problem that the application remained functional in the background after closing their application.
We had to close the duplicate application, close the application and close the background application.
If we did not close the application in the background, the application remained in communication with the DCM and drained the 12 volt battery.
Now, when we close the application, we close both.

In summary, to maximize the life of your 12 volt battery:
Always deactivate your remote control when washing your car, waxing it, sweeping it inside, shoveling snow, shoveling around the vehicle and/or being near your vehicle.
Always recharge your 12 volt battery after a visit to the garage or your dealership for oil change, tire change, service (inspection), repair, etc.
Always close your Toyota application correctly after use.
And optional, if you don't mind losing a little battery life, unplug the connector near the negative terminal of the 12-volt battery.
Finally, I recommend purchasing a BATTERY TENDER smart charger (better than NOCO) especially if you live in Canada.

 

My car (not a Prime) is still new. I'm still in toy-buying mode. This toy is nice:

 

I've seemed to have had similar issues. I bought my Prime in Feb 2025. It was delivered to the dealer Nov 2024 and mostly sat there until I bought it (it was in the mountains of Va). I was sitting in my car one day, car in accessory mode with the radio on. Suddenly "Battery Low" shown on the display and then went away. Weeks later, after washing my car, I got inside and ...flash flash, then nothing. No display at all. I disconnected the battery, measured it, found only 4v. Not wanting to wait for Toyota I went ahead and charged it with my Noco. Everything worked perfectly.

I brought it to my local dealer, the next day, and they found nothing. They said the battery was up to spec. No duh, I had just charged it the night before. I wanted them to dig deeper. After pouting and whining a bit, a senior "advisor" told me to bring it back the next day. I did and he wrote a ticket indicating it was happening consistently. They did a full diagnostic as a result and came back with .... nothing! "It all looks good".

I'm wondering how far away the FOB needs to be away from the car. I've read similar problems with Hyundai Sonotas and the keyfob. Do I need to put it in a shieled box or wrap it in tin foil (lol)? I hang it on a peg more that 30 foot away from the car. I'm going to give better consideration how I'm using the app.

I drive everyday. It's been a month now without problems but I'm waiting with bated breath for the battery problem to re-appear.

 

The key fob only needs to be three feet away. It has nothing to do with it.

Yes, this has been discussed to death.

The core reason for the Gen 4/Gen 5 12-V battery failures is: The Gen 4/Gen 5 12-V BMS antics.

And the contributing reasons are: (1) The car sits at the dealer lot for too long before purchase. (2) The car is not driven long enough frequently.

Note that once the 12-V battery voltage drops too low (e.g., because the car had sat for too long at the dealer lot), the buggy 12-V BMS will never successfully charge the battery to a satisfactory SOC.

And the remedy is: Charge it with a Noco Genius every now and then (every few months).

Since you have already charged it with a Noco Genius, you will never have a 12-V battery problem again if you keep charging it a few times a year just in case.

 

Another warm climate person with 12 V BMS problems. I'll be keeping an eye on my HEV this summer.

 

Le moyen le plus simple est de désactiver l'émetteur de votre télécommande.
Maintenez simplement le bouton verrouillé enfoncé et, sans le relâcher, appuyez deux fois sur le bouton déverrouillé.
Si cela fonctionne, la LED clignotera 4 fois.
De plus, cela augmente la durée de vie de la batterie de la télécommande.

Une batterie déjà déchargée est souvent endommagée et/ou sulfatée.
Pour connaître votre état de santé, vous devez mesurer votre SOH (état de santé) à l'aide d'un testeur.
Si la mesure atteint 100 %, l'appareil est comme neuf ; en dessous de 75 %, il est endommagé au point que je recommande son remplacement.
Comme indiqué précédemment, une batterie d'une capacité de 45 Ah avec un état de santé (SOH) de 75 % ne fournit que 34 Ah. (Batterie sulfatée)
Ces testeurs sont disponibles presque partout et ne sont pas très chers, par exemple :
http://www.amazon.ca/-/en/batterie-3Ah-200Ah-Analyseur-automatique-automobile/dp/B09PF18KL2?__mk_fr_CA=%C3%85M%C3%85%C5%BD%C3%95%C3%91&crid=PK78NXTB86XR&dib=eyJ2IjoiMSJ9.g1s2jAQqiweC9hNPhKf0BZmYp7maQHQIzXKXu0w2uwLl4ikNSAiMkcg4gWOg8LaA7WvoaSS405XBxmj1PV1lj3fmydmNxSXvluamQHW-bt1pe_uDmh-2YxsLAqWgE5aMCEstcy2291bMsF1IV7-Vl33uiuny3ihTnnvOMPix7Zk3rpT40n3MQYqgX922f4bFEdNgZ2L-BS-IGEfgKcz5fAxTxB-NqDpajwiEX8zkrqgdJEaksRpkJ9KAUw1GnKVx-r2_AXAVI1kp7tU1IXLhdmlGy7X9FsTkKpU4P5_HIvE.lu4FnVc9WWM4LpXsJPR9B1qMWOhez0J-Mv8VJg4skX4&dib_tag=se&keywords=soh+testeur+de+batterie+12+volts&qid=1768267007&refinements=p_76%3A3276484011&rnid=3276483011&rps=1&sprefix=soh+testeur+de+batterie+12+volts%2Caps%2C301&sr=8-129&xpid=-BdLdCKNPRMiR&language=en_CA

General PDP Template | Canadian Tire

Ces testeurs vérifient également le SOC (état de charge) de la batterie lorsque vous souhaitez savoir si elle est complètement chargée ou si elle doit être rechargée.
Il est peu coûteux de payer pour savoir où l'on va et pour ne pas se faire arnaquer par les vendeurs.

 

don't use accessory mode, use ready mode. that way, the 12 volt won't drain. that's been an issue for 25 years