12v battery (hybrid) runs low after sitting for a week

[Cordoc]

Noticed an interesting thing while on a trip today. Was on a major highway doing 125km/hr-135km/hr. Battery gauge read right in the middle of the gauge the whole time, which I hadn’t noticed before. Honestly, I haven’t looked at it in a while. When I went onto a secondary road and had to slow down the gauge dropped to where I usually see it to the left of the battery symbol. If I’d gun it to pass, it would jump to the middle of the gauge, slow down and back to the left. Does that seem normal?

I can not give you a technical answer to your question, but can tell you about my experience.
The battery state gauge you mention never, I think, goes above 50%. The object of this type of hybrid is to give the truck more power. For some reason, once it reaches about 50-60%, it starts to use the charge more liberally to increase power when the gas pedal is pressed.
My advice is to forget about it and enjoy the power!
I am sure a more knowledgeable member will correct me.
Let's see!
Good luck
Gaston

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[Tom Sellick]

I can not give you a technical answer to your question, but can tell you about my experience.
The battery state gauge you mention never, I think, goes above 50%. The object of this type of hybrid is to give the truck more power. For some reason, once it reaches about 50-60%, it starts to use the charge more liberally to increase power when the gas pedal is pressed.
My advice is to forget about it and enjoy the power!
I am sure a more knowledgeable member will correct me.
Let's see!
Good luck
Gaston

Thanks. I should have also clarified, it’s the 12v battery gauge I’m referring too.

 

[Cordoc]

I was referring to the hybrid large battery.
I

 

[StreeTaco]

I was referring to the hybrid large battery.
I

We Know!!

 

[MustardTiger]

@Tom Sellick I think this is pretty normal behaviour, and something I've seen myself. I'd like to think you're seeing the actions of voltage regulation. The lower 1/3 level reading is like 12.7v...which is fully charged. So when it goes to the 1/2 level reading...it's near 14v and charging. I've honestly given up worrying about the 12v battery...the truck hasn't gone dead yet, so I think it should be ok. Idk, thoughts?

 

[Tom Sellick]

@Tom Sellick I think this is pretty normal behaviour, and something I've seen myself. I'd like to think you're seeing the actions of voltage regulation. The lower 1/3 level reading is like 12.7v...which is fully charged. So when it goes to the 1/2 level reading...it's near 14v and charging. I've honestly given up worrying about the 12v battery...the truck hasn't gone dead yet, so I think it should be ok. Idk, thoughts?

You are probably right. The fluctuations are probably related to the load on the system or something. I’ve had mine go dead after sitting for 2wks which doesn’t breed confidence. But you are right, I haven’t paid attention to that gauge for a while and probably shouldn’t .

 

[StreeTaco]

I have one of these installed next to charging pad. It will answer all your questions!!

https://a.co/d/1MVXpI6

 

[AZTaco]

9 page summary.
Mustard Tiger had low voltage but no starting problems.
Max 911 had a dead battery, dealer replaced, no problems since.
Taco Freak has low voltage but no starting problems.
Tom Sellick has a dead battery, dealer charged it and said it was okay. No starting problems since.
Jt 2024 had a dead battery, dealer replaced it, and it's been okay since.
A couple non-hybrids chimed in with battery problems.
Toyota App may or may not put a significant drain on battery. I have a 2025 (with alternator!), has sat for two weeks with app running, and not had problems starting.
Not sure how many hybrid owners are on the forum and checking this thread to determine if this is endemic or a few isolated incidents.

 

[MustardTiger]

@AZTaco the battery in my truck was replaced probably right off the transport truck as it was dead and frozen in January, and the current battery doesn’t match the oem factory ones I’ve seen others post, but rather a parts counter oem battery. I also seen signs of the truck being jump boosted at the underwood terminal.

its highly possible there was a bad production batch of 12v batteries

 

[Tom Sellick]

9 page summary.
Mustard Tiger had low voltage but no starting problems.
Max 911 had a dead battery, dealer replaced, no problems since.
Taco Freak has low voltage but no starting problems.
Tom Sellick has a dead battery, dealer charged it and said it was okay. No starting problems since.
Jt 2024 had a dead battery, dealer replaced it, and it's been okay since.
A couple non-hybrids chimed in with battery problems.
Toyota App may or may not put a significant drain on battery. I have a 2025 (with alternator!), has sat for two weeks with app running, and not had problems starting.
Not sure how many hybrid owners are on the forum and checking this thread to determine if this is endemic or a few isolated incidents.

I’ve had no trouble starting since my dead battery issue because I always put a battery maintainer on it now when I won’t be driving it for more than a few days. Just wanted to clarify. Also, Toyota told me this happens all the time with new cars and recommended the battery maintainer when not driving for a while….

 

[AZTaco]

Also, Toyota told me this happens all the time with new cars and recommended the battery maintainer when not driving for a while….

Toyota or the dealer? A lot of dealers say "it's normal" because they don't want to deal with it.

We've had vehicles with electric start for over one hundred years and suddenly engineers can't figure out how to keep batteries charged?

 

[TacoFreak]

Toyota or the dealer? A lot of dealers say "it's normal" because they don't want to deal with it.

We've had vehicles with electric start for over one hundred years and suddenly engineers can't figure out how to keep batteries charged?

Yeah, that's the part I don't get either. It isn't just Toyotas or hybrids either, my first friend who's car couldn't keep its 12 volt charged was an Acura over 10 years ago. He had the battery checked repeatedly and it was always fine, but when he travelled his car wouldn't start when he got back. The dealer wouldn't do anything because they said the car was fine. He stopped buying Acuras but it seems like a common problem today with lots of brands. That is why there is a jump pack in my glove box, although my truck has never failed to start. But it is never over 1.2 volts, so I figure it is just a matter of time.

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