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.

 

[nielsd]

My 2025 Tacoma TRD Pro, with only 675 km on it, has been having battery issues since the day I got it. The battery has gone dead 3 times while parked in my heated garage and the truck wouldn't start. It would take about 4 days of being parked and not used for this to happen. So I tried another battery and had the same exact issue. I performed parasitic drain tests, charged the batteries using a 5 amp AGM charger, etc, etc, etc. What ended up being the problem was the Toyota Connected Services. I deactivated them and charged the battery fully. Now everything is good and the battery is maintaining a full charge. Who knows what the connected services are doing in the background, at night, etc.

For reference I forced the Toyota App on my phone to shut down before I did any of this so the app was not the problem for me.

I could have taken my truck to the dealer but that would have been a huge hassle. I would have had to wait days if not weeks for an appointment and they probably would have said nothing was wrong. Also I don't like them touching my beautiful truck. After reading the forums I'm glad I solved it myself.

 

[TacoFreak]

@nielsd Glad you found a solution. The hybrids do not keep the 12 volts adequately charged, which means that any parasitic drains like connected services can kill your truck. My battery is never about 12.1 volts with the engine off, but my dealer says it is fine and you probably would have wasted your time taking it to the dealer. Like you I love my beautiful truck and the less it sees the dealer the better off both of us will be.

 

[nielsd]

@nielsd Glad you found a solution. The hybrids do not keep the 12 volts adequately charged, which means that any parasitic drains like connected services can kill your truck. My battery is never about 12.1 volts with the engine off, but my dealer says it is fine and you probably would have wasted your time taking it to the dealer. Like you I love my beautiful truck and the less it sees the dealer the better off both of us will be.

It's hard to believe that the dealers are telling people that it's normal and acceptable for your battery to go dead if you don't drive it for a week or two. That's crazy. My old 1972 Ford pickup would start after not using it for a month. How is this an advancement?

Anyway, I don't care about the connected services. Android auto is more than enough for me. I just want my truck to start when I need it. They need to get their act together software wise.

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