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

[TacoFreak]

Not after the battery drained. No issues since I signed out after using it. Not sure but it may ping the vehicle.

Staying signed into the Lexus app ran down the 12 volt battery in my wife's Lexus electric SUV. Lexus said they were aware of the problem but had no fix and suggested that she shouldn't remain logged into the app. It has not died since she quit using the app.

Those Toyota/Lexus apps provide vehicle status and need to frequently ping the vehicle for their status. The charging systems in Toyota hybrids and electrics are so fragile that communicating with the server appears to be enough to drain the 12 volt battery. I have never used it in my truck, but the 12 volt is still frequently at a very low level. For me the convenience of the app is not enough to not know if my truck will start or not.

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[StreeTaco]

My truck does not behave that way, but maybe I just have a bad battery. The lowest I have seen it is 11.9 vdc or about 35% charged.

A couple of days ago it was down to 12.18 after driving for 90 minutes. I put my maintainer on it and it charged up to 13 vdc while charging or 100%. Without the charger it was 12.85, also 100%.

Sitting for 12 hours that dropped to 12.57 vdc - about 80%. Strangely the next morning it was up to 12.67 - maybe 85%. So I took it for a drive to see what happend.

After driving for 25 miles of mostly freeway it is now at 12.42 vdc or about 75%. So using the truck actually reduced the charge level. When it is first started it is higher than after just a few minutes of driving, falling from the middle of the gauge to just even with left of that little battery symbol. The instrument cluster gauge jumps around a lot between what looks like 70%-100% charge, but maybe that is normal.

So next I will drive it for a few days and see it it settles back into a constant state of less than 40% charge. If my maintainer can get it to 100% I don't understand why the truck's charging system can't, but again very little about this system makes much sense to me.

May I ask how you’re calculating your percent of charge?

 

[TacoFreak]

May I ask how you’re calculating your percent of charge?

I am measuring the voltage of the 12 volt when the truck is off, and using the chart I saw on a different thread which equated the voltage to the batteries state of charge.

Here’s a table showing the State of Charge (SOC) for AGM batteries based on their voltage levels:

Voltage (V)	State of Charge (%)
13	100% (charging)
12.85	100% (resting)
12.8	99%
12.75	90%
12.5	80%
12.3	70%
12.15	60%
12.05	50%
11.95	40%
11.81	30%
11.66	20%
11.51	10%
10.5	0%

This table helps you determine the SOC of your AGM battery by measuring its voltage.

My lowest resting voltage was 11.9, or only 35% charged according to this table.

 

[goin2drt]

@goin2drt hmmm interesting. Do you keep signed in to the Toyota app all the time?

no

 

[Jt2024]

Staying signed into the Lexus app ran down the 12 volt battery in my wife's Lexus electric SUV. Lexus said they were aware of the problem but had no fix and suggested that she shouldn't remain logged into the app. It has not died since she quit using the app.

Those Toyota/Lexus apps provide vehicle status and need to frequently ping the vehicle for their status. The charging systems in Toyota hybrids and electrics are so fragile that communicating with the server appears to be enough to drain the 12 volt battery. I have never used it in my truck, but the 12 volt is still frequently at a very low level. For me the convenience of the app is not enough to not know if my truck will start or not.

I still use the Toyota App, I just sign out when done. Thank you for the update.

 

[TacoFreak]

I still use the Toyota App, I just sign out when done. Thank you for the update.

Yep - that works and signing out is all you need to do.

 

[gpburdell]

Yep - that works and signing out is all you need to do.

and if that is truly the cause, it’s a fundamental design flaw that will keep me driving my 3rd gen until fixed.

I like the 4th gen based on my test drives, but this and some other matters give me pause. I think I’ll wait to see what changes are in store for 2026 if any

 

[SilverSurfer]

Or….don‘t get a hybrid?

 

[gpburdell]

Or….don‘t get a hybrid?

Why on earth would I drop $50k+ on a non-preferred config? That's a silly suggestion.

Costs a lot less to just wait until the matter is sorted out.

 

[SnowmanJPS]

I am measuring the voltage of the 12 volt when the truck is off, and using the chart I saw on a different thread which equated the voltage to the batteries state of charge.

Here’s a table showing the State of Charge (SOC) for AGM batteries based on their voltage levels:

Voltage (V)	State of Charge (%)
13	100% (charging)
12.85	100% (resting)
12.8	99%
12.75	90%
12.5	80%
12.3	70%
12.15	60%
12.05	50%
11.95	40%
11.81	30%
11.66	20%
11.51	10%
10.5	0%

This table helps you determine the SOC of your AGM battery by measuring its voltage.

My lowest resting voltage was 11.9, or only 35% charged according to this table.

This is the longest I’ve gone not driving my truck so far, about 7pm to now 11am and my battery is showing 12.27

 

[TacoFreak]

This is the longest I’ve gone not driving my truck so far, about 7pm to now 11am and my battery is showing 12.27

That does not seem right to me, but I have never had a vehicle that I had to check so often, so I don't really know how bad that is.

I have been checking mine pretty regularly, and it has been keeping it at around 12.5 volts. But after not driving it for 3-4 days this morning it is 12.1, which is way too low. I'll call soon and get a service appointment and I hope it is just the battery. But I have started to keep a jump pack in the glove box in case it goes completely dead, or the dealer tells me it is fine.

 

[SnowmanJPS]

That does not seem right to me, but I have never had a vehicle that I had to check so often, so I don't really know how bad that is.

I have been checking mine pretty regularly, and it has been keeping it at around 12.5 volts. But after not driving it for 3-4 days this morning it is 12.1, which is way too low. I'll call soon and get a service appointment and I hope it is just the battery. But I have started to keep a jump pack in the glove box in case it goes completely dead, or the dealer tells me it is fine.

I'm also using the app, and have my viofo dashcam setup for parking mode too, and it’s a 2025 with an alternator, so that doesn't help the crowdsourcing here

 

[TacoFreak]

I'm also using the app, and have my viofo dashcam setup for parking mode too, and it’s a 2025 with an alternator, so that doesn't help the crowdsourcing here

OK - I had missed that your truck was a 2025, so even with an alternator they don't work right - nice job Toyota. From my wife's experience with her Lexus battery electric I recommend not staying logged into the app all the time.

I love my truck but I am starting to have serious concerns about whether I can trust it or not. If my dealer doesn't replace the battery I might get an Optima yellow top and see if that fixes it. If not I may trade it on something else before the warranty is gone.

 

[Bravada]

Not that I'm a battery expert, but likewise I've been concerned about not keeping my Taco on the NOCO charger, as I don't drive it very often. I bought a battery data logger to keep an eye on things.

I drove it for the first time since installing the logger on August 1st. so on the days prior you can see the state of charge goes from 90% to 100% while it's on the smart charger. After running the errands and parking it for the day, the SOC drops to 75%, and will continue to drop till I hook up the NOCO.

Maybe I'm crazy, but after driving approximately 40 mins with a few stops, I would have expected the SOC to be back up to 100% after getting back home. I've left the charger off to see what it does. I'll perhaps go for a drive tomorrow just to see what it does, but I don't have a lot of confidence in the charging system. The logger is an Ancel BM300 Pro.

 

[Cordoc]

Before commenting on your post and to avoid misunderstandings, I want to add the following:
I do not know what you are doing, and I do not know what the acronyms you use mean. Still, I own a Trailhunter. It has close to 6000 miles now but, when it only had 340, I decided to take a long trip abroad and left it in a covered parking for 9 weeks, and it started the same as if I had only been gone 24 hours.
It does not have an alternator but it does not appear to need one.
Hope this helps you.

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