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

[Tom Sellick]

My Pro sat for 2 weeks. Dead as a door nail. Put the tester on it and it read 2.6V or something like that. Took it to the dealer. They put it on the charger and ran the test. Battery passed so they said they couldn't replace it. I complained exuberantly about how pathetic it was that a new vehicle couldn't hold a charge for 2 weeks. They said it's the norm nowadays.

Sponsored

 

[TacoFreak]

12.2VDC after a 30 minute drive is a bad battery (or bad charging system) regardless of AGM or lead-acid. Given the amount of self-discharge in two days it's likely the battery unless there's some significant parasitic draw.

That is what I thought too, and I will see what the dealer has to say. I have not added anything that is drawing down the battery beyond the many things that today's vehicles do, even when off. I assume that Toyota designed for that, but they did not account for that in my wife's battery electric, which struggles to keep the 12v battery charged.

I complained exuberantly about how pathetic it was that a new vehicle couldn't hold a charge for 2 weeks. They said it's the norm nowadays.

If your battery was down to 2.6 V it seems like either there is some excessive draw, or the truck is not keeping your battery charged. If it was me I would try another dealer because yours seems more lame than most car dealers today.

 

[Lennyz1]

@Lennyz1 My battery has never read at the levels you posted. I returned about an hour ago from a 30 mile freeway drive and it now reads 12.22 VDC. My last reading was 11.89 V after it sat for two days. I suppose it might be my meter, but the readings I get from it make sense when I use it for other things. It still starts fine but I guess I need to find the time to take it to my dealer and see what they say. I have not opened up the back seat to see if it is an AGM or not.

Here is a chart for AGM batteries. I am shocked at problems since Toyota has been using AGM batteries for hybrids for a long time.

Regular battery charging can break AGM batteries. Regular batteries need 15-17 volts to get the same amps. However, voltage greater than 15 volts can overheat an AGM and generate enough pressure to pop its safety valve.

 

[Tom Sellick]

That is what I thought too, and I will see what the dealer has to say. I have not added anything that is drawing down the battery beyond the many things that today's vehicles do, even when off. I assume that Toyota designed for that, but they did not account for that in my wife's battery electric, which struggles to keep the 12v battery charged.


If your battery was down to 2.6 V it seems like either there is some excessive draw, or the truck is not keeping your battery charged. If it was me I would try another dealer because yours seems more lame than most car dealers today.

Yeah. I'm monitoring for now. It's been fine but hasn't sat for long. We will see.

 

[gpburdell]

Here is a chart for AGM batteries. I am shocked at problems since Toyota has been using AGM batteries for hybrids for a long time.

Regular battery charging can break AGM batteries. Regular batteries need 15-17 volts to get the same amps. However, voltage greater than 15 volts can overheat an AGM and generate enough pressure to pop its safety valve.

It's possible for manuf to get bad batches of batteries from their suppliers. Happened not long ago to Ford for their Bronco Sport and Mavericks, enough so to generate a recall.

 

[Baltimore]

There was another post either here or on TacomaWorld in which some folks mentioned they were having battery drain issues when the vehicle was sitting and found the root cause to be Toyota Connected Services. When they disabled the services that provided constant updates to the phone app, their battery drain issues stopped.

I've never had an issue, but I also did not activate any of the subscription services, even those with 10yr trial periods. Never had it in my prior vehicle. Never needed it. If I never play with it, I can't get enticed to keep paying for it.

 

[TacoFreak]

@Baltimore My wife's Lexus battery electric 12v battery went low enough a couple of times that it had to be jumped.

It was just a few months old the first time it happened and I did not check the voltage before charging it, so I don't know how low it went. Lexus tested the battery and said it was fine and they contacted Lexus about the problem. They were told the draw was from the status updates requested from the server when she was logged into the app. They said it was a known problem and that they did not have a solution, other than to not stay logged into the app.

It has not gone dead again since she stopped using the app. I know that doesn't prove anything but it kind of fits what we see with Taco hybrids. It also seems like a possible reason why Toyota added an alternator to some of the hybrids in 2025.

Looking at the chart provided by @Lennyz1 it looks like my truck can drain the battery to a very low level after only a few days of not being started. Either my 12v battery is bad or my truck is not charging it properly and I'll let y'all know when I find out.

 

[MustardTiger]

I’ve definitely heard of the connected services (potentially) causing parasitic battery drain. I’m not logged in however, don’t use it.

I’ve left my truck parked for 2 weeks, starting yesterday. Directly to the battery I connected a smart charger (1.5 amp), ran the cord through the window, and it’s connected to a WiFi plug. I’ll let that run for 48hrs or so…then switch it off. When I get back…that damn battery better be like 12.6-12.8 resting volts…if it’s down to 12v or less again…I’m taking it in!

 

[Lennyz1]

There was another post either here or on TacomaWorld in which some folks mentioned they were having battery drain issues when the vehicle was sitting and found the root cause to be Toyota Connected Services. When they disabled the services that provided constant updates to the phone app, their battery drain issues stopped.

I've never had an issue, but I also did not activate any of the subscription services, even those with 10yr trial periods. Never had it in my prior vehicle. Never needed it. If I never play with it, I can't get enticed to keep paying for it.

My truck was recently at a body shop for a couple of weeks. It was started and moved only 3 times during that time and I looked in on the app a couple times per day to see windows open, hood open and which doors were. Battery never died. I did not check battery voltage, so it could have been low.
I have never put a meter on my battery to check. I think I will start doing some checks since I do let my truck sit for a few days at a time quite often.

All of this reads of parasitic draw somewhere. All new vehicles come with communication system built in and not all vehicles are having issues dead batteries. Probably solvable with a software update.

 

[Lennyz1]

It's possible for manuf to get bad batches of batteries from their suppliers. Happened not long ago to Ford for their Bronco Sport and Mavericks, enough so to generate a recall.

It is always possible.

Sponsored