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

[MustardTiger]

My truck does quite a bit of sitting in the garage, as I'm away for longer periods of time. I've been monitoring the state of the 12v battery...and after returning a week later, I unlock truck and open hood...let things settle for 15 mins...and battery shows 12.0v. I feel it should be at least 12.5-12.6 (like 80-85% charge). Before driving it, I connect a 7.5 amp NOCO charger and let it complete a charge cycle. The truck has never not started for me however.

A couple things to note:

-my truck was delivered in the dead cold of winter, it was like -30c temps. The day I went to purchase, it was out on the lot, -35c...they just started it...immediately backed up through a snow bank and into the shop.
-it had been sitting on the dealer lot for about 1 month (in said cold temps) before I seen it and purchased
-after getting it home, I noticed the little red flip cover on the boosting terminal was broken at the hinge...and it was obvious booster cables had been attached at some point, maybe during transport? hard to say

Maybe...my battery was in a very low state of charge for an extended period, in freezing temps, prior to me buying it, and it either froze or lost a significant amount of life/amps

Anyone else have a 12v issue like this? Particularly the drain after sitting for more than 5 days?

Sponsored

 

[Lennyz1]

You battery is an AGM. Charge should be 12.7 minimum at rest which is approx 12 hours since last charged. Typically 12.4 VDC or lower and it should be considered for replacement. If it is at 12.0, then it probably was at one point heavily discharged and damaged. I recommend having dealer test battery

 

[MustardTiger]

Charged the battery yesterday, then drove the truck for an hour...come home, left it in the garage with the hood up and doors locked. 12 hours later...battery shows 12.4v. Ugh...looks like i need to take it to the dealer, hopefully they replace the battery. I'm doubtful

 

[max911]

I just dropped my truck off at the dealer for a similar issue. Getting tired of having to pull out my booster pack every morning, or every time i have the tailgate down and doors open for ten minutes.
Will let you guys know how it goes.

 

[AZTaco]

Out of curiosity, do have a '24 or '25 hybrid? They added the alternator to '25 hybrids but only if you have the 4th Aux switch. The theory is that the alternator is needed if you are running all four aux switches at once.

I find it hard to believe that, after one year, Toyota received so much feedback for the need for a fourth aux switch that they went to the effort to add an alternator to make the switch work.

But if it were to solve a battery charging issue, why wouldn't all of the hybrids get the alternator?

FYI, the first Pro I ever saw (2024) was on the side of the road with the dealer. I stopped to look at it and 1. They were trying to jump start it directly from the battery by removing the rear seat, and 2. They said the battery died because it was just trucked in from Mexico (BS because Hybrids are shipped to the US and at most it was trucked from a port in CA to AZ).

 

[MustardTiger]

@AZTaco mine is a late model production 2024…I purchased it in early February 2025, and I think it was delivered to the dealer around mid January. I do have the aux switches (3), no alternator. I’ve been watching the volt meter gauge in the cluster as I’m driving…it doesn’t seem to apply a charging voltage (14+) all that often. Idk…the truck has always started…but I have been topping up the batter now and then with a charger

 

[TacoFreak]

My truck has always started just fine, but reading this inspired me to check the battery level. Checking it at the jump point in the fuse box it reads 11.89 VDC right now, which seems like a problem to me. I might put it on a maintainer for a while and see what happens and then talk to my dealer.

I don't think Toyota would have added the alternator just to get another AUX, I think the design is screwed up and all hybrids should have an alternator, not just trucks with 4 AUX switches.

 

[max911]

Picked up the truck. New battery installed on warranty.
(monitor recall performed as well)

Old battery was tested and would not accept/hold a charge properly.
I noticed on the drive home that the volt meter stayed at the mid point- 14v. Whereas before it would drop down to 12v after five or ten minutes of driving.

MustardTiger, I too picked up my truck from the selling dealer during a cold snap in December. -30ºC.
When new i found one of the push pin fasteners in the console and couldn't figure out where it came from. When accessing the battery under the seat I noticed one fastener missing. Guessing the dealer lifted the back seat to boost the battery. They might not have known about the +post in the fuse box under the hood for boosting as I think this was one of the first Hybrids that they sold.
Points to the possibility that the flat battery was damaged somewhat by the extreme cold.

Now I have to go through the procedure of shutting off some of the nannies that have been reset by default. No biggie. Happy i didn't have to shell out for a new battery myself.

 

[MustardTiger]

This doesn’t look like an AGM battery…

@max911 great news, glad it was no hassle for you to get it replaced! I’ll be leaving my truck sit for a good 2 weeks. May as well leave it as-is without a trickle charger attached…and if it doesn’t start when I get back, then a new battery will be indisputable

 

[MustardTiger]

My truck has always started just fine, but reading this inspired me to check the battery level. Checking it at the jump point in the fuse box it reads 11.89 VDC right now, which seems like a problem to me. I might put it on a maintainer for a while and see what happens and then talk to my dealer.

I don't think Toyota would have added the alternator just to get another AUX, I think the design is screwed up and all hybrids should have an alternator, not just trucks with 4 AUX switches.

I also think there’s more to the story with alternator addition. I wonder if the Grand Highlander MAX is experiencing a similar issue, and if it has an alternator or had one added too

 

[MustardTiger]

Hmmm…interesting results.

also…I would guess my selling dealer had already replaced the battery…as this one isn’t a factory one. It’s a Toyota (Canada) parts counter battery

 

[Lennyz1]

Could be different in Canada. AGM in my 24.

I believe this is the Toyota battery for hybrid.

Sponsored