12 volt battery failing to start the car

The battery apparently charges when charging the traction battery was my main point, trying to be diplomatic about writing it. Yes the battery drains when off. I don’t drive long enough every day for it. I had this on the gen 2 also but drove that one more, still had battery draining.
I also charged my 96 Toyota truck that sat for about a week unused. It has a computer but nothing as far as extra equipment. It starts extremely well so even a half charged battery is hard to tell. Like it starts in a second. It took a long time at 4 amps to bring it to full. I left it overnight and next morning it had stopped charging.
Neither car has an old battery.

 

right. that was documented by @Salamander_King quite awhile ago. sounds like you need a 12v maintainer

 

Since I have been summoned several times on this thread, I am going to chime in just to mention that the anemic 12v battery issue is nothing unique to Prius. I don't own a Prius anymore. But my current vehicle which is a 22 Ford Escape PHEV has the same problem. There is a long thread in the Escape forum discussing how to change the OEM flooded lead acid 12v battery with a larger AGM battery to prevent the frequent and repeated "Deep Sleep" mode warning when the 12v battery SOC becomes low. Ford's PHEV system works similarly to Toyota's, and the small stock 12v battery especially in HEV and PHEV models of Escape suffers from low or dead 12v battery frequently.

So far, I have not had a problem with mine. As I have discovered with Gen4 Prime, at least ~1 hr drive once a week keeps the 12v battery happy. If the car is not driven often enough and long enough, then monitoring the 12v battery using an onboard BlueTooth battery monitor would be good insurance. It's cheap, easy to operate, and doesn't even need to turn on the car to check. Yes, the unit draws a bit of amp from the 12v battery, but by itself, a draw of 1mA does not drain the 12v battery if it is healthy to begin with. I have it installed on every car I have owned since my last second PP (2020MY).

Dead 12v Battery?
How I chose to monitor "all" my 12V car batteries
Winter is coming -- 12V question

 

I have them. All the cars I have had for years now have to get charged. I thought what I was reading here said the 12 v doesn’t charge during hybrid battery charging. Have to look for the salamander king post.

Good info, I used only an obd reader, or multimeter. I was saying the same thing, many cars, essentially all cars, have this “problem.” That’s why stores are full of charger maintainers, because they sell them. I think people go to he is slamming the Prius, which I am not. Just the facts.
Any current best battery monitors?
Where is the thread about 12 v gets charged while charging the traction battery?
Thanks

 

You can find the Quicklynks: Battery Monitor II (BM2) in Amazon or other auto parts box store for ~$25: http://www.amazon.com/QUICKLYNKS-Battery-Monitor-Bluetooth-Device/dp/B01MT4583U

Disclaimer: While I believe the Gen5 PP also behaves the same, my analyses are all based on the Gen4PP. There maybe some differences in how the charging system behaves in your Gen5PP.

This is the most extensive thread on the 12v battery charging on Gen4 PP: 12v Battery Is Discharging, Re-Charge Now !
But if it is TL:TR, then I summarized my finding on the 12v battery charging system on Gen4 PP: Hopefully, ultimate 12v battery charging system discussion on PP

 

Thanks for the time answering. Got the monitor $22 happened to be on a deals. Plus Uncle Sam.

 

Thanks for posting those links. I haven't read through them yet but . . . has the minimum voltage necessary to get into Ready mode ever been established?

 

Practically speaking, in my old Gen 1, I usually had little trouble down to around 8, below that I would need luck, below 7.5, fuhgeddaboutit.

Several ECUs in the car will log voltage-related trouble codes, usually triggered at 9.5. The Toyota manuals will tell you if it's below 11, just go charge it, already.

What it actually takes to reach READY in a given car and situation will depend on a bunch of things, the temperature, the pressure in the brake system, yada yada.

 

See the above by @ChapmanF for a more technical answer. I never attempted to find out how low it can go. While I was monitoring the 12v battery in my Gen4PP with the BM2 that sat on our driveway for a prolonged time (up to a month) due to COVID restriction, I always charged up the 12v battery by connecting to an external charger when the resting voltage dropped below 12.4v. I even thought about installing an on-board 12v charger under the hood, but never did. In retrospect, I think that is probably overkill.

My feeling is that if the 12v battery never drains below 12.0v on resting voltage and is kept topped up in good condition, it will last a normal life span, which in our climate is ~5 years. I have had an original battery that lasted 10+ years in my older less electronically demanding cars. If the 12v battery gets drained to the level it does not allow the car to start, even a single time, then the chance that it will prematurely die will increase, I think.

 

The only time you’ll have voltage to the lighter socket is when the car is on, which will throw off your voltage reading.

 

Won’t this QUICKLYNKS cause a draw on the 12 volt battery itself?

 

Won’t this QUICKLYNKS cause a draw on the 12 volt battery itself?

 

Yes, as I stated in my comment above. It draws 1mA from the battery. If your battery is healthy, this additional draw is small enough that it should not cause a problem.

 

Yes, I know. As I explained earlier, the initial readings I listed are taken in ACC mode.

When first pressed it reads 11.6v then within a few seconds it jumps up to 12.0, so the initial dip indicates that there is a bit of current being drawn. After I start the car it jumps to 14.3v which indicates that the traction battery is charging the 12v battery. After driving a while it will eventually settle at 12.7-12.8v but if I make a stop and shut the car off it goes back to charge mode after a restart and reads 14.3v again for a while.

I ordered one of the Bluetooth units so I'll be able check voltage without anything on.

From what @ChapmanF says it seems that it probably takes very little voltage to get into Ready mode.

 

Using a battery monitor, I don’t see any 12 v charging when plugged in to charge the traction battery, on level 1or 2.
What I find is I don’t drive long enough to keep the 12v full. It slowly gets lower, charges up some in the 3-10 minute drives, but in the end I have to use a battery charger. It’s charging the 12 v right now, a Solar 4 amp. Oh well, if that’s lifes biggest problem…

 

that sounds about right

 

It took me long enough.

 

Sorry to revive an old thread but I thought my question would work here. For those of you trying to recharge or maintain the battery. Are you charging at the battery in the trunk and disconnecting the negative terminal per the manual?
I’ve seen others try and charge the battery through the jumping point under the fuse box under the hood.

 

I have ours near-constantly on a charger between uses, which is once or twice weekly, sometimes even less. Never disconnect the battery, and have a quick-connect wiring system installed for the charging. My connection is under the hood.

 

If your car is a Gen5 Prius Prime (2024) as indicated on your profile, then you don't have the 12v battery in the trunk. The 12v battery is under the hood in the engine compartment and there are no jumping points. You can disconnect the negative terminal from the battery post while charging for safety reasons, but as @Mendel Leisk pointed out, I also charged my Gen4 Prius Prime 12v battery without disconnecting, by directly connecting the charger to the battery posts without an issue.

EDIT: I just found out that the Gen5 12v battery is back in the trunk again. I thought it was under the hood as in the Gen4. Sorry for the confusion.

 

Thanks!

I ended up charging the battery in the trunk last night and it was difficult since I wanted to keep the hatch and doors closed to minimize any draw on the battery. I ended up running the charger cord through an open window in the back.

I only did this since I saw a low battery warning when sitting in the car last Sunday. Being proactive to keep myself out of trouble.

Maybe I’ll try the jump points under the hood next time.
Or get a tow and start a file with the dealer if there’s a bigger problem at hand.