Okay. Who HASN'T had a 12V battery issue with their 2023-24 Prime?

They are required by Toyota to test the battery as part of the PDS here in the US.

 

I think it is an issue for both versions of the Prius Gen 5. I've seen a few posts here by Hybrid only owners on the 12V issue.

 

My Prime has been on the 12 v charger all night. The car has been charging the battery well every day in the morning, since the temps went into forties overnight. Still the soc was 69% last evening after a short drive. It wasn’t charging during the drive. On the 12 volt charger it went. Part of my ownership now. The battery monitor makes it a fact driven process rather than guessing.
I haven’t checked into if a permanent 12 volt maintainer could be mounted under the hood. I would have to open the hood anyway and by now I am pretty quick about putting the clamps on and holding the hood up with my head to save the step of using the hood rod. I use a waterproof charger if it’s outside and wet.

 

I hope this does not also apply to my Gen 5 2025 Camry XLE.

 

It probably does. Toyota probably uses the same miserly, bug-ridden BMS algorithm in all of its new vehicles, especially the hybrids, in which they think they can squeeze an extra 0.1 mpg by limiting 12-V-battery charging.

 

After a night of charging, a 69% SOC makes no sense. As I told you, you shouldn't trust the SOC numbers shown by the battery monitor. They are calculated by a simple buggy algorithm using the battery voltage alone. There is no way you can calculate the SOC from the battery voltage alone without also monitoring the current. The best way of doing that would be to look at the resting voltage, but that’s not what the battery monitor does.

Also, there is no need to connect the battery maintainer everyday or even every week if the car is seeing half-hour trips a few times a week. Once every few months should be enough. The daily SOC loss is around 2.5%; so, if the car is not driven for longer than a week, the battery maintainer should be connected.

 

It was after driving a short trip in the evening in ev mode, no charging of the 12 volt. The soc is more or less ok, good enough for the purpose, regardless of what is said to the contrary. I look at monitor often and find no discrepancies that would indicate extreme oddball readings. Everything proceeds as expected when charging etc. I have the Ancel type on the Prius. I think there is more to it than you may know although it seems you know a lot. it would be like a cell phone or power tool battery soc readout I think. Do you question those? I read about it once but honestly it isn’t that important to remember. I think it works and it seems to be about right, however it works.

 

SOC stands for state of charge. Charge is the time integral of current, the latter of which is charge per unit time. Therefore, you need to measure the current as a function of time and then integrate it to obtain the state of charge (SOC). You cannot estimate the SOC from the voltage without making crude assumptions about the charge-vs.-voltage characteristics of your specific battery type.

Smartphone, power-tool, and traction-battery SOCs are calculated by the BMS, which also have current and temperature sensors. Likewise, battery maintainers have a BMS in them, which is equipped with current and temperature sensors, and as a result, they can reliably calculate the SOC. On the contrary, your battery monitor does not have a current sensor to be able to reliably calculate the SOC.

The Toyota BMS in your car actually does calculate the SOC, but to read that, you need to buy and connect a communication device. @Roy Peterson has an expensive one that can show the BMS-measured and calculated voltage, current, temperature, SOC, SOF, and SOH.

 

We currently have a rental from the dealer (Corollla Cross Hybrid AWD). Brought the Prius in on Saturday, and SM said that they would test the battery. If it were to “pass”, a replacement would not be covered by the warranty (about 700 out of pocket, I was told). The battery did indeed fail, but they did not have a replacement (hence the rental). We bought the car in October. Our car was manufactured in March 2024. I think the dealer received the car onto its lot late spring/early summer. Because of the door seal issues (referenced elsewhere on this forum), they couldn’t sell the car until late August/September (I think this was the timeline). Shortly after purchase, the car was flashing lights inside and all, but wouldn’t start. So, onto the tender it went, and I’ve been keeping it tendered most days (in a garage) since purchase). Always after being tendered, my smart tender gave me the green light. I’ve assumed that the battery is “good” or good enough (I thought that the tender would NOT give me a green light if the battery were bad - guess I’m wrong on this). Ultimately, I believe, the car never had enough time to fall below some charging point until my wife left it at the airport for the 3+ days. Short story is - I think you’re correct, and the battery was probably bad when we got it. As for when they’re going to get a new battery, they can’t tell us until Tuesday at the earliest. Hopefully a new battery does the trick. If not, this new Corolla Cross ain’t half bad.

 

So there are certain scenarios where the BMS will bring the SOC to a healthy level? Any idea what will trigger it?

 

The BMS likes to keep the SOC around 80% or so. But it will never be successful in bringing the SOC there after a deep discharge. You should hook up a battery maintainer if a deep discharge happens.

 

It appears to me the dealer did not follow Toyota's procedures for keeping vehicles on the lot. They are supposed to check & charge the 12V battery monthly.

 

Interesting point. I wonder how many of the cars with the battery issue sat on dealer lots for a while before point of sale.

 

I never would have thought SOC stands for state of charge. Well the soc value doesn’t change when the voltage changes on the graph. You must have seen this by now. When the car voltage goes up, soc does nothing. When car voltage goes down, soc does nothing. My power tools batteries show SOC when they are idle and unattached.

SOC doesn’t go up and down in response to the voltage readings. It seems fairly accurate and works for the purpose. When it says X soc it seems to inch up as expected % by % when charged by car or external means. When it gets to 100% the external turns off about the same, and goes into the other maintainer modes.

 

FYI, here are the guidelines.
https://dot.report/bulletins/MC-11005052-0001.pdf

 

Interesting. My suspicion with my 12V problem is that the pre-delivery routines were skipped.

 

Power-tool lithium-ion batteries have a built-in BMS inside that calculates the SOC. Again, the SOC is charge, which is the time integral of the current, and that is how the BMS calculates it—by taking the time integral of the current that goes in (positive, a rise in the SOC) and out (negative, a fall in the SOC).

Of course the battery monitor does not directly map the instantaneous voltage to the SOC, as this obviously would not work. It has some algorithm to correlate the SOC with the voltage history. It is far from being exact and reliable since the current is unknown to it.

 

Therefore the battery monitor I have has a bms. I think yours has a soc readout. I have one of those on a relatives car so haven’t been able to watch it like I do my two. Last night 88%, this morning 86%, car decided charging wasn’t happening this morning. I put in park as much as possible, wanting to catch as many lights as I could. The longer I wait at a light, the better, stress free living. On return, 6 miles total, battery climbed to 91%.

 

If it had a BMS, it would have to have a current sensor as well, which would require more than two terminals to connect to the battery.

 

I don’t remember any extra wires on battery chargers except the two.