I want to alert everyone of an issue with the auxiliary 12 volts battery in the new Prius prime 2024

It's been only a day so far, but mine has been staying at 100+% SOC. So, I am more worried about overcharging than undercharging. LOL Joke aside, I'm impressed how good the Prius Prime charging system is. But then, I have an AGM battery, which charges five times as fast as a flooded-cell battery. I think an AGM battery is a no-brainer in a Gen 4/Gen 5 Prius/Prius Prime. By the way, I found that your charging-at-park thing is not really the case, as it sometimes happens and sometimes doesn't. In my case, charging happens quite often, regardless of the gear selection and other driving parameters.

But, yes, a battery maintainer is a good idea for Gen 4/Gen 5, especially if you are not using an AGM battery.

Also, there is no evidence of parasitic drain so far.

 

Interesting oddity - above I said the Prius had a 55Ah battery. That came from the European manual, which says.

"Use a 12-volt battery that the case size is same as the previous one (LN2), 20 hour rate capacity (20HR) is equivalent (55Ah) or greater, and performance rating (CCA) is equivalent (345A) or greater."

The US manual for both HEV and PHEV says

"Use a battery that the case size is same as the previous one (LN1), 20 hour rate capacity (20HR) is equivalent (45Ah) or greater, and
performance rating (CCA) is equivalent (285A) or greater."

which is what people have shown here.

European manual error? Would seem odd to be using a different size here. Can LN2 even fit? I'm going to need to take a look later...

 

EN LN2/DIN H5/BCI 47 (typically 60 Ah) is 3.5-cm-wider than EN LN1/DIN H4/BCI 140R (typically 50 Ah, depth and height being the same). At least in my Gen 4 Prius Prime, there is minimal clearance in the battery tray (in other words, EN LN1 fits snugly in the battery tray); so, EN LN2 wouldn't fit. You can check what battery is installed in yours and how much clearance you have in your battery tray.

More importantly, if you replace your battery out of warranty, you should definitely upgrade to an AGM battery. The choice of a flooded-cell battery by Toyota was only done to reduce the production costs but at the expense of the owners having reduced reliability. That's one reason why Gen 2 and Gen 3 Priuses, which had AGM batteries, had fewer incidences of battery problems.

The only downside of an AGM battery is the increased weight (about 7-lb-heavier for EN LN1), but it shouldn't be a big issue.

 

Yep, my car has the expected 45Ah LN1 CCA 285A battery. I wonder how that manual error arose.

 

Mine 100% of the time charges in Park, and never, ever, has reached 100% oon it own unless I leave it in driveway charging in Park. Plus the minute I unlock and start car the soc is like 95% already. This is good, what I wanted to see. Now if we can see your three day etc graphs like I displayed mine. Maybe agm is the way to go.
Btw on monitor I changed my negative to the body instead of battery post. Didn’t change anything I could see how it works, but I thought the power used then goes through the sensor. I used the screw on the washer container, loosened it and slid the neg terminal in, then tightened.

 

Mine seems to be charging most of the time (above 13.0 V), regardless of the driving mode. Of course, 13.50 V is more like float charging. It also occasionally charges at 14.1 V. Sometimes, it will hover between 12.85–12.93 V, which means that it has suspended charging. I've seen all sorts of voltages, including charge suspension (12.85–12.93 V) when in park.

I wouldn't rely on the SOC estimates in the BM2 app. The best way to estimate the SOC is to look at the voltage after the car has been sitting for at least twelve hours without having been turned on and approached with the smart key (if you come too close to the car with the smart key, the battery starts draining). Mine has been over 13.02 V so far, which is 100+% SOC.

You should connect the battery sensor directly on the battery; otherwise, the readings may not be accurate at higher current levels because of the resistive losses in the connection path. It only measures the voltage and temperature and sends the data out through Bluetooth. Unlike the battery sensor in the car, it does not measure the current, which would require a shunt resistor (in series with the battery). If both the voltage and current were known, then the app could calculate the true SOC, SOH, and SOF from the voltage and current data, as that is what the car does.

Again, I find the battery sensor and battery management system in the Gen 4/Gen 5 Prius/Prius Prime to be very smart, and I don't think there is any shortcoming or negligence by the Toyota engineers. The failures are rare, and they are happening in extreme cases. The best way to prevent them is to buy a Noco Genius 1 (or higher) and fully charge the battery occasionally, at least once immediately after the car is purchased, as it quite likely that when you first buy the car, it could come with a very low SOC. In fact, I think this is the reason for the Gen 5 12-V battery failures so far: the 12-V battery having a very low SOC when you drive it off the dealer lot. If the battery has a very low SOC (say, under 50%), then the car's battery management system will not be good at recharging the battery at those low SOC levels, and you will need to hook it up to a Noco Genius or such battery charger/maintainer to get it back to 100% SOC and have the battery management system recalibrate itself. And, yes, an AGM battery works a lot better because it charges five times as fast as a flooded-cell battery, has much lower self-drain, can do deeper discharges more cycling, etc.

 

Ive never seen charging turn off in Park and of course car has to be in ready mode, not off. Not once, it goes on within one second. My graphs show the spikes. The monitor has consumption, I wanted all of it to go through the factory sensor. Somewhere online some expert said not to put it directly on battery so I changed it. Absolutely no difference in any way.
Yes people have already talked about battery sensors and monitors having a shunt to read amps. Quite a few times now. I’m comfortable with the soc readings as my need to know them is met with the device.
You could have had more data by recording the old battery, then seeing changes due to the new battery to compare flooded to agm. You seemed to charge the new battery a long time, 42 hours. I never have had to do that in my life, they usually are close to full. Maybe you overcharged your battery and are living off the over charge.

 

I see charging being turned off in park. As I repeatedly said, do not trust the app’s “charging” message and SOC indicators. Look at the voltage. If it is under 13.0 V, it is not really charging.

The current drain by the battery monitor is 1.5 mA. The AGM battery has an internal resistance of about 5 milliohms, and a flooded-cell battery is five times that; so, in any case, you are looking at a voltage drop in the microvolt range, and the battery management system wouldn't do anything about it if you pass the 1.5 mA current through the battery sensor or not. You will get a higher voltage accuracy at higher current levels because of Ohm's law if you connect the battery monitor directly onto the battery terminals (resulting in a smaller resistive voltage drop).

Noco is the best in the business. They know how not to overcharge a battery. What they did was to use constant-current charging at the end—I am assuming a very small current—and let the battery slowly reach 14.8 V. Everyone uses these chargers; so, I am not worried a bit.

 

It’s never under 13 volts when charging according to my monitor. I have said this maybe 10 times over a year or so. The charging is very strict right at 13 volts. If your battery is charged, maybe it goes off in park as it is finished charging. The soc is more complex probably than you know. You are locked into the chart. When you think your volts show 100 soc by your reckoning, hook up your noco and see if it charges and for how long. If your 100 is correct, the noco should show fully charged right away or soon. If it takes awhile, you know your 100% idea is incorrect.
I bought the Noco 4 amp for my daughter, nothing particularly special about it I can tell. There are a lot of makes.
So far a lot of words, but no pictures.

 

You are absolutely right that SOC is more complex. You need to measure both the voltage and current for a period of time. That is exactly why the BM2 app doesn't really know what the SOC is, as it doesn't measure the current. Did you know that you can customize the SOC level in the settings and change it from the default? Your best bet of estimating the SOC is looking at the open-circuit voltage after the car was off for half a day or so, with the smart key away from the car.

As I said, I will create a separate thread with pictures.

 

This afternoon I decided to check for the dark current (parasitic current) on my 2024 Prius Prime XSE Premium. OEM battery installed and car has not been driven since yesterday.

Starting voltage was 12.36 volts per the BM. Opened the hatch (powered hatch) to install my Fluke model 27 in direct current measuring mode. With the alligator clips I have, I was able to install the meter in series before fully removing the negative terminal. Installed a voltmeter across the terminals of the battery. Closed the hatch which results is about 4-5 amps then settled at 2 amps for about 7 minutes and slowly dropped off with time.

At 30 minutes, the voltmeter read 12.36 volts and the current would pulse between 20 and 60 milliamps at a frequency 3-5 seconds. At 120 minutes, voltage still 12.36 volts and current fluctuation 20-50 milliamps at about a 6 second frequency. I ended data collection. I have no explanation for why the current would fluctuate. I did note no change in the 12.36 volts during the current fluctuations.

For my driving, I noted that per the BM my at rest voltage is typically between 12.3X to 12.4X volts. After charging the traction battery, the aux battery voltage will be slightly higher at about 12.6X volts. However, after driving my typical trip lengths that vary from 10 to 20 minutes, the voltage returns to the above mentioned range. I attribute this to the fact that charging voltage during my trips starts at 14.1 volts but decreases within a few minutes or less to 12.8 volts for the remainder of the trip.

Toyota threshold for dark current is 120 milliamps or less per the troubleshooting procedure for a depleted aux battery. Thus I would surmise that a car as delivered with no after market items will reflect the current draw that I observed and this is in the expected range for current production high technology vehicles.

I purchased a replacement AGM battery and plan to install to monitor how it performs as a replacement for the OEM battery.

Cheers

 

A ~ 40-mA continuous drain in the sleep state sounds about right. If the car is plugged in, it will be much higher. There is also a much higher sleep-drain event (a few amperes or higher) that happens about once a day and lasts for a few minutes. So, the long story short, expect at least a 1% SOC drop per day. It could be 2% or higher as well.

I see the same voltage behavior as you do. 12.8 V you mentioned is 12.85–12.92 V in my case. The 14.1 V you mentioned is also there. There is also the 13.5 V when the car is in the park gear, which @Mr.Vanvandenburg takes advantage of and I verified, too. The Gen 4 and Gen 5 charging systems seem to be identical.

My rested open-circuit voltage for my AGM battery has so far been staying at 12.87 V, which is close to 100% SOC. Your 12.4 V is about 75% SOC for a flooded-cell battery. I think it is on the low side. Perhaps you have already experienced some battery degradation or you need to manually charge the battery for a couple of days. It takes about two days for Noco Genius to fully charge a battery.

How were you able to insert the ammeter in series without disconnecting the battery? That seems to be a logical impossibility.

 

Both leads installed in the meter ready to go. Hook the positive lead of the meter to the negative cable ( there is a crimp connection on cable). Loosen the negative cable clamp and lift the clamp up slowly enough to install the alligator clip on the battery negative terminal below the clamp. Then completely remove the negative cable clamp from the battery post. Voila. All battery current now flows thru the meter. I’ve done this before on other cars. Saves not having to reset functions in the car. On my Prime, if battery disconnected completely, just have to re-initialize the hatch system by manually closing the hatch so not a big deal really.

Cheers

 

Ah, yes, I see it, good technique! I didn't think of shunting the ammeter across the battery post and connector before you disconnected the negative terminal, but now it's obvious.