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

Doesn't matter - it's says it's always going to charge if it has "low SoC". Don't have to do that in one trip.

 

Charging isn't immediate. It won't continue while the car is off. So it is possible SOC won't reach 'full' before the next trip. Leaving the battery to sit below the charge level for best health. While the car will continue charging the next time it is started up, it is doing so after the start up drain.

People think the battery is charged up everytime the car starts, but short trips will result in the battery abused. If the battery's SOC is low to start, even longer trips may not be enough to get it back to full.

 

Okay, I thought you were talking about something specific to the Prius algorithm. If you're never turning it on enough to get out of the low SoC state at all, then the Prius's power-saving algorithm is irrelevant.

 

I still don't think you guys are understanding it.

Gen 4/Gen 5 Prius does not charge the 12-V battery to 100% SOC except once in every 20 driving hours—no matter how long you drive the car (unless you reach 20 driving hours).

At all other times, the 12-V battery is tried to be kept within a specified SOC target range—I am guessing roughly between 75–85% SOC, but it could be somewhat different—as long as the car is driven long enough. Of course, if the car is not driven long enough, the SOC could drift toward 0% while the car is sitting, as there wouldn't be sufficient time to charge the battery when it is driven. But in no case the 12-V battery is charged to above the maximum of the SOC target range (~ 85%?), except once every 20 driving hours, it is allowed to charge to 100% SOC if the car is driven long enough.

 

The car keeps track of driving time? What about when I sit in the driveway and charge the 12 v, by keeping car on and in park, which I have done? Am I driving in the driveway standing still? Now I wonder why they named it driveway, and who they are.

 

I am guessing that as long as it is in the READY mode, it is considered being driven.

 

It will be interesting to see how the AGM battery will charge. While the AGM battery has a voltage higher by about 0.3 V, it also has a lower internal resistance, which could help it charge faster, despite the low voltage sent out by the DC–DC converter. The charging current is equal to the difference of the charging voltage and open-circuit battery voltage divided by the internal resistance of the battery. It looks like AGM batteries have about 1/5 of the internal resistance of the flooded-cell batteries; therefore, the charging current should still be at least three times or so that of the flooded-cell battery despite the higher open-circuit voltage (like 12.9 V vs. 12.6 V) even if the DC–DC-converter output voltage is 13.3 V (corresponding to a 0.4-V difference across an AGM-battery internal resistance vs. a 0.7-V difference across a flooded-cell-battery internal resistance, which, according to Ohm’s law, gives a 5 × 0.4 / 0.7 ~ 3 times higher charging current for an AGM battery).

 

Yesterday, I charged the 12 volt battery with my Topodon charger in REPAIR mode. I believe this helps to possibly de-sulfate the battery. It took about 8 hours and ended with the battery reading 12.9 volts while on the charger at the end of the repair mode. Removed the charger and checked the battery voltage several hours later with my BT battery monitor. Voltage reading 12.79 volts. This morning I checked the battery voltage and it read 12.71 volts. Typically, I have been seeing a loss of less than 0.1 volt for an overnight. This would be expected from the dark or parasitic current that is constantly there. Toyota states a dark current of 120 milliamps or less is acceptable as part of their fault isolation procedure for a depleted aux battery.

Made multiple trips this morning. The first trip was short (about 10 minutes) and the battery voltage read 14.1 volts for 7 minutes of driving then decreased to 12.8 volts. The next trip was 14 minutes and I noted the battery voltage read 14.1 volts for about a minute then decreased to 12.8 volts. Basically same scenario on return trip. Summarizing, the battery voltage was at 12.8 volts for estimated 85 percent of my driving. After arriving home, let car sit for a few hours and then checked battery voltage to be stabilized at 12.57 volts. My experience has been that the battery voltage, i.e., charging voltage, while driving is predominantly at 12.8 volts from all my multiple observations. 12.8 volts is basically a battery maintain charge and I don't see it ever getting the battery to what would be considered a full charge.

I know from experience that very time you open the power hatch or a door the car wakes up and a drain on the battery for 10 minutes or more. I have confirmed this using an amp clamp on the negative battery terminal. If the headlights come on at car shutdown, the drain on the battery is quite significant. Thus, I no longer use auto mode for the lights. For my car, the last test I did indicated about 80 milliamps draw with the car asleep. This would account for the battery voltage slightly decreasing overnight.

One feature that sold me on the Prime was the ability to make short trips in EV mode. Looks like, from my experience, the aux battery system may not be up to the task. The number of dead aux battery complaints I feel is significant based on the low volume of primes sold to date. Toyota has a lot on their plate these days with multiple recalls across their offerings. Their corporate debt load is significant. The Tundra engine replacement costs are going to be monumental. The TSB for the prime cars regarding the depletion of aux battery if cable left plugged in is a puzzler. The use the term SOME cars. Well how do they know for sure that any car still could be affected ? The procedure in the TSB for the tech to determine the anomaly exists is not very clear as i read it. The warranty labor for the ECU firmware is 0.7 hours. I don't understand why Toyota didn't make it applicable for all Prime vehicles since the number of Prime cars delivered to date is relatively small. Pocket change in my opinion......

Cheers

 

IF so, then Toyota is abusing the battery at the owner's expense.

 

I'll report the actual findings when I install the new AGM battery and BM2 battery monitor tomorrow or so.

 

If the charging voltage is indeed 12.8 V, then this is below the open-circuit voltage at 100% SOC of the AGM battery I'll be installing (~ 12.90 V), which is not good. However, it is also possible that the charging system recalibrates itself according to the battery SOC detected and applies a higher voltage (like 13.1 V) in the case of an AGM battery. I hope it is 13.0 V or above. I'll see what the battery monitor shows. Did you check the calibration of your battery monitor with a more accurate multimeter? It is important to know that it is giving accurate readings.

I would think BEV driving vs. HEV driving should make no difference at all—as it is the same DC–DC converter charging the battery, which is always powered by the traction battery, regardless of the internal-combustion engine being on or off.

Also, from the tests and reports out there, desulfation and repair modes on battery chargers/maintainers don't seem to really work.

 

Yes, I did confirm the accuracy of the BT battery monitor AND the voltmeter plugged into the 12 volt outlet with a voltmeter reading at the battery terminals. Spot on. I was amazed even the plug in voltmeter at the 12 volt outlet was accurate to the multimeter though only reads to 1 decimal point versus 2 for the BT monitor and multimeter, i.e. 12.8 vs 12.84

My experience today was both EV and HV modes with both charging rate scenarios identical and at 12.8 volts.

My take on all this is to understand the charging system, battery capacity and I can manage 12 volt aux battery with what I know. I researched getting a larger AH capacity battery but for know will stay with OEM. Interested to see how an AGM battery will perform. I have an AGM battery that came with my 2019 Ford Ranger. The truck has a battery management system (BMS) that kept the battery at about 80 percent charge based on inputs from other owners in the forum. I simply disabled the BMS and start/stop system and got 5 years out of the battery. It was still showing OK but decided 5 years was good. I did often the repair program on the Topdon charger and the battery internal resistance dropped significantly. Seems like 4 megohm is what I would get and the capacity was close to rated CCA. For me, I feel using the repair mode won’t hurt and seems to improve battery performance.

Cheers

 

According to the owners' manuals, the Gen 5 is specced for a bigger battery - 55Ah compared to the 45Ah of the Gen 4. Presumably that's intended to cover the fact that they're aiming to maintain a lower percentage SoC. The actual absolute charge target may be the same or higher. (45Ah = 82% of 55Ah).

I'm assuming there that the Gen 4 didn't do this battery shenanigans. Can anyone confirm?

 

No, Gen 4 and Gen 5 batteries are identical, except for the central-vent lid on Gen 5's for trunk installation. They are both 45-Ah.

I have never seen a 55-Ah battery in the EN LN1 size. They are typically 50-Ah; so, the OEM-battery capacity is on the low side.

Help: Need Photos of Gen5 Prius 12volt | PriusChat

The new-car features manual explains that the battery is not fully charged to improve the fuel efficiency. And that's one reason why the Gen 4 and Gen 5 batteries have been failing often.

Yes, the same problems exist in Gen 4, especially for Prius Prime. The battery-control algorithm is the same. Gen 4 and Gen 5 are very similar cars.

 

Out of my depth, but I’ve heard that sitting idling, battery charging is minimal, maybe even “negative”.

 

He is reporting that he sees charging when it is in park. Perhaps the idling 12-V-system power losses are higher so that the DC–DC converter is going into the constant-voltage mode (~ 14.4 V) as explained in the new-car features manual. Normally, in Gen 4 and Gen 5, it stays in the controlled-low-voltage mode (~ 13 V) for most of the time.

 

He is saying

I am not the only one here who has said leave the car on and let the traction battery charge the 12 v. Of course it has to be in park. It will chime at you if you get out in drive. Or in park actually but that doesn’t matter. It may be a bigger chime if left in drive and get out. I can force the car to charge, 20 hours timer or not.

 

Here's some data to digest from my car operational experience regarding the aux battery and charging system.

Yesterday evening, per my BT battery monitor, battery voltage was 12.57 volts. Early this morning, voltage was 12.52 volts. About 3 hours later, 12.51 volts. Drove car on 2 short trips. First trip the charging voltage was 14.2 volts and then 2 minutes later dropped to 12.8 volts. Trip length 6.6 miles and @ 12 minutes. Return trip charging voltage showed 14.2 volts on car start and then decreased to 12.8 volts less than a minute. Allowing car to go to sleep, battery voltage now 12.46 volts. From my experience in my situation, the car battery voltage will eventually hover in the 12.3 to 12.4 volts range. Thus it appears that the charging voltage I see of 12.8 volts will result in the battery in the 12.3 to 12.4 volts for my car and how I drive it. Most of my trips are 20 minutes and under.

It is what it is.......still love the car -

Cheers

 

In my case of a dead aux battery, I believe it is related to the possible depletion of the battery if leaving the charging cable attached after car finished charging per the TSB Toyota released in June 2024. The only time I had a dead battery was when I left the cable attached for a long time. However, unable to convince dealer to update the firmware per the TSB warranty provision. I have a level 2 charger so the car typically is charged 3-4 hours. I left the cable attached for a pretty long time thinking it was no big deal.

I don't believe the tech was made aware of the TSB (which I gave to the SA) nor did he do a proper diagnosis. Almost willing to pay for the TSB if I felt this dealer was competent to do the upgrade.

Cheers

 

Mine was at 75% soc tonight and I put a charger in it. It’s charging. This will be the routine as long as I have the car unless more input from other drivers finds a solution to any issues, if there is one.