AGM battery for Gen 4/Gen 5 Prius/Prius Prime and observations on the 12-V charging system

-No I do not know definitively why the original Yuasa battery failed.

-I did observe resting voltages at 12.3 v many times.

-I had it 'tested' at the toy dealership several times. Each time they noted that it was still a good battery.

-The local dealership had the car for 3 months to replace the traction battery because of ac/heat pump gas leaks within the traction battery assembly. During its extended stay at that dealership, after about 1-1/2 months of waiting, I did visit the dealership one time to check on the car. It was parked in the back 40 and the battery 12v battery did not even have enough charge left in it to allow the fob to open the front drivers side door. They used the mechanical key from the fob to open it, trying to exclude that info from me. But I cought it. The car would not go into the ready mode at that time either.

I believe that a flood lead acid battery stored at 12.3 v or less is subject to accelerated sulfation. Something I learned from installing really big battery banks at a facility I worked at over the years and read about odyssey battery maintenance on their site. The battery university also has info on flood lead acid battery sulfation, as do many other battery maintenance sources.

 

I'll share some recent data I recorded on my 2024 Prime with the AGM battery.

2 November - charged the traction battery with my level 2 charger. Charge complete late afternoon

3 November - morning I checked the battery charge with the BM2 and it was 12.87 volts. Any surface charge should have been depleted by then. Drove 47 minute trip in HV mode. Initial 14.3 volts that transitioned to 12.8 volts in @ 3 minutes time. The remainder of the trip the voltage stayed at 12.8 volts and for most of the trip there was a slight discharge until the end portion where it was basically neither a charge or discharge. Battery temp was 60 F.
Drove a second 47 minute trip in HV on same route. Basically the charge voltage scenario same as first trip.

4 November - voltage in the morning was 12.68 volts. Did not drive the car that day.

5 November - voltage in the morning was 12.64 volts. Its getting colder here which will have some affect on battery capacity. I noted that the traction battery mileage estimate was decreased 2 miles from where it was in warmer weather
Charging voltage initial was 14.3 volts (@6 amps current) and stayed at that level for 21 minutes then gradually dropped off to 13.1 volts. Trip length 32 minutes in HV mode.
Return trip EV mode initial charge voltage 14.3 for 3 minutes then transitioned slowly to 13.1 volts for remainder of trip. Total trip time was 15 minutes. Took short way home
Checked battery voltage several hours after arrival. Reading 12.88 volts. I would expect that surface charge was depleted since I had opened and closed the rear hatch on arrival which has a fairly significant current draw.

So for my situation, the AGM battery is working to my satisfaction.

One side item I noted. I was recording aux battery current and DC-DC converter current. I noted that often during the transition from the 14.3 initial voltage, the DC-DC output current drops to zero for a few seconds and the battery current increases to a negative -20+ amps. basically picking up the DC requirements for a few seconds and then the DC-DC converter comes back to carry the current load. I believe this is one parameter the HV ECU measures to determine battery capacity, SOC and SOH.

Cheers

 

Well after I read this thread I went and checked my 2019 Prime and the 12V battery was 12.2V after about 2 hours of the car resting after my EV City mode drive from work - 6kms /3.75 miles. Put my intelligent charger on and it went to the de-sulphating program immediately. Fully charged this morning but I will keep an eye on it. Had no issues yet but I have only owned the car for two months - has 44000kms /27500miles on odo.
Thanks for all who shared their expertise and experience.

 

Whatever anyone else thinks about the Gen 4 Prime 12volt system, my take is that it's always better to monitor the system the way the car shows it in techstream or an app ( DrPrius, HybridAsstant, Torque or CarScanner ) are the ones I know show how the car handles the 12 volt system.
My understanding so far from reading posts here and monitoring my Primes 12 volt system is that 12.2 resting voltage reading is normal for Gen 4 Prime. As soon as the Prime is put into Ready mode the voltage I've seen goes straight to 14 or 14.x volts. Than depending which mode and which engine motor combination is selected the behavior can be very different during the drive session.

My 2017 Primes 2nd OEM 12 volt battery rests at 12.2 volts normally, at least when I've checked it with both a voltmeter and any of the ( apps I mentioned above ).

 

Did you charge (de-sulfating) the 12v battery in place or disconnect it from the Prius ?

 

Is it the stock battery a flooded lead acid battery or an AGM battery in your car ?

 

I’ve been watching the recorded graphs of the voltage many months now. I never see 12.2 volts resting. On pushing start button it does not immediately go over 14 volts. There is a big sharp dip to about 12.4 possibly lower, then it goes up. From usually 12.5x to 12.7x resting.

 

Good info about AGM deep cycle marine lead acid batteries.
Concord is the battery manufacturer, Made in the USA, West Covina, CA
Lifeline is the battery brand.
I used two of the GPL-31XT 12v AGM batteries in parallel in a 4WD popup camper for a few years. Charged them with a RedArc DC-DC charger from the vehicle battery/alternator and from Solar Panels. The setup worked exactly as specified by Lifeline and RedArc. Sold the camper with the Electrical System in place after using it for about 4 years.
Here's some info about that AGM Battery and Technical Battery info from Lifeline for their AGM batteries.
Lifeline GPL-31XT battery specs:
GPL-31XT AGM Battery - Lifeline Batteries
Technical Manual for Lifeline Batteries:
https://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101F-Lifeline-Technical-Manual-Final-5-06-19.pdf

For some reason I cannot cut and paste text from this PDF file. SO a screen shot will half to work:

AGM Battery Charging:


Charging continued:


Conditioning:


Deep Discharge recovery:


The logic of 'walking AGM battery life down' by not charging it to 100% SOC at nearly every recharge point is detailed in the screen shots above.
And this is what I believe will happen to AGM 12v batteries in Gen5 Prius Primes because the Toy battery charging Protocol in the gen 5 is not matched to the AGM battery charging needs.

If you charge it about once weekly with an ac automatic (3-stage) AGM battery charger I believe the battery will last longer than if you neglect to fully charge it on a routine basis.

 

Everyones got their own take on the 12 volt Prius systems. Enjoy the banter.

Here's a 7 year old thread some might like, others probably not so much.
12v Battery Is Discharging, Re-Charge Now ! | PriusChat

 

Yes my flood factory stock Lead acid battery performed poorly in my 2024 Prius Prime. Many have had this experience. 12.3v resting voltage is a battery killer in the short run (6 months). I figured the battery was under warranty and chose not to charge it with an ac charger maintainer because the vehicle should do that and if it does not, the warranty should stand. (It is a hassle and a pain to get a stock 12v battery replaced under warranty in a 2024 Prius Prime).

In my case I believe that the dealer installed vehicle theft deterrent system contributed to the stock battery failure. It's a KAAR SECURITY SYSTEM by SWDS. It makes the lights flash and horn honk upon locking the doors and if I try and open any door when the system is on. There is a blue light that continuously flashes brightly inside the cab when the system is on. I cannot get any info on its control or set features. But it does draw power when the doors are locked. This option was not a choice on my part. The dealership I chose to buy from included this system on every Prius they sold. There was no dealership markup on the MSRP price, but the dealer installed option was mandated and had a cost. That option cost was less costly than the $5000 dealer markup on the same car at my local toy dealership at the time.

 

Mine had a Spireon lojack. I found the fuse for it under the dash, inline fuse on a red wire. Someday I will remove the whole thing. Yours sounds more complicated, but there should be a way to disconnect.
I’ve settled in to manually charging when soc gets into the low seventies. Not much else can do.

 

I left it in place.