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.