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

I installed the UPLUS AGM battery to replace the OEM battery in my 2024 Prime several weeks ago. Based on scan tool data stream recording and data from the BM I have installed, here is a summary of what I have learned regarding the auxiliary battery charging system in regards to the performance of the AGM battery.

- When the car enters READY mode, the charging voltage is 14.2 / 14.3 volts. The AGM battery has a higher amperage charge rate compared to the OEM battery for this voltage. I only recorded a single drive event with the OEM battery installed. The initial charge rate is dependent on the aux battery starting voltage. The charging amperage is higher if battery starting voltage is 12.7 volts versus 12.8 or 12.9 volts.

- With the OEM battery installed, the resting voltage after car systems in sleep mode was typical 12.3X to 12.4X volts. With the AGM, the typical voltage range is 12.7X to 12.8X volts. The cranking voltage recorded for the AGM battery is @0.5 volts higher than the OEM battery

- I utilize a level 2 charger Typically it takes 3-4 hours for a full charge depending on how much mileage range was left. During bhe charging of the traction battery, the aux battery is being charged. It starts at 14.2 volts for @ 12-15 minutes then drops off to @13.1 volts for the remainder. This results in the AGM battery being fully charged. Yesterday the car charged for 3 hours and I disconnected the charge cable when receiving the notification. I checked the battery voltage 17 hours later and it was 12.88 volts. To my understanding, this indicates a SOC of 100%. This voltage is not an OCV as there is a constant parasitic drain of about 40 milliamps per my previous testing when car in sleep mode. The 17 hours and parasitic drain would have removed the surface charge post charging. I have seen this pattern of the AGM battery being fully charged to 100% SOC for the last 3 charging cycles I have monitored.

- When driving the car with a starting voltage at 12.88 volts (AGM full charge state), the charging voltage starts at 14.2 volts and within 2 minutes decreases to 12.8 volts. At 12.8 volts, the aux battery is not being charged and in fact has a significant discharge. With time, the discharge current decreases and and stabilizes at a charging rate plus/minus 100/200 milliamps. This was for a 28 minute drive cycle.

- With the battery starting voltage at 12.71 volts for a drive, the 14.2 volt charging rate stays significantly longer, then drops to 13.5 volts and stabilizes at 13.0 volts resulting in a slight positive charge current of about 0.2 amps into the aux battery.

- Today I drove 2 short trips ( minutes) with the voltage starting at 12.88 volts. After i returned home and car stabilized with systems in sleep mode, the battery voltage is 12.78 volts.

My take aways are (1) with the AGM battery installed, the BMS system will result in the state of charge voltage to be @12.7X volts. The concern about having to periodically fully charge the AGM battery with a dedicated charger doesn't appear to be necessary as the battery is showing to be 100% SOC after charging the traction battery as shown in the BM history snapshot.

I experienced a dead car syndrome with the OEM battery. Cause not positively identified but multiple short trips don't help.

For me, the AGM battery is meeting my expectations. If this changes, I will advise with a follow-up.

Cheers

 

No, I have not even used the level 1 charger. It’s hidden in the under floor storage in the trunk area

Cheers

 

My level 2 charger came with an adapter to allow use of 120 volts which is then basically a level 1 charger. I finally found the adapter to see how the car charges with a level 1charger. I would expect the same behaviour as with the Toyota provided charge cable.

The traction battery EV range has close to 85 percent remaining range. The aux battery voltage was stabilized at 12.88 volts. With the charge cable plugged in, the initial voltage was 14.17 volts for about 5 minutes and then decreasing to 13.07 volts. I left the cable in for about an hour and the voltage was at the 13.07 volts.

Made several trips and the EV range was now about 45 percent. Plugged in the level 1 charge cable (set the charge rate at 8 amps) and the initial voltage was 14.17 volts for 19 minutes and then dropped to 13.07 volts. Obviously it charges the traction battery much slower - probably 8-10 hours in lieu of 3-4 for a full charge. However, there seems to be a potential benefit of charging the aux battery at the @13 volt value for a longer time which may improve the battery charge level. These values noted are similar to what I see when using level 2 charger and this is for an AGM battery.

Cheers

 

UPLUS is the only battery I ever purchased that came with a user manual of instructions. That is awesome in my opinion. Regarding the 12.7 volt caution for an AGM, the way i read the instructions is that the caution statement is for a battery in storage or when the vehicle is not used for a long time (15 days or more).

My battery voltage this past week showed 12.68 volts in the morning. The traction battery had not been charged for a few days and I made only a few short trips. Drove the car on several trips and voltage with car in sleep mode was 12.74 volts. Decided to use the NOCO charger to charge the battery. The charger went GREEN within an hour and I then allowed it to stay overnite for the enhanced charge. After removing the charger next morning, voltage was 13.01 volts. Made 4 trips about 25 minutes long and 2 trips 8 minutes long. For the entire of all trips, the charge voltage showed 14.3 volts ( desulfation time ? ). Following day, the starting morning voltage was 12.88 volts. Within a minute of driving, the charge voltage went from 14.3 to 12.8 volts. That was the norm for the other trips I made that day. Next morning, battery voltage was 12.88 volts.

Keeping in mind that the voltages we see on the BM are not OCV but the battery voltage that is diminished by the parasitic drain current. For me, the 12.7 voltage caution in the user manual is related to long term not use. Since, for me, I drive the car on a regular basis, its not a concern if the voltage in the morning may be less than 12.7 volts. From what I am seeing to date with the charging system and AGM battery, its going to be a non issue. Keeping in mind the reason for the AGM purchase was the concern with the performance of the OEM battery and my experience with the unexplained dead aux battery back in August.

Cheers

 

I left the 120 volt charger plugged in overnight. This morning battery voltage was 12.95 with charger cable still installed. Confirmed again that my car is not victim to aux battery depletion with cable left installed per the TSB. Removed cable and turned on headlights for 10 secs to remove any battery surface charge. Waited 30+ minutes to ensure car was asleep. Voltage now read 12.90 volts.

What I’m seeing is that driving the car results in the battery voltage dropping to the range 12.7X volts based on the charging system voltages the ECU demands. That’s @85-90 % SOC. Charging the traction battery is resulting in a full charge so no need to use a dedicated battery charger periodically - at least so far it appears that way. So from my experience to date, the AGM battery is a big improvement over the OEM. Thanks for bringing your experience to this forum.

Cheers

 

Did some research on the 12 volt aux battery on Toyota hybrids and EV vehicles. The aux battery doing dead is a common complaint on the various models. One dealer to a CHR owner on YouTube recommended hooking up a solar battery charger to the OBD2 port - in the UK. Not sure how well that would work with the weather in the UK. When I bought my Prime, part of the “package” was an emergency kit which have a set of jumper cables. Guess Toyota knew something I didn’t at the time…….

Cheers

 

Can you post the more current voltage readings for your AGM battery, continuing the saga from 29-Oct to 11/04 ?
I take it that these voltage readings are taken after the car has been parked overnight and are the lowest volt readings just before you put the car in ready mode for the next days drive ?

 

I recently had the constant charge time and it was 99% when car turned off. Nothing stopped the charging during usual routine stop and go trip. It was beautiful. Well the deal is over and I manually charge again when in 70’s soc. If the car charges as it should when battery low , then there would be fewer if any dead battery reports.

 

as previously disclosed,
1. local Toyota dealer replaced my OEM GS Yuasa Flood-cell battery with a Truestart H4 (EN) flood lead acid battery under warranty. I no longer have the Yuasa battery to collect data from.
2. I still have not moved on an update to my excel program to plot data as you are currently doing.
3. AGM lead acid batteries are higher performance than flood lead acid batteries, as I have agreed with you previously.
In time, I will get excel to work again for me, will plot some battery voltage data. But it will not be Yuasa battery data.

 

I see that your AGM battery 'at rest voltage' continues to fall daily since the 100% SOC voltage greater than 12.89v was attained after a recharge apparently managed by the toy BMS at the 20 hours of Car in ready mode mark. I think you indicated that the charge voltage was 14.1v. That's good. I look forward to seeing how low that resting voltage drops before the next 20 hours of car in ready mode landmark occurs.

It has been my experience with Odyssey AGM batteries that they need a good 14.7-14.9 absorb charge occasionally to maintain their 100% SOC health and maintain the batteries warranted life.