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

What I am seeing is the typical voltage from overnight the next day or after short trips will be 12.7X to 12.8X volts. This is with the @ 40 milliamp parasitic draw current that I measured awhile back for my car. We have to take in consideration the SOC voltage chart data for being an open circuit voltage. It appears the BMS wants to keep the battery voltage at the 12.8 volt area. Potentially improved economy I would assume. A few days ago, I charged the traction battery for almost 4 hours. The following morning, the voltage was 12.91 volts. A few minutes after starting my trip, the voltage dropped to 12.8 for remainder of the trip. When the starting voltage for a trip is 12.7X, the charging voltage hangs at 13.2 volts for most of the trip which for me is typically 20 minutes or less.

I have an AGM battery in my truck and periodically I would do a full charge on it as Ford's BMS appears to keep the voltage in the 80-90 % SOC from what forum "experts" have said. I got good life from doing that periodic full charge. Expect to periodically fully charge the AGM battery in the Prius. Bit of a pain since battery location in in rear hatch area. To date, the AGM battery in the Prius is meeting my satisfaction. 12.7X - 12.8X volts sure is better that 12.4X - 12.3X volts from the OEM battery.

Cheers

 

Any Pics of the AGM battery install in a Gen 5 Prius Prime (in the back right hand corner of the vehicle, inside the cab of the vehicle) ?

 

I found the pics of the stock battery in your Gen 4 Prius:
( but did not find pics of the new AGM battery install in your Prius ).

 

Drove the 2024 Prime a few short trips yesterday. This afternoon, battery voltage was 12.72 volts. My truck has an AGM battery that is 6 months old and I just charged it a few days ago with the Noco Genius 5. Two short trips this afternoon. About an hour later, decided to check battery voltage. It was 12.72 volts.....

Drove the Prius for a 33 minute trip. Starting voltage 12.72 volts. For about the 10-12 minutes into the drive, charge voltage showed 14.2 volts. Subsequently it dropped to 13.5 volts and then stable at 13.1 volts remainder of trip. Checked voltage about 30 minutes after car shutdown and assumed it was in sleep mode. Battery voltage 12.97 volts. Was pleased to see this indicating he battery was taking a good charge for the relatively short trip. AGM advantage of taking a faster charge over the OEM flooded batterey.

Cheers

 

I see that the label on top of that battery says 'Stop Start Battery'. Its an AGM, not an EFB battery, as I understand, cannot be both. Have you any opinions about that ?

 

Definitely getting an agm battery when my battery shows weakness. It is easier to manually charge the one in the trunk instead of opening the hood all the time. Thinking of ways to make it easier.
More positive, it is a simple task to replace the Prius battery, much more than like in the VW Id4 video I watched. So if I get five years battery life, can live with that. Actually I am down to no more than two five year drivers license renewals in Calif. just to keep it real.

 

This is a description of the BMS used by Ford. Interesting and I find similarities to how the charging system appears to function on my 2024 Prime.

Just FYI fun reading

Cheers

 

I don’t know it it’s the same logic, but I disabled the fancy 12v charging system on my 2014 F150 many years ago, after experiencing a fairly short battery life from the original battery.

Yes I do own both a 2024 Prius Prime and an F150!

 

I own a 2019 Ford Ranger and also disabled the BMS to improve the AGM battery life. Got almost 5 years and it was still testing OK. Decided to change it anyhow as I felt on borrowed time

 

I have respect for and have used EnerSys Odyssey AGM batteries (EnerSys also makes NorthStar battery).
Over the years I have learned that an open circuit AGM battery voltage of 12.3v after sitting isolated for 12 hours is about a 50% SOC.
If the battery is left at 12.3v for an extended period of time, sulfate crystal formation on the '-' lead plates accelerates and becomes excessive. This results in difficulty charging the battery. A 14.7v bulk and absorb charge (>= 15v charge voltage is to be avoided) can sometimes recover the battery.

The '24 Prius Prime BMS charging protocol is based on a flood lead acid battery, not an AGM battery. My observation of the flood acid battery in my Prius and others observation of the PP BMS protocol using an AGM battery indicate that 14.1v is about the max the charging system ever provides to the battery. This results in a gradual drop in the resting voltage of the battery over time. Eventually the Lead acid battery fails prematurely and I believe the AGM battery will also fail prematurely.

An AGM battery in a PP charged with the BMS in the '24 PP will eventually drop in Open Circuit resting voltage. If it is not fully charged to 100% SOC at 14.7v its life will be reduced and will fail sooner than a properly maintained AGM battery.

The AGM battery will perform better than the flood lead acid battery over its lifetime, but will still fail early because it will never reach 100% SOC in the PP BMS setup.

So if you plan on installing an AGM battery in a '24 PP it needs to be routinely charged with an AC automatic AGM battery charger that brings its open circuit voltage up to 14.7v for an extended period of time. The reason being the BMS in the Prius Prime is not satisfactory for maintain an AGM battery.

Even if you install the AGM battery in the '24 PP, to maintain its life you will still need to charge it with an AC charger periodically (like every time its open circuit rested voltage approaches 12.3v).

 

What about leaving the car in ready mode for an hour or so to charge the 12 v?

 

Doing that brings the battery voltage up to about 14.1 v max in my experience for a very short time then drops to 13.x v and then down to 12.x v after some time passes. You can monitor the 12v battery voltage over time using a BM2 device and your phone.