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

So what is the consensus here... @Gokhan definitely standing behind AGM. @RandyPete and @Mr.Vanvandenburg not fully sold? and in any event, the 12V stock charging disappoints during normal use AND disappoints with parasitic drain while plugged in for AC charging.

I have a battery tender - i'll try to revive my 2020 OEM one more winter but am prepared to pull the trigger at any moment knowing i'm overdue. Assuming AGM is a better battery but nervous the shit BMS charging habits might shorten it's life.

There is a couple of reviews noting that the AGM warranty the dear leaders at UPLUS offer is actually dog shit if you want to, you know, have it honored. But for $110 right now when toyota wants 265 USD, I'll go through 2 of them before giving toyota that much loot for a 12V.

 

If you are paying for the battery out of your pocket (NOT WARRANRY REPLACEMENT), go for the AGM. Perhaps not the UPLUS, maybe you can find an AGM you can buy locally in the USA and get a better sense of warranty security. If its a warranty replacement, the flood lead acid will work (get one that has a warranty). Just my opinion. Perhaps Toyota will come up with a better BMS system as a maintenance item in the future.
As it stands right now, buy a smart battery charger/maintainer (min. 7 amp) and keep the new battery well charged, it will last longer. While you are at it, get a BM2 and monitor the battery voltage occasionally so you know when to charge it with the modern automatic charger.

 

thanks for the thoughtful and prompt replies!
I also see you recommended the everstart Maxx lead acid which many local wally-worlds carry, so provides another alternative with a US warranty, albeit $160. In any event, i obviously want this to last 3+ years and my chariot does go through spells of sitting idle for a week+ at a time... I'll give our friends at uplus a try given the overall positive feedback on AGM tech.

Appreciate this forum!!!!

 

"It is very important to install an AGM battery with a full charge, as otherwise, the charging system won't recalibrate itself properly and the AGM battery may never get optimized and initialized."

I think this is going a bit far. While it's probably GOOD PRACTICE, as you pointed out the system does periodic load testing to find out how much charge the battery needs. It's a safe bet to assume that if you put a new AGM in that has 90% charge (and is good enough to go up to 100%) the charging system of the car will see that in a load test and issue a proper charge to it. If the AGM is reporting below optimal voltage I see no reason the charging system wouldn't attempt to bring it up to a proper level.

 

The stock battery in the Prius Prime (2024) is a flood lead acid battery.
The Prius Prime (2024) has BMS for that flood lead acid battery.
How do you suppose the vehicle BMS knows you installed a AGM battery in pace of the flood lead acid battery ?

 

the real fun is experienced when you in stall two AGM batteries...


https://black.jmyntrn.com/2021/06/06/dual-85ah-batteries-in-my-toyota-prius-with-a-victron-bmv-712/

 

so that I could run two 2kWh pure sine inverters off each battery and have plenty of juice to power all of the addon electronics like a refrigerator, a couple of servers, cell phone signal booster, and other goodies which improves the quality of life while driving a Prius around the world... well.. so far 49 states and partially into Baja. A drive to Cabo is next then we set our eyes on crossing Darien's Gap.

 

Temperature has been dropping here in Eastern Washington and I have noted the morning voltage of my AGM battery is now typically 12.6X volts. I found this battery SOC table that factors in temperature. Car is parked in garage and temperature has been 45-50 degs F. This chart is based on open circuit voltage OCV so some adjustment would be needed to account for the continuous parasitic draw on the battery. Its a guide and I note no tolerance variable given for the various temperatures.



For the last 8 days, the charging voltage as read from a voltmeter that plugs into the 12 volt accessory port has been 14.4 volts continuous with one exception 4 days ago. On the 4th trip of the day (20 minute trip), the last 10 minutes the charge voltage dropped to 12.8 volts. However, for all trips for the last 3 days, the charge voltage was a continuous 14.4 volts.

This morning, the battery voltage per the BM2 was 12.62 volts. Installed my scan tool to record data. NOTE: engine run time is in seconds so this initial reading is a little more than 2 minutes after READY light on. Takes time for scan tool to link to car and then for me to choose paramters to monitor and record. This trip was in HV mode.

The following screen shot is at end of trip before placing in PARK mode.


This data from a little more than 26 minutes trip time. Battery temp reading was 46 F and aux battery SOC was 46 AH per scan tool data. FYI, a couple weeks ago, I used an Autel scan tool to confirm the data accuracy I was seeing with the Topdon unit. Same data readings. The scan tools just reads and display what the hybrid control ECU sends out.

During the entire trip length, the charge voltage was 14.4 volts per the accessory port voltmeter and scan tool data. The return trip home was 14 minutes and the charge voltage was continuous 14.4 volts. After a few hours post arrival, the BM2 voltage was 12.82 volts. I opened car door and turned headlights on for 10 seconds to remove any surface charge on battery, BM2 reads now 12.72 volts.

So what has changed ? This continuous charge voltage of 14.4 volts started with the recent drop in ambient temperature. We have been "enjoying" a temperature inversion. The daily temperature range has been low of 35 and high 38 degs F. Its the only variable I can associate this with.

Its supposed to warm up this weekend into the mid 40's F so will see if charging behaviour changes back to what has been observed previously.

Cheers

 

Nice camping prius setup !!!!

 

I was suspect of the charts SOC values especially with no plus/minus variations but it was all I could find regarding SOC vs temperature. The charts values are OCV. I considered disconnecting my battery to compare pre and post voltage values to learn impact of parasitic current draw but I decided not to at this time. I may email UPlus to see if they have data of their battery SOC vs temperature.

Cheers

 

Since my Gen4 (run almost exclusively in City EV mode) puts just over 14V on the 12V battery in Park ,I make it a habit now of leaving the car in Park when I get home from work and plugging the charger in when I intend to charge the car later that night. So for that 2-3 hours the car can do it's thing with the 12V battery. My charger is on an external timer, so it's off when I connect it to the car when I get home after work around 6pm. I have free power from 9pm till 12 with the timer set to switch on the charger during that time period, so the the car switches off (out of Park) automatically when the external timer powers up the charger. So no hassles of external 12V chargers. All very easy now, and my 12V battery is always between 12.6 and 12.7V resting as monitored with a MK B
Bluetooth battery monitor.

 

So you leave the car on - just sitting in Park and with the charger attached but without power? Presumably when the charger finishes the car turns itself off?

 

Yes the car just sits on in Park with "unpowered" charger plugged in.
No when the power of the charger switches on, the charger must communicate with the car, because the car then shuts down/switches off out of Park and the car starts to charge.

I also make it a habit now to put the car in Park when I am waiting for a very slow traffic light to change to green.