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

Do you charge by connecting directly to the battery or do you connect under the hood via the fuse block? Obviously the latter is far more convenient. Does it matter?

 

It doesn't matter, but the connection point under the hood is recommended and more convenient. I use the stud located there to connect my battery maintainer with ring terminals. The alligator clips detach, leaving the ring terminals.

 

I never had a gen 4, but my gen 3 had a clamping terminal under the hood.

 

I hooked up my digital charger this morning which has an AGM mode. The battery was reading 13.5 volts. Just for fun I put a P3 Kill A Watt meter in line so that I can monitor things like the charging time and power consumption. At present the charger is drawing only 19 watts and the voltage has risen to 13.7 after 15 minutes on charge.

Update: 3 hours and 50 minutes into the charge. The charger is alternating between 13.6 and 13.8 volts. It appears to be attempting to repair the battery as the status indicator is flashing red. This is a relatively inexpensive charger from Harbor Freight but is has served me well for several years.

 

Flooded cell batteries bulk charge to 14.2 volts and a float charge of 13.4 volts.
AGM batteries bulk charge to 14.7 volts and a float charge of 13.8 volts.

 

8:45 into this experiment the smart charger was reporting a steady green state and 12.9 volts. I plan to give it another go in the morning.

My charger is an inexpensive unit from Harbor Freight. I cannot say that I trust its display readings but it has served me well for several yeas while charging the lawn tractor in the off season and the inverter generator once a quarter. The latter has had an AGM battery since 2021 that needs a top off every few months.

 

I revisited this today. After about an hour the charger was in its green or happy state reporting 12.8 volts. I believe that the new battery has a 100% SOC. Does anyone check this occasionally or should I just assume that we are good to go for the next eight years?

 

I don't sweat the small things unless they give me some indication I should.

 

With my AGM, I did a full charge with my Noco Genius 5 a few weeks ago. The weather has been in the 30’s and 40’s up until today. For the past 3 months, the BMS charge voltage has been 14.4 volts. Even after the full charge with the Noco, the voltage was a solid 14.4 on every trip. I didn’t drive the car today and the voltage is 12.78.

Regarding the OEM battery, my recollection is the early morning voltage was 12.3X - 12.4X for late September. This would be after a full charge and driving short trips. I put the AGM in the car first week October. AGM battery big improvement over the OEM.

Cheers

 

Regarding the OEM battery, after I had a dead experience in August, I made appointment to have the dealer investigate. They tested the battery as OK. However, I saw a copy of the battery test print out and it stated something to the effect starting below 32 F may be marginal. Nor did they perform the fault isolation procedure that Toyota has for a dead aux battery. They basically charged, tested battery and made several successful starts. I was so disgusted with the dealer at the time I didn’t push the issue. If the battery is marginal below 32 F, that’s not a good serviceable battery IMHO. Made the AGM decision easy…..

Cheers

 

your occasional battery usage may be the Charcoal canister purge? I would go out into the garage and hear it humming fo a spell.

Found this:
What’s running long after car is shut off? | PriusChat
Is it 5 hours after you turn off the vehicle? Page 22 of the Owner's Manual:



#3fuzzy1, May 7, 2021

 

A little update on how the BMS functions with temperature. For past 2.5 months, temps here in eastern WA have been cold Below 40’s. During this time, the aux battery charge voltage has been a constant 14.4 volts.

Finally temps warming. Garage temp was 54 F this morning. About 11 minutes into drive, charge voltage went from 14.4 to 12.8 volts. Next trip charge voltage dropped within a minute to 12.8 volts. Exactly what the car did during warmer months. I was glad to see the BMS kept a constant 14.4 volts during the cold weather and thus kept the battery healthy.

Cheers