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.