2021 Prius. My 12V battery is starting to drop in voltage more than I'd like after the car is parked for a couple of days. I am considering replacing with AGM (Yuasa brand). I understand AGM has a slightly higher terminal voltage and a lower internal resistance (allowing faster charging on short journey). Sounds good. However are there any downsides to switching to AGM other than higher price?
Other than the charging algorithms are slightly different for the two types (and thus the AGM wouldn’t be receiving the correct charge) nothing that I’ve heard of. They are close enough you probably wouldn’t see anything, but for the Gen 4 not sure what benefits you would gain.
If you decide to get one, make sure it is designed for high heat. Even the heat tolerant versions may ultimately rely on some sort of aftermarket insulation and forced air cooling. Previous Prius generations relied on an AGM battery for that rapid recharge capability, and they were placed far from the ICE in the right rear corner of the car. AGM is more expensive also, making a premature failure due to heat a cost consideration. I have a separate beef with the replacement Toyota TrueStart battery. My OEM battery from the factory had removable cell caps so a person could keep the electrolyte topped up. The desert heat and aridity here will be pretty rough on this so-called maintenance-free battery replacement. Hope mine holds up.
I have been running a AGM in mine. Specifically the harbor freight thunderbolt magnum 12v 160cca one that i got for $40ish dollars when it was on deal. I had to make my own cables just to get it to attach simply because the + and - are reversed position. And the battery itself is a lot smaller so it fits in easy but had to fabricate a tie down. I used an aluminum strip and bolted to the previous oem bolt hole. It does work, and i been driving with it for the past 10,000 miles. At first i made my cables out of thin 16 gauge wires and it would try and turn on but acted like it was a dead battery. So i increased the size to 2 gauge and had no problems since. The prius doesnt need a lot of cca because its not using to crank a starter, but it does surge a lot of amps in a split of a second to power everything up, so a thick wire or strong connection is needed, as well as a battery that can handle that much current draw, which this one can.
