This is my second battery for my 2024 Prius

Moreover, especially with a Gen 4/Gen 5 Prius, once the lead–acid battery is fully drained, a jump start is not sufficient to restore it to a partially healthy state, as the battery management system (BMS) will be very confused. You need to manually charge it with a battery maintainer. Otherwise, you will be running it at around 30% capacity. A battery maintainer could perhaps bring it to 70–80% capacity, depending on the damage caused by the full drain.

 

I don't like the look of this.
It's not like I don't drive it every day.

 

This is actually not bad for the Gen 4/Gen 5 Prius HEV/PHEV. It is 78% SOC, which is close to the intended target for the 12-V battery management system (BMS).

 

I paid a little extra and upgraded to an ANCEL BM300 Pro Battery Monitor, which has several useful improvements I desired over the BM2 battery monitor. One is that it allows you to override the unknowable “intelligent” logic these battery monitors employ by default to guess at the SoC% and instead lets you setup and use your own custom values. So with that, I simply keyed in the same colorful chart values Gokan has shared here many times and that provides a nice consistent logic upon which the app now calculates and presents my battery condition. It's always made sense doing it this way to me so as to avoid the drama those red "low battery" banners would invoke when everything was perfectly fine.

 

It is quite inscrutable.

 

The table will be much more accurate than the BM2 algorithm, which makes some guesses based on the voltage history. It is not going to be 100% accurate, though, as the SOC also depends on the temperature and battery condition. The only way to determine the true SOC is to have both the voltage and current histories, since charge is the integral of current over time.

The 12-V BMS algorithm is buggy and could let the battery drain even while you're driving.

I noticed that my SOC was below 70% the other day, so I connected the Noco Genius for almost two days. It takes more than 48 hours for Noco to stop charging, but I had to drive, so I disconnected it after about 44 hours.