Has anyone tried replacing the 12 volt battery with a LiFePO4 12.8 V battery?

I see my battery monitor turns to “charging” only above 13 volts, 13.3 I believe. It seems consistent and quick to read charging versus not. Even with a maintainer and car off, until it reaches the value, it isn’t charging. But I wouldn’t go beyond AGM into lithium because I don’t try to out smart whoever designed the car.

 

In my experience with my AGM lead–acid battery, you don’t significantly raise the SOC of the battery even at 13.53 V, which is the voltage of the DC–DC converter when in the park gear. You need close to 14.0 V for significant current to flow through the battery. I expect similar behavior with a flooded-cell lead–acid battery.

I don’t know what is the minimum charging voltage an LFP battery requires, but I know that an LFP battery has a higher open-circuit voltage for a given SOC. It looks like you need 14.6 V to reach 100% SOC, which is higher than the maximum DC–DC-converter voltage, the latter of which is in the range 14.1–14.4 V, depending on the current demand. With a typical 14.1 V during aggressive charging, chances are that an LFP battery would never reach above 60% SOC. However, given how miserly the Gen 4/Gen 5 12-V battery-management system (BMS) is, I expect an LFP battery to hover around only 15% SOC most of the time. So, no, I doubt an LFP battery would work in a Gen 4/Gen 5 Prius.