https://teslamotorsclub.com/tmc/threads/800w-rms-amplifier-install-on-2022-model-y-with-new-15-5v-li-ion-battery-amd-system.280298/
this is the only thread you'll ever need.
basically it's always 15.5v, you need to get a 2nd battery to not cause any warnings/errors and hook it up parallel with...