Battery monitor (BMV/SmartShunt) showing wrong SOC

A battery monitor showing state of charge that doesn't match reality (says 80% but battery's clearly weak) usually needs a 'synchronization' — most shunts need the battery to hit a full charge state periodically to recalibrate their percentage tracking, and if that hasn't happened in a while, drift accumulates. Check when the battery last reached a true 100% charge. If it's been weeks, that's likely your answer — let it fully charge and hold at absorption/float before trusting the percentage again.