A vehicle includes a traction battery, circuitry including a switch and a capacitor, and a monitoring chip configured to draw power from a cell of the traction battery via the circuitry responsive to the switch being closed. The circuitry is configured to open the switch to disconnect the cell from the chip and power the chip via the capacitor for a predetermined period. Powering the chip via the capacitor prevents power down of the chip during the predetermined period. The circuitry opens the switch responsive to engine cranking.