FYI, the SCE was the Signal Conditioning Equipment, which converted various instrument data on the spacecraft into a 0 to 5 Volt DC signal that was then digital

FYI, the SCE was the Signal Conditioning Equipment, which converted various instrument data on the spacecraft into a 0 to 5 Volt DC signal that was then digitally coded for the telemetry downlink. That equipment needed a voltage reference to work correctly, of course, which normally came from the highly regulated main bus power. When Apollo 12 was hit by lightning it knocked a bunch of systems offline, including the 3 fuel cells that normally provided main power for the capsule. After this the capsule was instead forced onto battery power, but that resulted in voltage sag, causing the highly sensitive Signal Conditioning Equipment to start throwing bad data. Switching the SCE to its auxiliary power supply (which regulated the voltages being fed to it, with some loss of overall power efficiency) restored the telemetry data and enabled the ground crew to properly assess the situation on the spacecraft, work through the issues, and avoid an abort.