Check the tension on the belt; a loose belt will cause the battery voltage to go down because it may be slipping on the alternator and also squeals when you accelerate because the crankshaft is turning faster than the belt.
12 volts
a jeep Cherokee need 220 volts to run
Volts are the "electromotive force" that is needed to "drive" the electricity through the wiring.
No volts need a hard drive. However hard drives need a voltage. Usually about 5 for the data more for the physical movement of the drive.
Normal charging range is 13.0 to 14.5 volts
You can get gauges with instructions at auto parts stores Oil Pressure, engine temp, amps or volts
check your alternater
Have you replaced the PCM
About 2 seconds after key on, the asd relay shuts off it the computer doesn't see a crank sensor signal. So unless the engine is running or cranking the coil supply volts will turn off.
P0123 TPS volts high. Replace the throttle position sensor.
Usually only one wire there, but some are heated O2 sensors, that wire will have 12 volts while ignition is on.
There's a 12V and a 5V supply. The 5V is for the logic. The 12V supply is for the motor.