Python script running 'subprocess' using a lot of cpu on Rapsberry Pi Zero

Question

I have a short python script that monitors the battery voltage of a solar folding project I'm running, and shuts off the Pi if the battery voltage drops below a certain threshold. The problem is that the script is using a constant 60-65% of the Pi Zero's only core. I also have Grafana and a small script that enters values into a database running on the Pi, but these are only sipping 2-5% sporadically. The system works fine, but I don't understand why the script with subprocess is using so much cpu. Here is the script:

#!/usr/bin/env python

import subprocess
from ina219 import INA219, DeviceRangeError
from time import sleep
import RPi.GPIO as GPIO

SHUNT_OHMS = 0.1
MAX_EXPECTED_AMPS = 3.19

ina40 = INA219(SHUNT_OHMS, MAX_EXPECTED_AMPS, address=0x40)
ina40.configure(ina40.RANGE_32V)

while True:
   v40 = ina40.voltage()
   print(v40)
   if v40 < 19:
      subprocess.call(["shutdown", "-h", "now"])
      sleep(5)

Answer 1

This is because you're asking the pi to keep checking the voltage as fast as it can so it will use as much CPU as it needs.

Just put a short sleep eg. sleep(0.1) at the end of the loop and it will cut down the CPU usage dramatically.