My phone HTC Desire on cyanogen mod 7 shuts it down if the battery level is 18%.
Can I change this level to something lower?
CM6/Magic here; my phone complains at 15%. If you're shutting down at 18, something is wrong with your battery calibration.
Use your favorite recovery to wipe the battery stats, get a full charge, and then use the phone until the battery runs out.
Your battery life will appear to be worse, but it's really that the number is more accurate.