A low battery can cause unreliable behaviour. Operation of Live View can be compromised before the battery is exhausted which can lead to unusual error messages when running tests. If the battery becomes completely exhausted during a test, the test will fail and you will have to restart.
If possible, FoCal will advise you if your battery is getting low or warn you if the battery is almost exhausted. Some cameras do not report the state of their battery to the computer so these will be indicated as “unknown”.