While the Batt-O-Meter works on many devices, there are some that just don't work. The Batt-O-Meter has to place a load on the device in order to properly test it. Since not all devices are wired the same, this may behave differently between devices — resulting in some working and others not.
One thing to try when getting a PL6 or SLF error is to pull the Batt-O-Meter plug out just a little bit (so it's only partially inserted) and see if it works. If that does not work then the only other option is to remove the battery from the device and test it that way.