According to Garmin, the new firmware replaced "Lithium Ion" with "Lithium": http://www8.garmin.com/support/download_details.jsp?id=3709

Which makes sense. I think the "Lithium Ion" was either a long-term typo or some feature that was never rolled into many Garmin units.

If some have trouble running on full 1.7V L91 lithium primary batteries, there's no way a 7.2V LiOn battery pack is going to work, as there there doesn't seem to be any kind of "wide input" regulator to allow input voltage to vary much.