Why you should not use a Battery Tender® on a lithium battery
Here is the simple answer: A Battery Tender® can not measure State of Charge (SoC) of a lithium battery. It simply does not have this capability. While they work great on lead acid batteries, they are not able to accurately “read” lithium batteries. So, they will never properly charge a lightweight lithium battery.
Beyond SoC, the Battery Tender® is looking for a 6 cell battery, with each cell having roughly 2 volts. All lithium lightweight batteries are based on 4 cell packs, with each cell having roughly 3.3 volts. This sends the Battery Tender® a reading that is outside of the safe charging window, and contributes to the incompatibility.