I use my FT-818 either with internal battery or with 3S LiPO battery.
I noticed that the input voltage threshold under which low power is automatically selected is 11V.
However, one can still operate the pwr set button to restore whatever power he wants. After one year since I started operating the FT-818 (and no measurement so far, this is my fault!) I am not sure if it is possible to have the max power emitted even with input voltage lower than 11V (including internal battery) or not. In the former case the blinking low power icons should be intended just as a warning that your battery won't las for a long time...