I find that the low battery warnings turn on way too early on Macs and I'm left with months of staring at a blinking bluetooth indicator. I have yet to find a way to lower the threshold but I did discover how to remove the blinking indicator. I just copy over the low battery image in the system folder like so. The images are still being swapped out but since they're now the same it won't be noticeable. You'll still get the low battery alerts but at least the icon won't blink anymore.

cd /System/Library/CoreServices/Menu\ Extras/Bluetooth.menu/Contents/Resources/
sudo cp lowbatt.pdf lowbatt.backup.pdf
sudo cp Bluetooth_Low_Battery.pdf Bluetooth_Low_Battery.backup.pdf
sudo cp BlueTooth_Connected.pdf Bluetooth_Low_Battery.backup.pdf
sudo cp BlueTooth_Connected.pdf lowbatt.pdf