Does leaving your battery plugged in to the controller drain the battery when the controller switch is off?
I have a BL 36 kit with LiFePo4 battery and dont want to kill the battery.
That depends. If the off switch you are refering to is truely an off switch whereas, it is a power switch that is connected in line between the batteries and the controller then the switch is cutting all power to the controller. This is the way Currie is setup. They have an inline switch that the positive wire passes through on it's way to the controller. In the off position there is no power going to the controller and you can leave the batteries on the bike pluged in and actually charge the batteries in this state as long as the switch is in the off position.
I also have 2 Cyclone kits with the controller built into the back of the motor (never buy that again). On this setup there is a low voltage switch on the throttle that turns off the power to the controller. I am guessing the is a relay inside the controller connected to the LV switch. I have left the batteries connected and this switch in the off position for weeks and not had a problem ether. Again I caution you to check how your switch is wired though. Not all bikes are the same.
EDIT.....Take a voltage reading of the pack after a short ride to drop the voltage t o3.2v per cell. Leave the bike sit with the swith off. After about 30 mins check the voltage and write it down. Wait a few hours and check it again. If it is the same then the switch is truely an off switch. I would recommend that you recheck the voltage again in a few hours thoug hto be sure. Actually I would recommend checking it several times over the first 24 hours just t obe 100% sure.