Note that if the keyboard backlight drains the battery, it does not necessarily mean that you should avoid having a keyboard backlight.
The amount of energy saved with a lower brightness setting is likely to be minimal - most power consumption from the microcontroller will come from I/O with the hardware device being controlled. Outputs can consume most of the current when they have been switched on. If only low-brightness levels are needed, there may be better ways to save energy by turning off features such as screen updates and generating quicker pulses for switching devices on and off quickly without losses in speed or quality...