The TT21 is very low drain at only 125mA. You can read my original post describing my test at:

https://groups.google.com/forum/?hl=en&fromgroups#!searchin/rec.aviation.soaring/tt21$20battery$20drain/rec.aviation.soaring/KS5ANH41SOo/23HPf6ruNvYJ

or if that's scrambled up, just search RAS on TT21 battery drain.