Time Machine Running while on Battery Power

2010 mbp 15" i5 running 10.6.3.
time machine doesn't seem care fact notebook running on battery power. keeps on performing backups. i've tried restarting, enabling , disabling time machine....nothing seems work...

isn't time machine supposed delay backups when laptop on battery power..?

 

why don't decrease backup interval time machine helper or turn off backup drive altogether?

 

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