High DC current makes strong magenetic fields, thus obviously interfering with the magnetometer. You must keep power wiring way from the APM and further, never make any loops. When you added the battery you moved the wiring and it's magenetic fields.
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Today i test magnetic field and clearly i have some noise.
I can't move my apm board further. Is there any other way to reduce magfield?
http://img407.imageshack.us/img407/7740/img0476k.jpg
https://www.youtube.com/watch?v=NNeameEo7hc
High DC current makes strong magenetic fields, thus obviously interfering with the magnetometer. You must keep power wiring way from the APM and further, never make any loops. When you added the battery you moved the wiring and it's magenetic fields.