Problem with battery Replacement

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Hi Dear Friend's
i have a X10 mini phone ...
after using about 1 week my phone battery wase damaged!
the phone was restart after i pushing the sim card!
so that i try to write original rom on my E10i and i was success
but that way was not true!
so that i try to replace a Sony Ericsson S500 battery for my phone
there is a small board with connector on Original battery
but i don't use that (the red box in the image)
Now my problem is : My phone decharging is very very fast! for example : about 40 minutes on music player!!!
can anyone help me ?
thanks (sorry for my english)