If you're getting bright light at Apple logo and then dim once it's booted, the backlight is probably fine since the Apple logo is lit. Can you see the backlight is on if you take it into a dark room?[br]
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If so, the screen is just SUPER dim possibly due to issues with the ambient light sensor, which is used to detect auto-brightness levels. In the iPad Air 2, this sensor is part of the headphone jack. If you replaced the battery, the cable for that is along the top edge of the battery. On the long arm of the logic board. [br]
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Assuming you took the board out to remove the battery, that cable could have just not been connected in the reassembly process. Or is not fully seated in the connector. Or the alignment of the actual display is off and the sensor is obscured.
Check and see if you can just turn the brightness up, or turn auto brightness off. I might also verify that your headphone jack works to start.[br]
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