Did it get lost inside the barrel shroud hand guard on the front? If so, just pull back on the big ring at the back of the hand guard, and the top or bottom of the hand guard should just pop right out with a little tug. That's the area the battery would normally go in, so it's meant to be accessed easily.
If the backing has gotten lost in another area, well, the TM M4 does open just like a real M4, so you should be able to find it in there, though I can't imagine where you could have lost it in there.
Hope that helps.
-Spat
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