If you can take a picture that would help. I don't know about the FRD but the 2015 AR 3 can accommodate either internal or external batteries. The Seat post does come with the necessary clips to mount the Di2 battery inside it and there are screws on the down tube just above the bottom bracket.
I just looked at the picture of the FRD frameset on Felt's web site and it looks like it is the same as the AR 3.
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