MM esc ?external BEC necessary w/ a 2s config?

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Just saw some random blurbs about how you should use a external BEC w/ the MM esc on a 3s lipo config... does this also hold true for a 2s config? I will be running a medusa 36x50mm 4800kv motor in my new Ofna Hyper STe and was planning on just using the internal BEC. Again I will only be running 2s lipo.

Thanks!