This paper describes an analysis performed at 250 GeV centre of mass energy for the reaction e+e- -> bbbar with the International Linear Collider, ILC, assuming an integrated luminosity of 500 fb-1. This measurement requires determining the b quark charge, which can be optimally performed using the precise micro-vertex detector of the detector ILD and the charged kaon identification provided by the dE/dx information of its TPC. Given that the forward backward asymmetry is maximal for e-L (Left-handed electron polarisation), it has been necessary to develop a new method to correct for unavoidable angular migration due to b charge mis-measurements. This correction is based on the reconstructed events themselves without introducing external corrections which would induce large uncertainties. With polarized beams, one can separate the Z and photon vector and axial couplings to b quarks. The precision reached is at the few per mill level, and should allow to confirm/discard the deviation observed at LEP1 on the ZbRbR coupling. Model independent upper bounds on the tensor couplings, F2V and F2A, are derived using the shape of the angular distribution.
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