I don't think the 90 has extra battery cells at all and the change is due to chemistry and the introduction of silicon into the graphite anode. The weight is pretty much the same. The chemical change as I understand it will result in sustaining a higher voltage longer - I think in practice this will mean it holds on to its performance at lower states of charge. It seems that some p85d owners are waiting to see if the ludicrous upgrade delivers the same performance as a p90dL so there must be potential for a difference. I would expect the 90D to be as fast if not faster than an 85D