Why aren't warp drives able to use charge instead of negative energy?

[Endercreeper01]

I've heard that warp drives need negative energy in order to contract space-time in the front of a spacecraft. But, space-time expands when you have charge, doesn't it? In the Ressner-Nordstrom solution, the metric is given by

[latex]ds^2=-\left(1-\frac{2GM}{rc^2}+\frac{Q^2G}{4\pi \epsilon _0 r^2 c^4}\right)dt^2+\left(1-\frac{2GM}{rc^2}+\frac{Q^2G}{4\pi \epsilon _0 r^2 c^4}\right)^{-1} dr^2 +r^2(d\theta ^2 + r^2sin^2\theta d\phi ^2)[/latex]

Which means space-time is contracted by charge. Why is it that warp drives are not able to use charge instead of negative energy?

[MigL]

IIRC the negative energy of exotic matter is needed to keep wormholes open and stable.