Power relays are great for passive shield tanking on Minmatar ships, as projectile weapons require no capacitor. Flux coils will deliver less recharge than they state, because of their reduction of total shields (remember that recharge time is independent of shield capacity, so as shield capacity is reduced, so is shield recharge). So with that said, shield power relays are the way to go for a passive shield tank, but neither should be used for an active shield tank.