If you're recharging batteries it will impair diesel efficiency, I agree that a crash dive should cancel this and apply the full power of the diesels to pushing your boat under water,
However, this is not moddeled in the game, the stop recharging order is not linked to the C key with the crashdive order, if you want full efficiency for your crash-dive take the extra second to tell your Chief Engineer to stop charging batteries, then press C, this will solve the problem.
Personally when crahsdiving, I first order ahead flank, then wait a few seconds for the boat to get up proper acceleration, then crashdive...
this can be done if you have RWR, or against ships not when a plane is already attacking
There is a technical reason why crashdiving on one diesel is the right thing to do when recharging batteries. this is because the gearing involved in routing the diesel through an electric engine and using that was fairly complex and couldn't be switched back in a few seconds...
I can't substantiate this from any sources, :hmm: I read it somewhere some time ago, I'm sure one of the real U-boat technology-buffs can tell you more about this.