I'm not an expert but as far as I know you need to hook the turbines in because BRGC will adjust your reactor to output exactly what the turbines need, without the turbines hooked up it could be confused.
The other possibility is that you need to use the GUI, to go to the battery page and click charge, by default it will try to produce the same amount of power that you are currently using therefore keeping the battery at a steady level instead of charging to max.
I'm not sure of exactly what the button names are so those are approximate instructions.
Hope one of those things