I have been using a C40 to divert a 48V solar setup. I have noticed that the C40 does not start to divert based soley on the voltage. If the voltage shoots up quickly, the voltage will get higher before it starts to divert than if the voltage rises slowly. I appears to me it uses an alogrithm to decide when to dump that takes into acount the voltage and the time as factors to decide when to throw the switch. You can adjust the set points a bit, but I'm assuming you've already looked at that. If your generation is larger relative to your battery bank, this will be more of a problem.
One way around this might be to build a soley voltage dependant switch based on an op-amp comparator circuit with a zener diode on one leg and a voltage divider on the other with a potentiometer on one of its legs to adjust it, and a bit of hysterisis by feeding back a bit of power from the output to the voltage divider leg through a resistor to prevent it dropping the voltage below the threshold voltage on the input as soon as it is turned on. That prevents it from continuously switching at the threshhold voltage.
Kind of like the 12V battery monitor example on this page, but with the transistor circuit to throw a relay on the output like in the auto fan examble, but with a more robust transistor, and a relay capable of switching the amps for a dump load:
http://www.uoguelph.ca/~antoon/gadgets/741/741.html
I'm not a circuit guru of the board like some who frequent here, but I have built a few simple op-amp circuits, and in my experience they work reliably and predictably well. Rich