Capacitor Array calculation to simulate high-voltage Regen in Electric Vehicle
You just need to dump excess energy.  I gather you don't want to actually regenerate into the battery yet, because your tech isn't there yet.  What you need is a dynamic brake grid or basically a big fat resistor.  

Blindly connecting the resistor in series with the battery won't work. In off-grid solar panel systems that use batteries, the solar charge controller has a pair of terminals called "Dump". Examples of such controllers are Morningstar or Midnight Electric.  When the controller is receiving more energy than the battery can absorb, the controller routes the excess power to these "Dump" terminals, which then powers something like a water heater.  Ideally, your controller has a "dump" terminal that does the same thing.  This is where to connect your dynamic brake grid. 



Home Depot sells 2000W, 240V Cadet heaters for $50. They are bulletproof and have plenty of surface area, so they will convection cool just fine.

 2000W @ 240V = 8.333 amps, repeating of course.
 8.333 amps @ 240V E=IR 240=8.333R.   R=28.8 ohm. 


Your voltage won't exceed 480V so use them in series pairs*, giving 57.6 ohms.  Say Regen floats battery voltage up to 380V.  

380V into 57.6 ohms is E=IR 380=57.6I.  I=6.6 amps.  


Five such pairs should be able to dissipate 33 amps at your expected voltage.  It would be very hokeydokey, and bulky, but will work - and it's hard to argue with the price. 

If money is no object, you can do the same thing much more compactly with Chromalox style strip heaters, but then you'd need to actively cool them. It would help if you could find a fan happy to run on 380V. 



* they are not listed for series connection, so this would violate NEC 110.3b. However NEC is inapplicable to automobiles.