I do not know the absolute maximum rating of a 36v controller like that, the 48v version of the same controller will handle a 60v lead acid pack with an absolute maximum of 75v
I’m guessing your wanting to use a 45v nominal lithium pack like you would find in a Chevy Volt?
Considering those controllers are only worth about $99 used I would get a lightbulb and bench test the controller at various voltages to see if it shuts down.
Most Curtis controllers will shut down in over voltage situations and a minor over volt normally won’t damage only cause shutdown
YMMV, it is also possible you will just let out magic smoke, tread carefully if you can’t afford to loose it.
I have 1204x-4408 which is rated at 24-36v and 400 amps. I am looking for info on replacing the Mosfets with higher voltage units for instance IRF640's like those in the Curtis 1221 which is rated at 120v and 400 amps. I'm not sure what else would need to be upgraded. Cheers, Norm
www.diyelectriccar.com
It appears 60v is the absolute maximum so if it improves battery life don’t charge past that level