wesnewell said:
I've got a 48V controller that draws about 30A and gets 25+mph using a GM 500W rear hub. I've ordered a cheap 72V controller to try on it, but am thinking about modding the shunts on the 48V one I have now. It has pads for 3 shunts, but only 2 are used. I want to get ~40A draw from the controller, but while I'm at it, I'd like to add a toggle to effectively limit the amps by running one of the shunts through the switch. Does this sound right?
As long as the switch is low-resistance, too, then it should work, but it's possible for a switch to be higher resistance than a shunt, so it won't end up the total resistance you expect.
Also, it will increase as the switch gets hotter from current passing thru it; remember that in theory, assuming the shunts were all equal value, the switched one would see 1/3 of your total battery current flowing thru it at any instant. Since the switch will add some resistance, possibly double it, it may only see 1/4 or less, but that's still a lot at high loads.
How many FETs does your 48V controller have, and what kind are they? I'd guess it's a 12FET for 30A (assuming constant), as that seems about right for both of the 12FETs I have here. 40A is a third more than original, whcih is a lot. My 12FETs will allow up to 60A momentarily, but not for long (a second or two?), just as I start from a stop. Haven't tested it on a steep hill yet, under high throttle (I try to avoid those).
Too few FETs with too low a rating could end up blowing some, running at 40A continous; I'm not sure what rating you need for this, as it depends on the phase currents they see (very different from the battery currents, which is what the shunt measures).
It does drop back to 35 amps really fast and quickly drops lower yet as I accelerate. Not many runs on it yet so the jury is still out on longevity..
Also it is used with a 9C 6x10 hub in a 20 inch rim. I really have to try hard to hit 55 amps. Usually when the front wheel is off the ground at startup 
