I'd disagree with Brad. 70 parameters? I'd be surprised if you get any valid
output. Way too many dimensions, at least for my taste in nonlinear models.
I'd look for another approach -- reparametrize, impose restrictions, etc.
I would like to fit some data to the equation for a
rectangular hyperbola (y = mx/(b+x)). Can I use
Excel to fit data to equations that I enter?