Fit the data with Bessel functions using FindFit?

[keunglee1996 (Qiang Li)]

Consider the following code:

fp = {{1.9, 27.9}, {3.42, 42.8}, {4.94, 53.2}, {6.46, 56.8}, {7.98, 
    60.3}, {9.5, 61.4}, {11.02, 63.1}, {12.54, 66}};
FindFit[fp, {1/
   1000*\[Sqrt](R*3.08*10^19*(0.045*10^10*1.48*10^3*(3*10^8)^2*(
          R*3.08*10^19)/(
          2*(1.9*3.08*10^19)^3)*(BesselI[0, (R*3.08*10^19)/(
              2*1.9*3.08*10^19)]*
             BesselK[0, (R*3.08*10^19)/(2*1.9*3.08*10^19)] - 
            BesselI[1, (R*3.08*10^19)/(2*1.9*3.08*10^19)]*
             BesselK[1, (R*3.08*10^19)/(2*1.9*3.08*10^19)]) + 
         0.045*10^10*b*(3*10^8)^2*(R*3.08*10^19)/(2*1.9*3.08*10^19)*
          BesselI[1, (R*3.08*10^19)/(2*1.9*3.08*10^19)] BesselK[1, (
           R*3.08*10^19)/(2*1.9*3.08*10^19)]) + 
      c/2*(3*10^8)^2*R*3.08*10^19 - 
      1.1*10^-52*(3*10^8)^2*(R*3.08*10^19)^2), {10^-40 <= b <= 10^-37,
    10^-29 <= c <= 10^-26}}, {b, c}, R]

mathematica gives me a obvious wrong result, I don’t know, maybe FindFit can’t fit for Bessel function?

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