documentclass{article}
  usepackage{tikz}
  usepackage[american,siunitx]{circuitikz}
  usepackage{latexsym,amssymb,amsmath}
  usepackage[active,tightpage]{preview}
  PreviewEnvironment{tikzpicture}
  setlengthPreviewBorder{50pt}%

  begin{document}


  begin{tikzpicture} [auto, circuitikz]

  %produces error. Without i=$I$, it works
  draw (0,0) to [R=$R_1$, v_=$v_1$, i=$I$, *-*] (2,0)
              -| (3,-1)
              to [R=$R_2$, v_=$v_2$, *-*] (3,-3)
              |- (-1,-4)
              to [battery, l = $V$, *-*] (-1,0)
              -- (0,0)
        (1,-2) node[scale=6]{$circlearrowright$}
        (1,-2) node{$I$};

  end{tikzpicture}

  end{document}

documentclass{article}
  usepackage{tikz}
  usepackage[american,siunitx]{circuitikz}
  usepackage{latexsym,amssymb,amsmath}
  usepackage[active,tightpage]{preview}
  PreviewEnvironment{tikzpicture}
  setlengthPreviewBorder{50pt}%

  begin{document}


  begin{circuitikz}
  %produces error. Without i=$I$, it works
  draw (0,0) to [R=$R_1$,v_=raisebox{-10pt}{$v_1$}, i=$I$, *-*] (2,0)
              -| (3,-1)
              to [R=$R_2$, v_={$v_2$,,}, *-*] (3,-3)
              |- (-1,-4)
              to [battery, v_ = $V$, *-*] (-1,0)
              -- (0,0)
        (1,-2) node[scale=6]{$circlearrowright$}
        (1,-2) node{$I$};
  end{circuitikz}

  end{document}