Perhaps I should have mentioned that you also have to split the component into two paths to implement two different line widths,
Note, you probably could reuse \linethickness instead of \tempwidth. A quick search of sty files only found it used in geometry and gkpmac(?). Besides, local changes won't get out of tikzpicture.
\documentclass{standalone}
\usepackage{circuitikz}
\newlength{\tempwidth}
\makeatletter
\ctikzset{bipoles/battery/width/.initial=.1}
\pgfcircdeclarebipole{}{\ctikzvalof{bipoles/battery/height}}{battery}{\ctikzvalof{bipoles/battery/height}}{\ctikzvalof{bipoles/battery/width}}{
\pgf@circ@res@step = -\ctikzvalof{bipoles/battery/width}
\pgf@circ@Rlen \divide \pgf@circ@res@step by 2
\tempwidth=\pgfkeysvalueof{/tikz/circuitikz/bipoles/thickness}\pgfstartlinewidth
\pgfsetlinewidth{\tempwidth}
\pgfpathmoveto{\pgfpoint{\pgf@circ@res@left}{\pgf@circ@res@up}}
\pgfpathlineto{\pgfpoint{\pgf@circ@res@left}{\pgf@circ@res@down}}
\pgfusepath{draw}
\pgfsetlinewidth{2\tempwidth}
\pgfpathmoveto{\pgfpoint{-\pgf@circ@res@step}{.5\pgf@circ@res@up}}
\pgfpathlineto{\pgfpoint{-\pgf@circ@res@step}{.5\pgf@circ@res@down}}
\pgfusepath{draw}
}
\makeatother
\begin{document}
\begin{tikzpicture}
\draw (0, 1) to [battery] (0, 0) {};
\end{tikzpicture}
\end{document}
