Dynamically loading image based on function output

I'm trying to load a .png to display a battery icon in conky. So far I've tried nesting the function call, using eval but I can't seem to find a solution. Here is my non working conky.text:

conky.text = [[
  ${image ${lua battery_icon $battery_status $battery_icon}}
]]

Where my battery_icon function looks like this:

function conky_battery_icon(battery_status, battery_percent)
  if (battery_status == "charging")
  then
    icon = "charging-battery.png";
  else 
    ...
  end

  return("/path/to/icons/" .. icon);
end

I checked that the function works and I can get the correct path to the image by adding the following line to my conky.text:

${lua battery_icon $battery_status $battery_percent}

How can I use the returned path of battery_icon to load the file in conky's image? Related question: [Dynamic conky variable argument](https://unix.stackexchange.com/questions/193883/dynamic-conky-variable-argument/193906#193906)