I have no idea whether this will give you what you want, but in ~/.icewm/preferences there are these lines:
# Show APM/ACPI/Battery/Power status monitor on task bar
# TaskBarShowAPMStatus=0 # 0/1
Try uncommenting the second line so it says this:
TaskBarShowAPMStatus=1 # 0/1
Let us know if you get a battery monitor. I don't have a battery in my laptop because it's old and the battery expired long ago, so I removed it and a replacement is too expensive. Otherwise I'd try out the suggestion above and report what happens. I don't know if you need a battery monitor utility installed in order for this to work.