If you have a laptop with APM installed, try the following PROMPT_COMMAND to create an environment variable ${battery} you can add to your prompt. This will indicate if AC power is connected and percentage power remaining. AC power is indicated by a "^" (for on) and a "v" (for off) before the percentage value.
function prompt_command {
# As much of the response of the "apm" command as is
# necessary to identify the given condition:
NO_AC_MESG="AC off"
AC_MESG="AC on"
APMD_RESPONSE="$(apm)"
case ${APMD_RESPONSE} in
*${AC_MESG}*)
ACstat="^"
;;
*${NO_AC_MESG}*)
ACstat="v"
;;
esac
battery="${temp##* }"
battery="${ACstat}${battery}"
} |