battery-mode-line-format

battery-mode-line-format is a variable defined in `battery.el'.
Its value is

"[%b%p%%]"

Documentation:
Control string formatting the string to display in the mode line.
Ordinary characters in the control string are printed as-is, while
conversion specifications introduced by a `%' character in the control
string are substituted as defined by the current value of the variable
`battery-status-function'. Here are the ones generally available:
%c Current capacity (mAh or mWh)
%r Current rate of charge or discharge
%B Battery status (verbose)
%b Battery status: empty means high, `-' means low,
`!' means critical, and `+' means charging
%d Temperature (in degrees Celsius)
%L AC line status (verbose)
%p Battery load percentage
%m Remaining time (to charge or discharge) in minutes
%h Remaining time (to charge or discharge) in hours
%t Remaining time (to charge or discharge) in the form `h:min'

You can customize this variable.