I found this script on the i3 forums and thought that I'd share it from here. It's a cool little script that checks for battery strength and warns you via the nagbar if your battery is running low. Helpful if you run the i3 status bar on the bottom and don't always notice if the battery is low!!

bat_test.sh

! /bin/bash

SLEEP_TIME=5   # Default time between checks.
SAFE_PERCENT=30  # Still safe at this level.
DANGER_PERCENT=15  # Warn when battery at this level.
CRITICAL_PERCENT=5  # Hibernate when battery at this level.

NAGBAR_PID=0
export DISPLAY=:0.0

function launchNagBar
{
      i3-nagbar -m 'Battery low!' -b 'Hibernate!' 'pm-hibernate' >/dev/null 2>&1 &
          NAGBAR_PID=$!
        }

        function killNagBar
        {
              if [[ $NAGBAR_PID -ne 0 ]]; then
                  ps -p $NAGBAR_PID | grep "i3-nagbar"
                      if [[ $? -eq 0 ]]; then
                          kill $NAGBAR_PID
                      fi
                      NAGBAR_PID=0
              fi
        }

        while [ true ]; do

          killNagBar
          if [[ -n $(acpi -b | grep -i discharging) ]]; then
              rem_bat=$(acpi -b | grep -Eo "[0-9]+%" | grep -Eo "[0-9]+")
             if [[ $rem_bat -gt $SAFE_PERCENT ]]; then
                 SLEEP_TIME=10
             else
                 SLEEP_TIME=5
             if [[ $rem_bat -le $DANGER_PERCENT ]]; then
                 SLEEP_TIME=2
                 launchNagBar
             fi
             if [[ $rem_bat -le $CRITICAL_PERCENT ]]; then
                 SLEEP_TIME=1
                 pm-hibernate
             fi
             fi
          else
            SLEEP_TIME=10
          fi
            sleep ${SLEEP_TIME}m
            done
          fi
        fi
      fi
      done
   fi
  fi