Hi qc-zhao,
First take a voltage reading on your charger it should read over 8 volts static. If its under that replace the charger.The average battery life on general application Ni-Cd batteries is 18-24 months.If you have good voltage output on the charger next take a voltage reading on your battery.A good charger should stimulate the battery enough to get voltage reading of 7.2-8.0 volts.If its below the 7.2 volts you have a shorted cell and need to replace the battery.Hope that helps.Good luck.
Tinker