Check the voltage of the pack with a multimeter. If the battery is below 9V the stock charger won't charge the battery. I found a trick around it though, but be careful and only try this if your battery is very close to 9V. I made 2 short pieces of wire hooked them up to the battery and to the charger's power supply (basically bypassing the "brains" of the charger. After only about 30 seconds the voltage of my battery jumped at least a whole volt, which then let me safely charge it with charger hooked up properly.