There is no clear cut answer as to Yes or No on using the charger normally used for the 4 cell batteries. It is easy to tell for sure if you have a voltmeter or multimeter. If the open circuit voltage is greater than 7.2 volts, it will charge the battery. Now, if you have the multimeter, measure the current that flows when you have the battery connected to the charger. If it at least 0.1C it will charge the battery fully, just take longer. How long? Divide the battery capacity by the current you read with the multimeter and multiply by 1.6 to get the hours it will take to fully charge the battery if it were fully discharged at the start of the charge cycle. It has been my experience that most of these chargers that come with a radio system WILL fully charge a 5 cell battery. Your batteries will also last longer (more charge/discharge cycles) if done at a lower rate of charge. The only reason for using a faster charge is if you are using a charger that depends on a voltage drop to trigger cutoff of the charger. Remember, to get that voltage drop, you had to intentionally overcharge the battery to initiate that voltage drop which in itself does a very minor bit of acceleration of battery deterioration internally.