I am not sure if your igniter will work with a 5 cell, however, you can always add a voltage regulator to the system. The nice thing about a regulator is that with a 5 cell battery, if one cell goes south on you, the regulator will open up and continue to provide 5.1 – 4.8 volts keeping your airplane under control and letting you replace the bad battery without loosing the whole plane. The other option is to put the regulator on only the igniter however that’s not how I would do it.