Keep the batteries for your FC-A field controller ready to go with Pyrodigital's FC-A Charger.
DO NOT CHARGE batteries while operating a field controller. In general, chargers cannot detect a fully charged battery while current is being drawn from it.
As the battery will never be detected as being fully charged, chargers will not stop pouring juice into the battery, even though the batteries are already full. Overcharging batteries too many times will drastically reduce their lifetime!
For more information, check out the operating instructions for the FC-A field controller.