The diode is a nice safety but it should be necessary if the ignition is rated at 6V (NiCAD).

Five cell NiCAD packs top off at 7.2V off the charger. They drop to 6.9V fairly quickly and then spend a lot of time between 6.8V to 6.0V

A Life battery pack spends 85% of it scapacity at 6.6V, well within what a NiCAD is delivering.