Don't do it Doug!!!! there's a reason Wren specifies an 8 cell pack and it is that a 6 or 7 cell pack will not drive the starter motor up to the 35k rpm point where you disengage the starter. You must push the engine up to that rpm on the starter, for it won't self-sustain below it.

You switched the starter off at 35k rpm. From then on it played no part in the running of the engine. The engine accelerated on its own from 35k to 55k rpm. Then it flamed out. That proves the flame out had nothing at all to do with the starter motor or its battery. You have air in the line, or low pump voltage causing insufficient fuel flow or something like that. The little battery provided with the engine is only going to have enough capacity to run the pump for one or two flights, it will need frequent re-charging. Did you re-charge it after the first successful run? Have you properly cycled it from new with a timed 14 hour C/10 charge then discharge a couple of times to get it working. Some nimhs can take a handul of cycles to get up to speed.

It's a beast at 190,000rpm isn't it