If weight is not super critical, I'd stick with multiple nicad packs. You could power your PWM board (what's that ? ) with a separate battery from the receiver battery or power the servoes separately from the PWM and spark system. And/or you can have battery packs in parrallel for redudancy and more capacity. Max current is probably not a problem with nicds. Even little 600 mah packs can put out beaucoup amps, just not as long as bigger packs. What is important is CAPACITY, or milliamp hour rating (mah), to give you the endurance you need for your project.

Eight digital servoes could pull a lot of current; I don't remember the specifics, but seems like I read that stalled, they could pull a few amps a piece, so lets estimate 30 amps peak and 10 amps average load from 8 servoes. Will this overload the regulated output of the PWM board? May be another good reason to power the servoes separate from the PWM; just get the servo signals from the PWM, not the power. Obviously you'll need heavy duty wire; 20 ga or better. 10 A avg load means you'll need 1000 mah capacity for each 6 min of run time. Throw in a good safety margin and we're talking a bunch of packs. Maybe my guesstimates are way off; put a meter in line with your equipment and measure the loads.

One last thought, It would probably be a VERY good idea to power your spark system with a totally separate battery pack to avoid interference with the rest of your system.

Check out FMA's clearance for some great prices on nicad packs:
https://www.fmadirect.com/site/fma.h...roducts&cat=15

Joel