I hope I didnt miss someone saying this already. LI poly batteries are designed to put out a steady power for a long period of time. If you are igniting simple estes igniters, maybe a few copper heads that is perfect. BUT they do not hit as hard under a big load, meaning they wont work as well for Clusters etc.
For situations were you need HIGH amperage you are better off with Ni cads. but then they wont last quite as long. one more thing to think about is temp. I hear Lipolys really do bad in really cold temps.
just my .02
Would that it be so simple. Ir isn't!
The best bang for the buck for a launcher comes from a lead acid gel cell. You can get a 12 V, 7 AH lead acid gell cell for $21 and it will deliver over 100 amps on a relay system if your wiring resistance is low enough.
https://www.batteryspace.com/sealedleadacidbattery12v75ah20hrsforupsseascooterande-bikes.aspx
You can get good information on all rechargeable batteries here.
https://batteryuniversity.com/index.htm It will take you several hours to read all of it, but if you want to get fairly well educated on battery technology, this is a good place to start. (And only to start as much of the data is 5 years old data and the NiMH and Li battery technology has got a lot better since this was written.)
The temperature ranges of the most common rechargeable batteries are listed here, but read the fine print.
https://batteryuniversity.com/partone-3.htm While NiCads have the lowest rated operating temperatures, the rest of the pack is not far behind. The rest are rated down to -20 C, and if you insulate a big battery well, it will stay warm for a long time.
Also Li-ion Mn and Li-ion-FePO4 batteries source more current unit capacity C (capacity), and last longer for a given current drain on a per unit C rating, than NiCads, but Pb-acid gell cells are so inexpensive relative to all other chemistries, that they win hands down for high pulsed current delivery (several seconds long).
If weight is not an issue, Pb-Acid gell cells always wins in a practical, low cost launcher, and if you're really cheap, you don't even need to buy a charger. Just get a cigarette lighter plug and charge it from your car on your way to and from the launch field. Your car's charging system already had the protective circuit built-in to prevent overcharging.
Bob