CroDriver said:
Don't worry, there is enough space. The TunderSkys will be replaced with Headways or A123s, I'm still not sure since the price difference isn't small and a Headway pack is easier to make since they have screw terminals.
You could be just fine if you kept the ThunderSky's just like they are, and secured an A123 20Ah pouch cell across the terminals of each cell. This kinda makes for an ideally cost-optimized battery, as the ThunderSky's carry the energy you need very cheaply, and the A123 pouch cell's are there to get the Ri down to avoid voltage sag, and to give the pack an extra 600amp capability. It's neat how cells of identical chemistry function when directly parallel'd. The A123 carries the brunt of the current during hard discharge, but as this means the SOC voltage is always higher in the TS cell, it's continuously re-charging the A123 cell as quickly as it can, and they work to always equalize state-of-charge between each other. Cheap range from the Thunder Sky battery bank you've all ready got, and for $50/20Ah A123 cell, you get tons of the power you were lacking, and a low Ri pack that will hold its voltage solid.
Double win!