https://radpowerbikes.zendesk.com/hc/en-us/articles/1260800194270-RadMission-Chain-Tensioner-Replacement-Guide

This shows the removal and replacement, but it's fundamentally similar. You want to unbolt the tensioner from the frame. Then if you see step #2 under "Install" that there is an indexing pin on the tensioner that fits into one of 5 holes in the frame. This is how you adjust the tension, by selecting the next hole that tightens up the chain.

It's likely more of a current limitation out of the battery or thermal conditions in the motor. Most Li-Ion cells, depending on internal resistance, are rated for 0.5C discharge (or less). That means about 1A per string of cells. Since the RadMission battery is smaller and less parallel, you can't get as much current out of it. You'd need the larger battery.

Additionally, I think there is less thermal mass around the RadMission motor, which means less heatsinking. Since it's a Permanent Magnet motor, you really want to keep the magnetic material under its thermal limit or you get permanent degradation of the binding material and then less magnetic field. So it's more of an aging and power degradation rather than a wire/heat problem.