I am looking at the sudden drop in pack voltage when I start from a stop on my nice 15 degree slope streets in my neighborhood. This is the XM3500 with the 40AH TS cells which, from what I can find and also tell from my own measurements, they have about 5 mohm internal resistance each. This along with the wiring and copper straps is about 120mohm total resistance so about a 9V drop with 2C from the stack and takes away the accelerating/starting torque. I have found when my pack is below 50% SOC and I try to start from a dead stop going up the 15 degree slope, the torque cuts out and the bike sometimes shuts down (I assume from UVC).
So, I started thinking about putting a super capacitor in parallel with enough capacitance and low ESR to give me a few seconds of hold-up voltage to get started on these slopes in these situations (after a couple of seconds the bike speed is enough that the back EMF reduces the inrush current needs so it is fine and the capacitors are then discharged during this event and the full current is from the pack again). However, I figure I need 10F minimum and it has to go across 72V peak.
There are the Maxwell boostcaps but I would need a BMS on them and run a string broken up piecewise as you can not put one cap per cell since they are 2.5-2.7V and the cell can go to 3.65V during charge. Also, they are expensive ($50 each for the 650F and I would need 28 and then a BMS just for these).
So, I have found car audio boost caps that can run 30F each and they are designed (some) to take 24V with balancing done internally. They also have ESR ratings of 1.5mohm. I can get three of these across cells to keep 24V on each, have 3 of the 30F in series to get 10F, and the cost is about $100 each. Has anyone ever considered this and do you see anything wrong with this?