Good day all, I'm toying with the idea of using a small supercapacitor bank to buffer short bursts for acceleration/regen of an electric vehicle (likely an e-bike).
It seems wasteful to put a capacitor bank in parallel with the battery stack (both with independent balancing systems for their respective unit/cell voltages) because the capacitor bank would only provide/accept energy over the small voltage fluctuation when the battery pack provides/accepts current.
What cheap alternatives are there to use the entire capacitor bank capacity over a small voltage fluctuation? An active solution would be a bidirectional DC-DC converter that can fully discharge the capacitor bank when it sees the battery pack voltage sag and then recharge the capacitors when the battery pack voltage jumps up. I imagine this being nonideal though because the DC-DC converter would need to be high power (and therefore heavy/expensive) to handle the relatively infrequent use case of start/stopping.
For the sake of making this fun for us engineers let's throw in some numbers with a more likely situation: we want 20kW for 10 seconds to help accelerate an electric car. 200kJ stored in a supercap bank that has a 5Wh/kg density is about 11kg of capacitors, totally reasonable for an EV! This page cites Maxwell's supercaps as capable of charging/discharging in under 10 seconds: https://batteryuniversity.com/learn/article/whats_the_role_of_the_supercapacitor
To highlight the problem, if we have a sad battery pack with 200mOhm of internal DC resistance cough Leaf, a load of 20kW on the battery would still only sag it's voltage from 360V to about 349V so a supercap bank sized for 200kJ in parallel (3.08F @ 360V) would only provide 12kJ, about 6% of the energy, hardly the capacity we could get if we could extract down to 0V.
Is the only other solution to use a DC-DC converter capable of converting 20kW bursts and tracking a huge voltage swing (following the capacitor charge/discharge voltage profile)?