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My biggest problem is when I discharge a supercapacitor, let's say 100F 2.7V, I use a boost converter, but all boost converters have a minimum input voltage of about 0.9V. But the capacitor still has a lot of energy, about 40%.
It is frustrating because I'm not able to use this energy so my real useful capacity of capacitor is only 60%.
Does anyone have some idea how to fully discharge a supercapacitor?
The energy stored in a capacitor is given by :
$$E= \frac{CV^2}{2}$$
Fill in the numbers for both 2.7 V and 900mV:
$$E_{\text{full}} = \frac{100 \text{F} \cdot 2.7 \text{V}^2}{2}\approx365 \text{J}$$ $$E_{\text{end}} = \frac{100 \text{F} \cdot 0.9 \text{V}^2}{2}\approx41 \text{J}$$
In other words, we have \$41/365\approx 11\% \$ of the full capacity left when your converter dies.
Making DC/DC converters that can extract (part of) that 11% capacity left in the capacitor, without losing it all to lower overall efficiency, is an active and challenging topic within research.
I use a peltier element on a heatsink for discharging mine. Just connect red and black leads of peltier to either side of capacitor and you should be able to get to your desired 0V.
Use a charge pump IC @ <10mA to boost the uC Vdd then, no problem. An interim Cap to store enough energy is needed until the supercap is drained.
