# How to fully discharge supercapacitor?

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?

• I'd be more concerned about 0.9 volts representing 40% of the full charge energy and why this figure isn't significantly lower than 40%. If you charged the cap to 2.5 volts (full charge) then 0.9 volts contains an energy of 13%. It's, a squared ratio; not linear. – Andy aka Jun 2 '20 at 12:07
• Yes, am sorry, my misake – Sahasrar Jun 2 '20 at 13:42

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.

• Sorry for my mistake, you are right with calculating – Sahasrar Jun 2 '20 at 13:42

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.

• This does answer the question in the title, but I think the OP was wanting to use all the stored energy for their device, rather than looking for a way to fully discharge it. – Colin Jun 3 '20 at 7:49
• On rereading the question and particularly that second paragraph I realise my answer may not be as useful for the OP as was intended. – TheDarkVice Jun 3 '20 at 8:12
• Yes, mate. Fully discharge and waste energy is pretty easy :D – Sahasrar Jun 4 '20 at 9:51

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.

• Does anyone have some idea how to fully discharge a supercapacitor? .Why does some incompetent person downvote without a question? in comment reply – Tony Stewart EE75 Aug 28 '20 at 6:31
• Thanks for advice, I have used IC TLV61200 to boost main converter TPS61088 and it works pretty well in my application – Sahasrar Sep 10 '20 at 12:03
• Maybe someone downvote your answer because your answer is not specific, also my case in one question :D – Sahasrar Sep 10 '20 at 12:07