Variable super capacitor battery provides power on the go


Instructable user [EngineeringShock] got sick of buying batteries for his devices all the time and has instead opted to build himself a super capacitor bank that can be used to power common household items.

His “forever” rechargeable capacitor bank is made of two large super capacitors rated at 400 farads apiece. It is charged through a LM317-based charging circuit that is adjustable to allow for slow or fast charging, the latter of which he admits, is slightly dangerous.

Since the super caps are only rated at 2.7 volts, they are wired through a DC-DC booster circuit that allows him to adjust the output voltage from 4.3 v to 34 v. The adjusted voltage is then passed through a digital display that allows him to see what the output voltage is at any time.

He says that the super cap bank can power his computer’s speakers for about two hours before requiring a recharge, which takes just a few short minutes, depending on how he is charging them.

While it’s not exactly cheap, the capacitor bank could be useful for those requiring quick portable power for relatively short periods of time. If we were to build one ourselves, we would likely fit all of the components into a small project box to protect the caps from accidental discharging, and top it off with a couple of solar cells to charge it for free during the day.

Keep reading to see a quick video demonstration of his super cap “battery” in action.


  1. Brad says:

    This is good stuff. I may have to incorporate this in to a project. Thanks for posting it.

  2. Arran Short says:

    Nice project, what sort of current can this provide?

  3. Ben Choy says:

    It will be nice to see it power msp430!

  4. mess_maker says:

    great post and project. The only thing was that the video got hard to watch… he stammers a lot which is pretty distracting. It was like he either wasn’t prepared to show it or was distracted. Otherwise, it was very informative.

    I will have to build something like this as well :)

  5. DanJ says:

    Interesting. Expensive but good for some applications. Using the equation U = 0.5*C*V^2 we get about 2500 Joules in a 5V 200F capacitor bank. The website lists the amount of energy in typical batteries. A single AAA Alkaline cell has about 5100 Joules and a D Alkaline cell has 75000 Joules.

  6. Life2Death says:

    I’d put that in a box, would hate to accidentally discharge it

  7. DanJ says:

    Anyone interested in using ultracapacitors should probably read this app note from Maxwell. For one thing, balancing circuits should probably be used for strings of the capacitors to prevent a capacitor from seeing an over-voltage condition during charging that could destroy it.

  8. lwatcdr says:

    @Ben Choy I was thinking the same thing. Maybe with a small HAM radio and some sensors and some solar cells. Put it in the middle of no where and see how long you get data from it.

  9. microHacks says:

    Nice idea. Pretty cool to have 2x400F caps on a board. One thing – long term you may have trouble with your caps. When connecting in series, you need a way to balance the charge on them. At some point after several charge & discharge cycles, they will become unbalanced – one will have more charge than the other. When this happens during discharge, the weaker cap will experience reverse charging from the stronger one and you’ll start to damage it. Unbalance will also reduce the run time. On the other hand – maybe you never discharge it any where near empty….

  10. Harvie.CZ says:

    Two interesting links:

    big f**king cap (of unknown capacity):

    farads to mAh (&vice versa) conversion:

    BTW what about making some really big (in means of capacity) DIY capacitor?

  11. @DanJ – Good info! I was thinking the same thing.

    …and think about making sure this circuit is “single fault tolerant” to an over voltage, over current or reverse polarity condition. A shatter resistant enclosure would be good too.

    I couldn’t find any videos of a supercapacitor exploding… I might have to make one, although the largest value I have is 1.0F 5.5V on hand. They all might “explode” with the same force, due to the relief stampings in the case tops these days, but don’t count on it ;-)

  12. nes says:

    @DanJ: I was thinking the same thing. Even a single Li-ion cell from a laptop has about 10 times as much energy capacity. Of course it takes about an hour to get the full amount of energy into it but you could probably put 2.5kJ in in a couple of minutes if you watch the temperature doesn’t rise too much.

  13. dan fruzzetti says:

    I love this hack, personally.

  14. Scuzz says:

    I think I need to build one of these, as this is something which I need friggin’ all the time.

    I guess the big reason he left balancing off is because balancing severely limits the usefulness of the circuit. Passive and active balancing would both cause an extra drain, even when the circuit is “off.”

    Presumably, even if he doesn’t balance, so long as he doesn’t create too much of a charge disparity by having a *lot* of current (either in or out) he really shouldn’t have too much to worry.

    I would have to sit down when I’m not too busy and do out the math to see how much current would actually make a problem to defend my point, but I think it’s fair game for discussion as is!

  15. tim says:

    i did the math, that’s 280mAh

  16. tim says:

    or 1W during 1/2 Hour

  17. sparki says:

    I really like this hack. You could potentially split the idea in two. A stationary unit that charges a cap bank and a portable cap bank with DC-DC booster that you can potentially charge in seconds from the first bank.

  18. Dan says:

    As pointed out above, the capacitors only store about 200 F * 5.4 V = 300 mAh, or a fraction of the energy stored in a typical cellphone Li-Ion battery.
    The useulness of this circuit must lie in the fact that it recharges quickly and that the capacitors won’t degrade as fast as a Li-Ion battery if they’re used correctly. I hope this circuit does that.

  19. Tachikoma says:


    they are rated between 1 to 10 mF, 800 to 3000 VDC.

  20. WestfW says:

    400F is tiny. Electronics Goldmine has 2600F Maxwell “Boostcap” caps:

  21. HackerK says:

    I asked the question why I can’t use a Cap to replace battery when I was a kid. The answer I was told is cap will not hold nearly as much energy in a battery for the same size.

    Looks like this still hold true today.

    I kinda like this hack because it is really a PoC (proof of Concept… Not Piece of C***). But not really for “power on the go”…

  22. asdasd says:

    Why not swap the caps with 1 or 2 LiFePO4? The charging circuit could stay the same.

  23. Thomas says:

    A balancer could be a divider with an op-amp voltage follower. That would not use much power, if built with a very low power opamp.

    If you can tolerate some voltage difference, just two transistors (NPN and PNP) and the high-Z divider would work.

  24. Wagner says:

    Mike, take off your metal ring and wrist watch while handling high capacitance, with thousands of Joules circuits. You have no idea what happens if the ring melts in your finger.

  25. cypherf0x says:

    This is the same guy who made a ‘green’ capacitor bank without balancing resistors or a circuit which will kill the caps, not to mention there seems to be a general lack of electrical safety. Even at low voltages supercaps _will_ hurt you.

    He’s just posting stuff to try and sell people overpriced unsafe crap. If you want cheap supercaps go here.

    Same place he likely gets them and they don’t charge over double the price.

  26. andynonymous says:

    Reminds me of an older Hackaday post where someone replaced a 9v battery with a supercap wired into some sort of circuit.

    I’m not going to post the link, as, last time I looked, the link to the original instructions were dead. I was able to track down a link, but I lost it.

  27. I’m really interested in this technology to power our micro quadcopters as well as the 808 cameras. Both as a main power source and\or as a supplementary power supply. You think this is feasible in these two applications to implement and maintain safety? We’re talking 4.1 to 5V and 300 mAh capacity. Lipo batteries weigh 7-9 grams for the quad and 4 grams for the 808 camera. Every gram reduction in weight is precious. Reducing the quad’s weight 4 grams will make it perform and last much longer in flght.

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