I am trying to make a super capacitor bank that I can charge is a couple minutes. It will output a 5V USB charge (aka a power bank.)

My materials: a 15V 8A power adapter, 2 Maxwell 2.7V 3000F supercapacitors, a high power buck converter and small circuit boards to produce the 5V USB.

I was wondering if it was safe to connect 15V to the buck converter, step it down to close to 5.2V and connect the buck converter directly to the capacitor.

I DID do some experiments with the power adapter and it seems that it has a short circuit protection as when I short the leads, it cuts off, then a couple seconds later it comes back on which repeats again and again. What will happen if I connect the capacitors? Any ideas of what I can do with it?


3 Answers 3


You don't want to put caps in series without parallel balancing resistors because otherwise you can never be sure how the voltage between the caps is split up — it would depend on ESR and ESL variance.

For your supercaps, it's worse. You are only 0.2V away from blasting the things, and the currents are so high your balancing resistors had to be very low-ohmic to have any effect. That would be a huge waste of energy.

Do yourself a favour and get a 2.5V converter.


Why can't you read the specs?


  • Maximum ESR_DC, initial, rated value 0.27 mΩ
  • Leakage Current at 25°C, maximum 12 mA
  • Absolute Maximum Voltage 3.25 V
  • Absolute Maximum Current 2,200 A
  • Maximum Continuous Current (ΔT = 15°C) 130 A_rms
  • Maximum Continuous Current (ΔT = 40°C) 210 A_rms

0.27 mΩ is what the Supply will see with a voltage difference.

Therefore you need a SMPS a regulated current limit and voltage limit like a battery charger.

  • Assuming 15V*8A=120W source,
  • Pout=V²/ESR = say 100 W * 0.27 mΩ = 0.027 = V² , V=
  • thus if V initial = 164 mV I=609A
    • but that would exceed the spec for > 5 seconds.
  • Thus you must have a good charger limited to around 100 Amps max or anything less that is a current source, not a voltage source, but voltage limited.
  • You also need a cell Voltage balancer to limit V to 3.25V absolute max./cap.

A USB charger output is spec at 5V +-5%; 4.75V to 5.25V.

If you have the supercap charged at 5V and uses it to charge your cell phone, the charging will take place between 5V to 4.75V. Cell phone will stop charging once detect a lower voltage (depending on phone manufacturer), which makes the remaining charged 4.75V supercap useless. It is possible to add a boost converter to overcome this problem, however you'll still face the same issue when voltage drop below the converter minimum required voltage to function.

For a portable high energy storage, a powerbank is a better candidate. For application where there will be short high current demand, use Super cap. I used 6 supercap parallel to my car battery. These Japanese made cap have a variance of 0.02V when I charge them in series without protection circuit.


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