I've been experimenting with ultracaps for a while now and I have a very specific problem:

I use the ultracapacitors to compensate for line inductances and general power filtering. When I install a capacitor (16F to 33F) in a vehicle (14Vdc) I obviously have to precharge them to avoid the huge inrush current. However this is not a permanent solution because when the battery is disconnected for service the capacitor would discharge and would have to be recharged again.

My plan is to use a high power resistor to limit the current until the voltage has reached a certain level. Then a Mosfet/SSR/Relay will short the resistor, connecting the bank to the system.

Now my question is, what is the simplest most robust (not cheapest) solution that can be left unattended.
Obviously once the system is charged I want the lowest possible current draw and resistance.

Maybe an opamp as an comparator switching a mosfet? How do I avoid the ohmic region?

Heres the datasheet for one of my typical caps: https://cz.mouser.com/datasheet/2/257/2_7_100F_ds_3001959_datasheet-1517387.pdf

  • \$\begingroup\$ How fast or how long the charging should take? Or what is the current you want to charge at? \$\endgroup\$
    – Justme
    Mar 4, 2021 at 18:41
  • \$\begingroup\$ a time constant of under one minute would be nice. I do not want to exceed 10A, or whatever the resistor can handle. ESR of one cap is around 10mOhm, I usually have 6 or 7 in series and 1-4 in parallel. I want a circuit for one with one parallel and 7 series. So 70mOhm in total. \$\endgroup\$
    – Flowx
    Mar 4, 2021 at 18:53
  • \$\begingroup\$ Define the current limit that you wish to use. 33F stores 3.4kW-seconds or 14.2 W per Amp-second. So charge time is controlled by your current limit of fuse. \$\endgroup\$ Mar 4, 2021 at 18:57
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 around 10A for now. I need a circuit, I can scale the other components accordingly once I have that. \$\endgroup\$
    – Flowx
    Mar 4, 2021 at 19:01
  • \$\begingroup\$ 10A will take 45 s to charge 3.4kW \$\endgroup\$ Mar 4, 2021 at 19:02

1 Answer 1


A 12V 50 W high beam+low beam bulb in series will limit current to 2.4A as the Cap charges up. dt= 33F * 12V / 2.4A = 165 seconds. The bulb will draw more current as it dims below visibility and speed up a bit.

  • \$\begingroup\$ I have not thought about that. Good idea. But when the Capacitor has reached sufficent voltage, how do I short the bulb? Like with a relay or a mosfet. I need a circuit that gives a logic signal once a voltage, say 13V, is reached. Im thinking a voltage divider with an opamp maybe? \$\endgroup\$
    – Flowx
    Mar 4, 2021 at 21:13
  • \$\begingroup\$ yes anything like that or a transistor with <0.7 Vbe with base R to limit current and NPN to bulb on low side and Collector goes low when charging. and pulled up to Vdd when ready to bypass using a 10A relative and low inductive litz wire to load from cap and gnd. \$\endgroup\$ Mar 4, 2021 at 22:42
  • \$\begingroup\$ Flowx... Just as Motor Off current burns out relay contacts, so do Cap charge ON currents, so ensure you check discharge contact ratings and consider a 25A relay so 25A*70 mOhm = 1.75V @Justme 70A? you sure ? \$\endgroup\$ Mar 5, 2021 at 8:51

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