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I have two motors in parallel and a battery with a switch in series with both motors to stop and start them. They are DC brushed motors and the inrush current is huge. While my 3S LiPo handles this with no problems, I would like to use a capacitor to limit the initial burst, and take it easy on my battery. I am also using MOSFETs to keep the high current away from the tiny switch, but that isn't relevant here so I've left it out for simplicity.

I have learnt from my research that putting a capacitor in parallel to the battery and leaving the rest of the circuit as it was means it will get charged when nothing is happening and then I can use that stored charge to help the motors get going when I close the switch.

Does this mean there will always be current flowing when I plug in the battery? Is there anything fundamentally wrong with how I plan to wire this? When the capacitor is charged, and the switch is open, is anything happening, or is it idle, waiting for me to close the switch? And finally, what happens if the capacitor is charged, and I remove the battery?

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  • \$\begingroup\$ You should forget about the capacitor and use the mosfets to control the motor current. \$\endgroup\$ – Charles Cowie Apr 23 '18 at 17:34
  • \$\begingroup\$ I'm afraid I really have little experience with this. Do you mean using a H bridge? Currently I just have the one mosfet acting to prevent arcing by keeping the current out of the switch, and that is it. I don't know how to control the current with a mosfet \$\endgroup\$ – Damon1707 Apr 23 '18 at 17:39
  • \$\begingroup\$ If you are keeping the current out of the switch with the mosfet, you must be controlling the current to some extent. One problem with a capacitor is the high charging current. Another is the voltage dropping as it discharges. Also, allowing the motor to start with as much current as it draws at full voltage is really not good for the motor. The classic DC motor starting method is to start with two or three resistors in series and short them out one at a time. You may want to study that and then learn to design a PWM controller. \$\endgroup\$ – Charles Cowie Apr 23 '18 at 18:18
  • \$\begingroup\$ What's the inrush current in ampere? \$\endgroup\$ – winny Apr 24 '18 at 7:10
  • \$\begingroup\$ Sorry for the late reply, I was away. It is ~70A \$\endgroup\$ – Damon1707 Apr 26 '18 at 17:15
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What you propose seems reasonable to me. Indeed, you can use the capacitor in parallel with your battery to limit the inrush current for the battery. This is exactly why every IC on a PCB board should have a separate capacitor assigned to it.

If you connect the capacitor in parallel with the battery, the battery will charge the capacitor until the cap is charged (= same voltage as battery). There won't flow any current after that. When you close the switch, the motor 'sees' the battery and capacitor in parallel. Because the capacitor has a lower initial resistance than the battery, most inrush current will be drawn from the capacitor.

If the battery is removed after charging the capacitor (switch open), nothing happens, as the cap just stays at the charged voltage. (Ok, leakage currents will discharge it very slowly).

I hope that makes things clear.

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