Anti-spark system / Inrush current limiter

Today I decided to make an Anti-spark system for my bicycle.  The problem was that every time I connected the battery to the controller there is a big spark on the connector. This happens because the electrolytic capacitors in the  controller want to drain as much energy as they can as quick as possible to charge themselves, so there is a big “rush” of current flowing on the wires hence the spark.

One may say: “can’t you just leave the battery connected adding just two more wires for the charger?”. Well, I can do that, but my bike is a work-in-progress and I need to remove the battery often.

Here below there are a couple of videos (sorry the quality is low) showing the difference between with and without the Antispark system in place.

Antispark OFF:


Antispark ON:


The  Anti-spark is simple to make.  In my case (check what you need it for and don’t just copy me, it can be dangerous!) I used a 380 Ohm 3.75W resistor that was sitting around in my little lab. The resistor is soldered to the green wire you see in the video. Instead of closing the circuit connecting the two red wires I close the circuit with the green wires. This allows the current to flow slowly through the resistor from the battery to the capacitors. Then I can connect the red wires safely and without any spark. Note that you will need a “power” resistor because the high current flowing trough it will need to dissipate power in the form of heat. The more the power (in Watts) a resistor is rated, the more heat it can tolerate/dissipate. In other words a common 1/4W resistor could burn, don’t use it!

Bike data: The battery is 36v nominal (42v fully charged) 24Ah Li-ion. The controller is 36v 30A.

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