RC Calculations Logo "No Spark" Resistor

Hook-up Spark

When attaching a battery pack to an ESC, there can be a substantial initial spark/arc caused by the charge current required by the ESC capacitors. This spark can pit the contacts and eventually degrade performance due to increased contact resistance. Also, the snapping noise and spark can be startling to the user. This connection spark is not generally of great concern at around 14v (4s lipo or 12 cells) or less because the spark is not as severe as the spark from a higher voltage setup.

Fortunately, there is a low-cost method to eliminate this initial spark; all you need is to use a small "pre-charge"resistor to charge up the ESC capacitors at a slightly slower rate.

Important Note: Do NOT use a pre-charge resistor on the Castle Creations "Mamba" line of speed controllers. Due to the way these controllers are designed, using a pre-charge resistor may render the unit inoperative. This has been confirmed by Mr. Patrick del Castillo, president of Castle Creations.

Resistor Value

For most ESC's with around 1,000uF of total capacitance, use a 1Ω 2watt resistor when using up tp 25v (6s lipo), or 10Ω 5watt resistor when using voltages over 25v. This value will charge the capacitors slow enough to avoid the hookup spark, yet fast enough to still allow the ESC to arm properly (under half a second). The power rating is low because the only time the resistor has to dissipate the maximum power is on initial hookup. After that point, the differential voltage between the supply and the capacitor charge decreases quickly, which reduces current and therefore power dissipation.

Wiring Diagrams

Pre-Charge Resistor Diagram Using Deans or Any Similar Dual-Pole Connectors
  1. Connect the small polarized connector
  2. Wait the appropriate amount of time
  3. Connect the main Deans connector
Pre-Charge Resistor Diagram Using Bullet Connectors
  1. Connect the large negative bullet connector
  2. Connect the small bullet connector
  3. Wait the appropriate amount of time
  4. Connect the large positive bullet connector

In both diagrams, the small connector should remain connected to prevent accidental shorting against any conductive surfaces, and to keep the contacts from getting dirty while running. Even though the small connector is hooked up at the same time as the main connector, the main path of current flow will be through the main connectors.

Resistor Calculator

If you want a specific charge time or for a specific capacitance, the calculator below can assist you in selecting the proper resistor value. This will also show you the capacitor voltage, charge current, and resistor power dissipation for various points over the charge cycle.

Enter total battery voltage (?):
Building List...
Charge time (?): seconds
Total ESC Capacitance (?): uF


Resistor resistance (?):
Charge levels over time (?):