Introduction: Simple Spot Welder Using Car Battery for Building Lithium Ion Battery Pack

About: A hobbyist who loves to explore new technology & a proud HAM Radio Operator with call - VU3PYN

This is how I made a Spot welder with car battery which is useful for building Lithium Ion(Li-ion) Battery Packs. I have succeeded to build 3S10P Pack and many welds with this spot welder.

This Spot Welder instructable includes,

  1. Functional Block Diagram
  2. Part List to build.
  3. Spot Welder Flowchart.
  4. Spot Welder Arduino Code.

Caution: Do Not Try this at home if you are unaware of the Risks involved with Lithium Ion Batteries.
Disclaimers: The author is not responsible for any misuse of the information contained herein and accepts no responsibility for any damage caused by the use or misuse of this information.

Step 1: Part Required for the Build

1, High Current Relay /Motor Cycle Starter Relay(12V )

2, SPST Momentary Switch

3, Arduino Board(UNO,Mega etc)

4, MOSFET Board(to trigger the High Current Relay) (you may use a 12v low current Relay instead)


6, Copper rod - as Spot welding Leads.

7, 200A Fuse

Step 2: Spot Welder Functional Block Diagram

The Functional blocks illustrates, how to connect the components together to form the Spot Welder.

  • Blue path is the high current flow path.
  • Orange path carries low current which is responsible to switch ON the High current Relay.

Make the connections as shown in the Functional Block Diagram.

Step 3: ​Arduino Mega 2560 Connections

  1. Connect Arduino PIN 10 to a N Channel MOSFET's Gate.(you may use a 12v low current Relay instead)
  2. Connect Arduino PIN 3 to Momentary Switch.
  3. Connect other side of momentary switch to GND.

Step 4: Spot Welder Flow Chart - Arduino Code

The code attached as ".txt" format in this section shall be directly loaded to Ardunio Mega 2560, You may choose any simpler version of Arduino board for this purpose.

The Flow chart of the code is attached.

The multiple function of this Arduino Program are as follows,

Main Functions:

  1. Triggers the MOSFET upon press of the Momentary switch.
  2. Switches ON the MOSFET for 40millisecond

Sub Functions:

  1. Allows Spot welding only if the Switch is not pressed during POWER UP.
    • To avoid mis trigger of MOSFET if the switch is pressed by mistake
    • To avoid mis trigger of MOSFET if the POWER to Arduino drops due the weld and the switch is held.
  2. Triggers the MOSFET only to the set "period", Even if the switch is pressed for long time.
  3. Debounce mechanism, This is to avoid mis triggering the MOSFET due to switch bounce.

Step 5: Spot Weld on Nickel Strip and 18650 Cell

12V Lithium Ion(18650) Battery Pack was built using this Spot welder and illustration on how to build shall be found from below link,

Step 6: Spot Welder Demonstration at a Forum

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