2 Added some clarifications, added paragraph breaks
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There is almost certainly a short circuit on the left hand battery.

The motor driver is described as a "H-Bridge" meaning it switches both motor terminals, so it doesn't use a split positive and negative supply to enable it to reverse the motor, instead it routes positive to the motor negative terminal and negative to the motor positive terminal.

A quick check with a continuity tester should confirm that battery -ve connects to logic input ground on the motor controller.

I would suggest the following: 1:

  1. Use the motor board for the Arduino's ground (unless using an isolated DC-DC converter)
  2. Add a Series resistor in Arduino positive (unless using a DC-DC conv.)

If you can confirm the Arduino is grounded through the motor driver then don't make a second ground connection to the battery. Leaving this off prevents a situation where the heavy ground from the battery gets interrupted and the motor current flows through the lighter hookup wire and Arduino groundplane. The Arduino's current consumption should be low enough to easily pass through the lightweight control lead ground. 2:

If not using a DC-DC converter I would suggest adding a series resistor in the connection from the battery to arduino (remembering to take 12V from the mid point NOT 24V). You can probably "afford" 10-100 ohms depending on how many LEDs are in use, and this will act as a cheap barrier to prevent switching noise from the motor driver reaching the Arduino. I'm assuming you can drop 5V and still meet the 7v requirement.

When using an isolated DC-DC converter things get simpler, as now the only common ground between the motor driver and the Arduino is through the control connector. This means that DC faults in the motor side shouldn't be able to pass back through to the Arduino.

I'm also a little concerned that a "12V" battery may actually be nearer 14V when off load and may exceed the Arduino's stated max input voltage, but if a suitably rated DC-DC is used this will be irrelevant.

There is almost certainly a short circuit on the left hand battery.

The motor driver is described as a "H-Bridge" meaning it switches both motor terminals, so it doesn't use a split positive and negative supply to enable it to reverse the motor, instead it routes positive to the motor negative terminal and negative to the motor positive terminal.

A quick check with a continuity tester should confirm that battery -ve connects to logic input ground on the motor controller.

I would suggest the following: 1: If you can confirm the Arduino is grounded through the motor driver then don't make a second ground connection to the battery. Leaving this off prevents a situation where the heavy ground from the battery gets interrupted and the motor current flows through the lighter hookup wire and Arduino groundplane. The Arduino's current consumption should be low enough. 2: I would suggest adding a series resistor in the connection from the battery to arduino (remembering to take 12V from the mid point NOT 24V). You can probably "afford" 10-100 ohms depending on how many LEDs are in use, and this will act as a cheap barrier to prevent switching noise from the motor driver reaching the Arduino. I'm assuming you can drop 5V and still meet the 7v requirement.

I'm also a little concerned that a "12V" battery may actually be nearer 14V when off load and may exceed the Arduino's stated max input voltage.

There is almost certainly a short circuit on the left hand battery.

The motor driver is described as a "H-Bridge" meaning it switches both motor terminals, so it doesn't use a split positive and negative supply to enable it to reverse the motor, instead it routes positive to the motor negative terminal and negative to the motor positive terminal.

A quick check with a continuity tester should confirm that battery -ve connects to logic input ground on the motor controller.

I would suggest the following:

  1. Use the motor board for the Arduino's ground (unless using an isolated DC-DC converter)
  2. Add a Series resistor in Arduino positive (unless using a DC-DC conv.)

If you can confirm the Arduino is grounded through the motor driver then don't make a second ground connection to the battery. Leaving this off prevents a situation where the heavy ground from the battery gets interrupted and the motor current flows through the lighter hookup wire and Arduino groundplane. The Arduino's current consumption should be low enough to easily pass through the lightweight control lead ground.

If not using a DC-DC converter I would suggest adding a series resistor in the connection from the battery to arduino (remembering to take 12V from the mid point NOT 24V). You can probably "afford" 10-100 ohms depending on how many LEDs are in use, and this will act as a cheap barrier to prevent switching noise from the motor driver reaching the Arduino. I'm assuming you can drop 5V and still meet the 7v requirement.

When using an isolated DC-DC converter things get simpler, as now the only common ground between the motor driver and the Arduino is through the control connector. This means that DC faults in the motor side shouldn't be able to pass back through to the Arduino.

I'm also a little concerned that a "12V" battery may actually be nearer 14V when off load and may exceed the Arduino's stated max input voltage, but if a suitably rated DC-DC is used this will be irrelevant.

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source | link

There is almost certainly a short circuit on the left hand battery.

The motor driver is described as a "H-Bridge" meaning it switches both motor terminals, so it doesn't use a split positive and negative supply to enable it to reverse the motor, instead it routes positive to the motor negative terminal and negative to the motor positive terminal.

A quick check with a continuity tester should confirm that battery -ve connects to logic input ground on the motor controller.

I would suggest the following: 1: If you can confirm the Arduino is grounded through the motor driver then don't make a second ground connection to the battery. Leaving this off prevents a situation where the heavy ground from the battery gets interrupted and the motor current flows through the lighter hookup wire and Arduino groundplane. The Arduino's current consumption should be low enough. 2: I would suggest adding a series resistor in the connection from the battery to arduino (remembering to take 12V from the mid point NOT 24V). You can probably "afford" 10-100 ohms depending on how many LEDs are in use, and this will act as a cheap barrier to prevent switching noise from the motor driver reaching the Arduino. I'm assuming you can drop 5V and still meet the 7v requirement.

I'm also a little concerned that a "12V" battery may actually be nearer 14V when off load and may exceed the Arduino's stated max input voltage.