18650 Battery Holder

The holder is what turns two loose cells into the car's battery pack: it clamps them, wires them in series, and gives you a red and a black lead to screw into the L298N.

Check it is a series holder

Two-slot 18650 holders are sold in two wirings that look identical from the outside.

• Series — the cells add up. Two 3.7V cells give 7.4V. This is the one the car needs.
• Parallel — the voltage stays at 3.7V and only the capacity doubles. The car will not run on it: the L298N would give the motors about 1.7V and the Arduino's regulator would never start.

Where the two leads go

Both the motor driver and the Arduino feed from the same pack, and they must share a ground.

• Red → the L298N +12V screw terminal, and on to the Arduino Vin pin
• Black → the L298N GND screw terminal, and on to an Arduino GND pin

Leave the L298N's 5V-enable jumper in place. At 7.4V the on-board regulator is happy to make its own logic supply from the pack.

Put a switch on the red lead

The holder has no switch, so as soon as the cells are in, the car is live. Cut the red lead and put a small rocker or slide switch in the gap, rated for at least 3 A. Without one the only way to stop a runaway car is to catch it and pull a cell out, which is exactly as awkward as it sounds.

Fitting it to the car

• Mount it on the lower deck. It is the heaviest single part and the car corners better with the weight down low.
• Double-sided foam tape holds it well enough, and unlike bolts it lets you slide the pack fore and aft to balance the car.
• Load each cell negative end first, against the spring, then press the positive end down. Forcing the positive end in first bends the flat contact.
• Check the tiny + and − marks moulded into the plastic at each slot. In a series holder the two cells point in opposite directions, which looks wrong the first time you do it.

Every part of the car