18650 Lithium-Ion Batteries

Two 18650 lithium-ion cells in series make the 7.4V that runs the entire car — the motors through the L298N, and the Arduino through its Vin pin.

The numbers that matter

• Nominal voltage: 3.7V per cell, so 7.4V for the pair
• Fully charged: 4.2V per cell — 8.4V for the pair
• Empty: 3.0V per cell. Below about 2.5V you are damaging the cell.
• Honest capacity: 2000 to 3500 mAh
• Size: 18 mm across and 65 mm long, which is where the number comes from

Flat top or button top

Cells come with a flat positive end or with a small raised button. Most two-cell holders have a spring at one end and will take either, but flat tops make a smaller contact patch and are the more common cause of a car that cuts out over bumps. If you have the choice, take button tops.

Protected cells have a tiny circuit board under the wrapper that cuts the cell off when it is over-discharged or shorted. They are the safer buy and worth the extra. They are also 2 to 5 mm longer, so check they still drop into the holder.

Why two cells and not three

7.4V is the number that makes everything else in the car work.

• The L298N eats about 2V, so the motors see roughly 5.4V — inside their 3V to 6V rating.
• The Arduino's Vin regulator wants 7V or more to hold a steady 5V, and 7.4V clears it.

Three cells give 11.1V, which puts about 9V across a motor rated for 6 and makes the Arduino's regulator run hot for no gain.

Safety, and this one is not optional

• Never let anything metal bridge the two ends of a cell. A loose 18650 in a pocket with a key or a coin is the classic way these catch fire.
• Charge them in a proper lithium-ion charger that stops at 4.2V. Never wire a phone charger or a bench supply straight onto a cell.
• A cell that is dented, hot, or swollen goes in the recycling, not in the car.
• Take the cells out of the holder when the car is put away for a while.
• Buying, charging and inspecting the cells is the adult's job in this build. Everything else on the car can be handed over.

Every part of the car