Battery Sizes for Makers
Every maker project eventually picks a cell, and the labels lie in predictable ways. This is the table behind the picks: what each format measures, what voltage it actually is, what capacity is realistic, and how much current it will really deliver.
The table
| Format | Size (mm) | Nominal | Realistic capacity | Typical max discharge | Notes |
|---|---|---|---|---|---|
| 10440 (AAA size) | 10.3 x 44.2 | 3.6 V | 300 to 350 mAh | 1 to 2 A | Tiny Li-ion; easy to over-discharge, always protected |
| 14500 (AA size) | 14.1 x 50.3 | 3.6 V | 600 to 1,000 mAh | 1 to 3 A | Fits AA holders but is 3.6 V, never a drop-in for 1.5 V |
| 18350 | 18.3 x 35.0 | 3.6 V | 700 to 1,200 mAh | 5 to 10 A | Short and stout; common in vapes and small lights |
| 18650 | 18.4 x 65.2 | 3.6 V | 2,000 to 3,500 mAh | 5 to 10 A | The workhorse. Above 4,000 mAh is marketing |
| 21700 | 21.2 x 70.2 | 3.6 V | 4,000 to 5,000 mAh | 10 to 15 A | Same voltage, more capacity, needs a bigger holder |
| 26650 | 26.2 x 65.4 | 3.6 V | 4,000 to 6,000 mAh | 10 to 30 A | Heavier; mostly lights and e-bikes |
| LiPo pouch 1S | size varies | 3.7 V | by volume, 1,000 to 10,000 mAh | 1C to 20C | The radio-control pack; needs a proper charger |
| LiFePO4 (e.g. 32650) | 32 x 65 | 3.2 V | 5,000 to 6,000 mAh | 3C | Safer chemistry, lower voltage, flat discharge |
| CR2032 coin | 20.0 x 3.2 | 3.0 V | about 220 mAh | about 0.2 mA pulses | Sensors only; cold halves the usable capacity |
| CR2450 coin | 24.5 x 5.0 | 3.0 V | about 620 mAh | about 0.4 mA pulses | The bigger coin; same rules |
| AA NiMH | 14.5 x 50.5 | 1.2 V | 2,000 to 2,500 mAh | high (several amps) | Rechargeable and forgiving; 1.2 V not 1.5 V |
| AA alkaline | 14.5 x 50.5 | 1.5 V | about 2,800 mAh at low drain | low, sags under load | Fine for sensors, poor for motors and radios |
Capacity ranges are for genuine cells. Li-ion nominal is 3.6 to 3.7 V, charged to 4.2 V, and wants a cutoff around 3.0 V. Coin cells are for microamp loads; asking one for 100 mA is how projects get flaky.
Three rules before you wire a pack.
1. Never charge unattended and never over-discharge below the cutoff. Li-ion cells vent when abused; treat every pack like it can.
2. Match cells before paralleling. Same model, same age, same charge. Mismatched cells trade current between themselves until one dies early.
3. Use a BMS on any multi-cell series pack. It balances charge and cuts off on over/under voltage, which is the entire difference between a pack and a fire starter.
FAQ
FAQ
Why do protected cells have lower current ratings?
The protection board adds a cutoff MOSFET and a small resistance, and it trips above its rated current. A protected 18650 might be limited to 5 A even when the bare cell handles 20.
Can a 21700 replace an 18650?
Electrically yes, both are 3.6 V, but it is 21 mm wide and 70 mm long, so it needs its own holder and space. Capacity is roughly 40 percent higher.
Is a 9,900 mAh 18650 real?
No. The best 18650s reach about 3,500 mAh and the best 21700s about 5,000. Higher numbers on the wrapper are fiction, and the cells behind them are often recycled.
What is a C rate?
Discharge current relative to capacity: 1C on a 2,000 mAh cell means 2 A. A 10C pack delivers 20 A, and a CR2032 is happiest in the microamp to 0.2 mA range.
Series or parallel?
Series adds voltage, parallel adds capacity. Two 18650s in series are 7.2 V at 3,000 mAh; in parallel 3.6 V at 6,000 mAh. Series packs always want a BMS; parallel packs still want matched cells.
Picked a cell? The IoT battery life calculator turns capacity plus duty cycle into days.
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