Battery form factor for headlamps. 14500 vs 18650 vs 21700.

LED Flashlights – General Info
1

ChatGPT :slight_smile: here:

14500 Battery: 1100 mAh
18650 Battery: 3500 mAh
21700 Battery: 5500 mAh

We will assume a nominal voltage of 3.7V for all batteries.
Energy Capacity Calculation

Energy capacity (Wh) is calculated as follows:
Energy Capacity (Wh)=Capacity (Ah)×Voltage (V)Energy Capacity (Wh)=Capacity (Ah)×Voltage (V)

14500 Battery:
Capacity (Ah)=1100 mAh1000=1.1 AhCapacity (Ah)=10001100mAh​=1.1Ah
Energy Capacity=1.1 Ah×3.7 V=4.07 WhEnergy Capacity=1.1Ah×3.7V=4.07Wh

18650 Battery:
Capacity (Ah)=3500 mAh1000=3.5 AhCapacity (Ah)=10003500mAh​=3.5Ah
Energy Capacity=3.5 Ah×3.7 V=12.95 WhEnergy Capacity=3.5Ah×3.7V=12.95Wh

21700 Battery:
Capacity (Ah)=5500 mAh1000=5.5 AhCapacity (Ah)=10005500mAh​=5.5Ah
Energy Capacity=5.5 Ah×3.7 V=20.35 WhEnergy Capacity=5.5Ah×3.7V=20.35Wh

Volume Calculation

Using the same volumes as before:

14500 Battery:
    Volume: ≈7.7 cm3≈7.7cm3
18650 Battery:
    Volume: ≈33.18 cm3≈33.18cm3
21700 Battery:
    Volume: ≈48.5 cm3≈48.5cm3

Weight Calculation

Using the same weights as before:

14500 Battery:
    Weight: 20 grams20grams
18650 Battery:
    Weight: 45 grams45grams
21700 Battery:
    Weight: 70 grams70grams

Energy Density Calculation
Energy Density by Volume

14500 Battery:
Energy Density (by volume)=4.07 Wh7.7 cm3≈0.529 Wh/cm3Energy Density (by volume)=7.7cm34.07Wh​≈0.529Wh/cm3

18650 Battery:
Energy Density (by volume)=12.95 Wh33.18 cm3≈0.390 Wh/cm3Energy Density (by volume)=33.18cm312.95Wh​≈0.390Wh/cm3

21700 Battery:
Energy Density (by volume)=20.35 Wh48.5 cm3≈0.42 Wh/cm3Energy Density (by volume)=48.5cm320.35Wh​≈0.42Wh/cm3

Energy Density by Weight

14500 Battery:
Energy Density (by weight)=4.07 Wh20 grams≈0.204 Wh/gramEnergy Density (by weight)=20grams4.07Wh​≈0.204Wh/gram

18650 Battery:
Energy Density (by weight)=12.95 Wh45 grams≈0.288 Wh/gramEnergy Density (by weight)=45grams12.95Wh​≈0.288Wh/gram

21700 Battery:
Energy Density (by weight)=20.35 Wh70 grams≈0.291 Wh/gramEnergy Density (by weight)=70grams20.35Wh​≈0.291Wh/gram

Summary Comparison
14500 Battery

Volume: 7.7 cm³
Weight: 20 grams
Energy Capacity: 4.07 Wh
Energy Density:
    By Volume: 0.529 Wh/cm³
    By Weight: 0.204 Wh/gram

18650 Battery

Volume: 33.18 cm³
Weight: 45 grams
Energy Capacity: 12.95 Wh
Energy Density:
    By Volume: 0.390 Wh/cm³
    By Weight: 0.288 Wh/gram

21700 Battery

Volume: 48.5 cm³
Weight: 70 grams
Energy Capacity: 20.35 Wh
Energy Density:
    By Volume: 0.42 Wh/cm³
    By Weight: 0.291 Wh/gram

Recommendations for Headlamp Use
14500 Battery

Pros: Very lightweight and compact.
Cons: Lower energy capacity, lower energy density by weight.
Suitability: Suitable for low-power headlamps where compact size and low weight are crucial. Limited runtime due to lower capacity.

18650 Battery

Pros: Balanced size and weight with high energy capacity. Highest energy density by weight among the three.
Cons: Heavier and larger than the 14500 battery but still manageable for most headlamps.
Suitability: Ideal for high-power headlamps requiring longer runtime without adding too much weight. Most commonly used and widely available.

21700 Battery

Pros: Highest energy capacity, good for very high-power applications. Highest energy density by volume.
Cons: Heavier and larger, which might be less comfortable for headlamp use.
Suitability: Suitable for very high-power headlamps where maximum runtime is required, and the extra weight and size can be managed. Less common in headlamp applications due to bulk.

Conclusion

For lightweight and compact headlamps: The 14500 battery is a good choice if minimal weight and size are paramount and lower energy capacity is acceptable.
For balanced performance and longer runtime: The 18650 battery remains the best overall choice due to its superior energy density by weight, making it suitable for most high-power headlamps.
For maximum runtime and high power: The 21700 battery is suitable but may be too heavy and bulky for some headlamp users, making it less common for this application.

The 18650 battery is generally the best recommendation for headlamps, providing a good balance of weight, size, and energy capacity.

5 Thanks
2

That’s pretty weird values for the capacity here… Assuming 1100 mAh 14500 seems okay, but 3800 mAh for 18650 is way too much, and 4300 mAh for 21700 way too low. I’d probably go with 3000 and 4800 or 5000, so 11 Wh 18650 and 18.5 Wh 21700. That changes the energy density calculations by a huge amount.

Doesn’t change much about the bottom line though, a 21700 on the forehead is not comfortable. I don’t even like 18650 headlights unless the battery pack is separate on the back of my head.

1 Thank
3

Recalculated, but its almost the same answer :slight_smile:

4

I didn’t bother to do the maths, common sense says that given identical chemistry a bigger battery will have longer run-times so just get the biggest you’re comfortable with carrying.

5

Another important factor is price. They all cost about the same , lets say 4$ (NKON)

  1. 21700 Battery: 5.0875 Wh/$
  2. 18650 Battery: 3.2375 Wh/$
  3. 14500 Battery: 1.0175 Wh/$
6

And this is why I prefer 21700 cells.
I have an enormous noggin, and I would get a 21700 headlight if I could find one that appeals to me.
I know that a 21700 headlight is too big for most people, but it might be just fine for me. :slightly_smiling_face: