Flashlights are one of the most commonly used lighting tools in our daily lives, for activities such as walking at night, cycling at night, camping, outdoor exploration and etc.
In 1899, British inventor David Misell registered the first “flashlight” patent in the United States, using a cardboard tube, a brass reflector, and a carbon filament bulb, powered by early dry battery. Because the battery life was short (only a few minutes), requiring frequent switching on and off, it was named “Flash Light.”
With technological advancements, flashlight bulbs have evolved from the initial tungsten filament bulbs to LED bulbs, significantly increasing brightness at the same power level. Similarly, batteries have progressed from dry batteries to lithium batteries, extending the flashlight’s operating time, and due to their rechargeable nature, becoming more environmentally friendly by eliminating the need to frequently discard used batteries.
Modern flashlights typically use rechargeable 18650/21700 cylindrical lithium-ion batteries. So, how should we choose replacement or spare batteries?
First, you need to determine the battery model.
18650/21700 refers to the size of the lithium-ion battery. 18650 means the lithium-ion battery has a diameter of 18mm and a height of 65mm, its common capacity ranging from 1000mAh to 3500mAh. 21700 means the lithium-ion battery has a diameter of 21mm and a height of 70mm, its common capacity from 4000mAh to 5000mAh. A very small number of flashlights use 14500 lithium-ion batteries, which have a diameter of 14mm and a height of 50mm. The specific model of the lithium-ion battery used in the flashlight can usually be found on the product details page.
Secondly, you need to determine whether the flashlight’s compatible battery is flat top or button top.
The originally cylindrical lithium batteries are flat top, meaning both the positive and negative terminals are flat and without protruding metal parts. This type of lithium battery is suitable for flashlights with a spring contact on the positive terminal, ensuring effective contact with the battery’s positive terminal.
Button-top cylindrical lithium batteries have a cap-shaped metal piece welded to the positive terminal, making the positive terminal more prominent. These are used in flashlights with a flat positive contact area without a spring.
Whether it’s a flat top or button top 18650/21700 lithium battery, the purpose is to effectively connect the circuit. The correct choice should be made based on the design of the flashlight itself.
Finally, determining the power output:
Manufacturers produce flashlights with different power outputs depending on the intended use. Below, I’ll show you how to calculate the battery’s power to better match the flashlight’s power requirements.
Power = Voltage × Current
The Nominal voltage of a single lithium-ion battery is 3.7V. Why do we use the Nominal voltage of 3.7V in the formula, instead of the charging voltage of 4.2V or the cutoff voltage of 2.7V? This is because the voltage of a lithium battery fluctuates and gradually decreases during operation, but most of the operating range is concentrated around 3.7V, so 3.7V is defined as the standard voltage.
The current in the calculation refers to the max continuous discharge current. If this value is not available in the battery specifications, we can calculate it using the discharge rate. The discharge rate is represented by C, such as 1C, 2C, 5C, 20C. Current = Capacity × Discharge Rate.
Now we have a 15W flashlight and need an 18650 lithium-ion battery. Here are two options; which one should we choose?
No. (1) 18650 lithium battery 3.7V 2200mAh (2.2Ah), discharge rate 1C. Its power is 3.7V × 2.2Ah × 1C = 8.14W. Clearly, this power is a bit low for a 15W flashlight. If used, it will not allow the flashlight to reach its designed max brightness.
No. (2) 18650 lithium battery 3.7V 2200mAh (2.2Ah), discharge rate 2C. Its power is 3.7V × 2.2Ah × 2C = 16.28W. This power is bigger than 15W and can be perfectly used with this flashlight, allowing it to reach its designed max brightness.
Of course, when using a high-power flashlight, considering its battery life, you can choose a larger capacity lithium battery.
So how do we choose flashlights with different powers according to our own needs? Here is a simple formula to convert LED light power into lumen value: LED 1W=100 lm (Please Note: this is not an official formula, but an empirical formula based on measurement data, which is convenient for people to estimate. Different LED light measurement values is different)
Here is our summary of the required flashlight brightness for different living scenarios, hoping it will be helpful for your selection.
| Brightness range | Applicable scenarios | Feature |
|---|---|---|
| 1-30 lumens | Home use at night: getting up at night, finding things, reading. Camping: tent lighting. | Moonlight/low brightness. The light is soft and not dazzling, and will not disturb others’ sleep. |
| 30 – 100 Lumens | Everyday carry: Walking at night, checking vehicles, walking the dog. Maintenance and inspection: Checking computer cases, under furniture, etc. General camping: Cooking at the campsite, organizing equipment. |
Low-brightness. Sufficient for most daily close-range lighting needs, offering a good balance of battery life and brightness. |
| 100 – 300 Lumens | Outdoor hiking/night running: Illuminates the path ahead and helps identify obstacles. Searching: Finding items in a warehouse or room. Strong floodlight for work lighting. |
Medium brightness. This is the most commonly used brightness range for outdoor activities, providing a clear and wide field of vision. |
| 300 – 800 lumens | Professional hiking/mountaineering: For use in complex terrain. Exploration and caving: Requires illuminating a large area. Search and rescue (as a supplementary tool). |
High brightness. Powerful light that can illuminate a long distance. |
| 800 – 2000+ lumens | Search and rescue, law enforcement: Requires wide-area, high-intensity illumination. Extreme exploration: Absolutely dark environments such as caves and deep mountains. Special tasks (such as hunting, inspecting large areas). |
Extremely bright, suitable for special needs, rarely used in daily life. |
