Batteries play an integral role in flashlights. They not only power the flashlight but also directly affect its performance and safety. Choosing the right flashlight battery is essential. Here, you will find the comprehensive guide of how to choose a flashlight battery.

How does a Flashlight Work?

The working principle of a flashlight is simple. It’s the process of converting electrical energy into visible light. Let’s explain in detail:

1. Battery: The heart of the flashlight. It supplies the juice to keep your flashlight shining bright.

2. Driver Circuit Board: The brain of the flashlight. It manages the flow of electricity, allowing you to switch between brightness levels and modes.

3. Light Emitter: The LEDs light up when current flows through them.

4. Reflector: Properly shapes the light to create a spotlight or floodlight effect depending on your needs.

5. Switch: Controls the on/off of the light.

What are the Main Types of Flashlight Batteries?

Flashlights come in various shapes and sizes, but one thing they all have in common is the need for power. Let’s explore the different types of batteries that keep our trusty torches shining bright:

Rechargeable Lithium-ion Batteries

In the world of flashlight batteries, rechargeable lithium-ion batteries reign supreme. They are commonly available in sizes like 18650 and 21700. Due to their high capacity and long battery life, these batteries are the go-to choice for most flashlight enthusiasts. Besides, they feature fast charging and a long recharge cycle life of 500 to 800 times. So, they are perfect for everyday carry and outdoor adventures.

Disposable Alkaline Batteries

Disposable Alkaline Batteries are widely used due to their convenience and affordability. These dry AA and AAA batteries are available in almost every local store. And they have a long shift life. So they are the best choice for short-term use or emergency preparedness. However, compared to rechargeable batteries, they have lower energy density and are not environmental-friendly.

AA and AAA Ni-MH Batteries

This type is considered to be the best replacement of traditional disposable Alkaline batteries. They offer higher energy density and current. Most importantly, they are rechargeable. So they are ideal for daily use and environmentally conscious users. They are primarily suitable for some small handheld flashlights.

CR123A Batteries

The CR123A batteries has the same size with the AA or 14500. It is non-rechargeable lithium battery type. With higher capacity and voltage, it is also another ideal replacement for the traditional disposable AA Alkaline batteries.

Flashlight Battery Characteristics and Selection Factors

As we all know, brightness and battery life are the two most important aspects of LED flashlights. They directly impact user experience. Then, what affects the brightness and battery life? The battery performance. So, be sure to consider the battery characteristics when investing in flashlight batteries.

1)Battery Size and Compatibility

Different flashlight models often require different battery sizes. Usually, you can find the information on manufacturers’ websites or user manuals. Don’t use batteries other than those indicated for the flashlight.

2)Discharge Current and C-Rating

The discharge current and C-rate of lithium batteries are crucial factors in determining whether the battery suits your flashlight. Check if the discharge performance meets your needs before considering capacity.

Discharge current comes in continuous and peak forms. Continuous discharge means the battery can sustain that current for an extended period under standard conditions, while peak current is the occasional, momentary current the battery can handle. It is not suitable for continuous discharge at peak current levels. Here are four ways to decide what discharge current you need.

• Method 1: Check what LED does the flashlight uses

Some manufacturers indicate the LED model on their flashlight specification, datasheet or user manual. For example, you find a flashlight uses CREE XM-L2 LED, then go to visit the CREE official website and read the datasheet of this XM-L2 LED. You will find the required maximum continuous discharge current there.

• Method 2: Calculate based on the flashlight’s maximum brightness output

Generally speaking, the typical power efficiency of LED flashlights is between 100 to 175 lumens per watt. Let’s assume the flashlight’s maximum brightness is 1000 lumens. If loss is not considered, according to the formula:

I (Current) = Power(Watt) / Voltage

1000lm is roughly equals to 10W. If a battery voltage of 3.7V, we can get:

10W / 3.7V ≈ 2.7A

That means, you need a 3.7V Li-ion battery with max discharge current of at least 2.7 amps.

• Method 3: General Rule

There is also a general rule that single emitter rarely needs continuous discharge current of more than 10A. Multi emitter gets into 15A. If you are buying a beast, then 20A and up is appropriate. This is a general reference range. Specific battery requirement would depend on the design and specifications of the flashlight.

• Method 4: Research from flashlight reviews

Usually, it’s easy to find flashlight reviews on YouTube, forums or other blogs. Some flashlight enthusiasts tested each lighting level of the flashlights and the discharging current. You just need to find the actual discharge current at the turbo mode. Then buying a flashlight battery with higher amps than that value.

In addition to discharge current, some battery specifications also mark the discharge rating(C-rating), such as 5C and 10C.

The discharge rating(C) = Capacity / 1000

For example, for a 3000mAh lithium battery:

5C →5 x (3000/1000) = 15A discharge current

10C →10 x (3000/1000) = 30A discharge current

For AA and AAA batteries, due to their relatively low power, there is no need to pay attention to the maximum continuous discharge current and C-rating. These batteries typically meet the needs of most AA and AAA flashlights.

3)Battery Capacity

Battery capacity determines how long the battery can power a flashlight. Among the same lithium-ion battery sizes, a higher capacity means a longer battery life. The highest capacity for 18650 battery currently can reach 3600mAh to 4000mAh. Claims of capacities higher than that are often overstated. For 21700 batteries, the highest capacity can reach 5000mAh, and for 26650 batteries, it currently stands at 6000mAh. Higher capacity means higher battery price, so, finding a balance of battery capacity and price is important. Consider the actual usage needs and frequency when investing in a flashlight battery.

Usually, the actual capacity is lower than the rated capacity, because the battery rated capacity is measured in lab conditions.

4)Battery Internal Resistance

Internal resistance is crucial for lithium batteries, as it determines their overall performance. According to the formula that P=I²R, high internal resistance batteries struggle with high-current discharge, leading to sharp increase in battery temperature and power consumption. This affects their discharge performance.

For 18650 size, the quality high-drain batteries are usually below 14Ω, while the high-power batteries have much higher internal resistance. Due to the size limitation, a battery cannot get both high capacity and performance simultaneously. The battery discharge performance is more important.

As batteries age or unused for extended periods, their internal resistance gradually increases. This is shown by reduced power output and shorter runtime. If you install this battery into the flashlight and turn on the high lighting mode, the flashlight will be in dimmer light.

5)Flat-top or Button-top

When purchasing cylindrical batteries, check if they’re flat-top or button-top. Button-top batteries have a protruding positive terminal. You can enquiry the flashlight manufacturer or unscrew the flashlight to check which battery type it needs. If the flashlight’s positive terminal has a spring, it is capable of both flat-top and button-top batteries. But some compact flashlights may only support flat-top batteries. Without a spring on the positive terminal, flat-top batteries can’t contact the terminal very well. In this case, the flashlight needs button-top batteries.

6)Protection Circuits

Some flashlights come with protection circuits, while DIY flashlights and some low-quality models usually don’t. When using an unprotected battery in a flashlight without protection circuit, there would be a risk of battery over-discharging and battery lifespan damage. Since most users can’t tell whether their flashlights have battery protection or not, it’s safer to choose protected batteries for EDC flashlights.

But for the super bright flashlights which loved by flashlight enthusiasts, unprotected batteries would be the prefer choice. The reason is protected batteries have higher internal resistance and limited discharge current, while the flashlights usually require large current of more than 15 amps. For example, the Imalent MS18W flashlight has max output 100,000 lumens and uses 8 x Samsung 40T 21700 batteries.

Note that, batteries with protection circuit boards are 2-5mm longer than unprotected batteries, which would not fit some flashlights or chargers.

How to Choose Batteries According to Flashlight Types

Flashlights have different characteristics depending on their purpose: everyday carry, industrial, tactical, or outdoor activities. The most commonly used flashlights mainly fall into four categories:

• Handheld EDC Flashlights: It is the most common types. EDC flashlights prioritize portability and functionality, accommodating various battery sizes from AA to 18650. Opting for a size readily available in stores adds convenience.

• Outdoor Exploring Flashlights: For outdoor use, the runtime and brightness is important. So, it’s advisable to choose high-capacity rechargeable lithium-ion batteries to ensure sufficient power.

• Headlamps: These head-mounted lights typically support AA, AAA, or 18650 batteries, providing convenient lighting for outdoor activities like cave exploration, hiking, camping, and different work conditions.

• Camping Lanterns: Usually use 1 to 4 AA and AAA batteries or a single 18650 battery.

Besides, there are flashlights tailored for professional purposes:

– Tactical Flashlights: These types are made for military or police use. They usually boast a compact size and emit strong light. 18650 or 21700 batteries are widely used to offer higher brightness and longer runtime.

– Search and Rescue Flashlights: Built for brightness and endurance, these flashlights use multiple 18650, 26650, or 32650 high capacity batteries.

– Penlights: Small and shaped like pens, these are widely used in the medical industry with 1-2 AA or AAA batteries.

– Super Bright Flashlights: Providing more than 10,000 lumens at great distances but for a short duration. These flashlights require multiple unprotected 18650 or 21700 batteries or high capacity battery packs. They attract enthusiasts interested in extreme brightness and technological innovation.

XTAR Flashlight Battery Comparison Chart

XTAR, as a reputable brand in rechargeable lithium-ion batteries, chargers and flashlights, developed various high-quality flashlight batteries in different sizes and capacities. Here is a parameter comparison table:

Battery Type	Capacity	Voltage	Max Continuous Discharge Current	Protected	Size（Diameter x Length）
Ni-MH AA	2500mAh	1.2V	1.25A（Quick discharge current ）	No	14.5±0.7mm x 50.5±1.5mm
Ni-MH AAA	900mAh	1.2V	0.45A（Quick discharge current ）	No	10.5±0.7mm x 44.5±1.5mm
Li-ion AA	2500mAh	1.5V	2A	Yes	14.3±0.3mm x 50.3±0.5mm
Li-ion AAA	750mAh	1.5V	1.5A	Yes	10.3±0.3mm x 44.5±0.5mm
Li-ion AAA	1000mAh	1.5V	2A	Yes	10.3±0.3mm x 44.2±0.5mm
14500	800mAh	3.7V	2.25A	Yes	14.3±0.3mm x 50.5±0.5mm
16340	650mAh	3.7V	1.95A	Yes	16.0±0.1mm × 34.0±0.5mm
CR123A	1400mAh	3V	1.4A	No	17±1mm x 34.5±1mm
18650	2600mAh	3.7V	4.5A	Yes	18.4±0.2mm x 68.5±0.5mm
18650	3500mAh	3.6V	10A	Yes	18.4±0.3mm x 69±0.5mm
18650	3600mAh	3.6V	10A	Yes	18.4±0.3mm x 68.8±0.5mm
21700	5000mAh	3.6V	10A	Yes	21.5±0.3mm x 74.5±0.5mm
26650	5200mAh	3.6V	10A	Yes	26.5±0.3mm x 68.2±0.5mm

FAQ

Which battery lasts longer in a flashlight?

Answer: In flashlights, lithium-ion batteries like 18650s generally last longer.

Can I use any 18650 battery in my flashlight?

Answer: Not all 18650 batteries work with all flashlights. Choose compatible and reliable ones for your flashlight.

What kind of battery do most flashlights take?

Answer: Most flashlights use common batteries like AAA, AA, or 18650.

Are lithium batteries better for LED flashlights?

Answer: Yes, lithium batteries are often considered the best power source for LED flashlights as they provide stable current and long-lasting power.

Will lithium batteries make my flashlight brighter?

Answer: Lithium batteries themselves won’t make the flashlight brighter, but they provide stable current allowing LED flashlights to perform better in terms of brightness.

What if the battery discharge current doesn’t fit the flashlight requirement?

Answer: Using a battery with a discharge current lower than the flashlight’s requirement may result in inadequate power supply, causing the flashlight to operate at reduced performance or even malfunction. It may lead to dimmer light output, shorter runtime, or overheating of the battery due to excessive strain. It’s essential to use a battery that meets or exceeds the flashlight’s current requirements to ensure optimal performance and safety. 

Using a battery with a discharge current higher than the flashlight’s requirement is generally safe. The flashlight will draw only the current it needs from the battery, and the extra capacity of the battery won’t be utilized fully. However, it’s essential to ensure that other specifications of the battery, such as voltage and capacity, are compatible with the flashlight to avoid any potential issues.

Should I take out the batteries after using the flashlight?

Answer: It’s good to do this. The best practice is to remove the batteries when the flashlight is not be used for an extended period of time This helps prevent battery self-discharge and extends the battery life. Store the batteries in a dry, well-ventilated place afterward.

Conclusion

Choosing the right flashlight battery not only boosts performance but also ensures safety. Consider factors like power requirements of flashlights, capacity, discharge current, and safety when selecting batteries.

Quality matters too. Choosing batteries from reputable brands for stable performance and longer lifespan. Don’t skimp on cheaper options, as high-quality batteries ensure your flashlight operates correctly.