Best Battery for High-Powered Flash Lights

The best battery for a high-power flashlight is the Reliance INR21700-RS40, and the best battery for a mid-range flashlight is the EVE INR21700/50PL. Lithium-ion 21700 cells that use NMC (INR) chemistry have the most power and energy density compared to other common flashlight batteries. When it comes to finding the best battery for a high-powered flashlight, you will need to consider two important factors: capacity and resistance.

How To Pick The Best Flashlight Battery

You will often find that there is a trade-off between capacity and resistance in a battery cell. In general, high-capacity cells tend to have higher internal resistance than “power cells,” because internal construction has to balance electrode area, current-collector design, and how much active material you can physically pack into the can.

There’s something about how the cells connect on the inside and how they can be packaged that can result in either a very low-resistance connection to the chemistry or lots of room for the chemistry. And it makes sense because it's all about the contact area, right? If you've got more room for very large electrodes that connect to many points (or just a large contact area to the chemistry), then you're going to have less room for the chemistry itself.

So that means there's really no way to overbuy in an attempt to underworry. You have to know how much brightness you need to know how much current you need. Once you know how much current you will need, you have to know how long the flashlight can last at that brightness to know how much capacity you need.

If you try to just buy the highest-capacity, lowest-resistance cells, you will run into a problem because those goals are often at odds. So you kind of have to know which one you need (capacity or ampacity / low resistance) more than the other so you can optimize your purchasing decision for that use case.

Doing the Math

That means we're going to have to do some math to make any sense of this article. We're going to have to determine how much power a given brightness uses — like the most popular brightness or the most desirable brightness, or two different desirable or popular brightness levels. We have to find out how many watts those things use.

And then we also have to find out how hot — or at least how many waste watts — are going to be generated as a result of the imperfections and resistance in the battery. Once we determine how much power is going to heat as waste, and determine how much power is going to light, then we can determine the ratio between those two. And we can optimize for one or the other.

Flashlight Power Categories

First, let's break this down into three different categories: low end, average, and high power. We'll run the numbers for the average “high brightness” flashlights and also high-end, high power flashlights. We won't spend any time on the lower-end flashlights.

Commodity-level low-end flashlights output somewhere around 200 to 600 lumens. When it comes to “high brightness” flashlights — regardless of what the packaging may claim (and some claim as high as a million lumens) — your average low-cost flashlight is going to output somewhere between 800 and 1,200 lumens initially.

Then, at the other end of the spectrum, you have those extremely bright flashlights that can throw massive walls of photons for extremely long distances. Those operate at about 2,500 to 5,000 lumens. If you've got 2,500 lumens, you're going to have a very nice, strong, powerful beam.

Example Calculations

We will base these examples on 1,000 lumens and 3,000 lumens. 110 lumens per watt is a good mid-range figure to use for LED efficiency. These flashlights are operating in what's called a 1S configuration because there's not any more than one battery cell, so there cannot be any cells in series. So that means they're operating at the nominal voltage range of 3.7 volts.

Considering these numbers, a mid-range high-brightness flashlight will draw about 9 watts (2.46A) from the battery. For one of those extremely bright flashlights that can light up entire fields at 3 a.m., you will draw around 27 watts (7.37A) from the battery cell.

For our cell comparison, we’ll use:

• EVE INR21700/50PL: ~5000 mAh (minimum 4800 mAh), ≤ 7 mΩ AC impedance
• Reliance INR21700-RS40: 4000 mAh, ≤ 4 mΩ ACIR

Resistance and Power Loss

For the mid-range flashlight that draws just 9 watts, the voltage drop experienced by the 7 mΩ cell is:

• Vdrop = I × R = 2.46A × 0.007Ω = 0.017V
• Power wasted inside the cell = I² × R = (2.46²) × 0.007 ≈ 0.042W

That’s only about 0.5% of the total power burned off inside the cell as heat — basically negligible.

But if you use that same 7 mΩ cell in the high-brightness flashlight that draws 27 watts:

• Vdrop = 7.37A × 0.007Ω = 0.052V
• Power loss = (7.37²) × 0.007 ≈ 0.38W

That’s about 1.4% of the total power. Still not catastrophic, but now it’s enough to contribute to extra cell heating and a little more voltage sag (which can reduce peak brightness, especially on “turbo”).

Now let’s see what happens with the 4 mΩ cell.

For the mid-range flashlight:

• Vdrop = 2.46A × 0.004Ω = 0.010V
• Power loss = (2.46²) × 0.004 ≈ 0.024W (almost nothing)

And then with the high power flashlight:

• Vdrop = 7.37A × 0.004Ω = 0.029V
• Power loss = (7.37²) × 0.004 ≈ 0.22W (under 1%)

So at higher currents, lower-resistance cells waste less power as heat and hold voltage better, which helps sustain brightness and reduces thermal stress on the cell.

Factoring In Capacity

But that's just current performance. Once you factor in capacity, you can gain an even greater understanding.

In our example the 7 mΩ cell (EVE INR21700/50PL) is a 5000 mAh battery (4800 mAh minimum). The 4 mΩ cell (Reliance INR21700-RS40), on the other hand, is 4000 mAh.

For the mid-range flashlight (2.46A draw):

• EVE 50PL (5000 mAh): 5.0Ah / 2.46A ≈ 2.0 hours
• Reliance RS40 (4000 mAh): 4.0Ah / 2.46A ≈ 1.6 hours

In the high-brightness (27-watt) flashlight (7.37A draw):

• EVE 50PL (5000 mAh): 5.0Ah / 7.37A ≈ 41 minutes
• Reliance RS40 (4000 mAh): 4.0Ah / 7.37A ≈ 33 minutes

So in mid-range use, capacity tends to dominate the decision. In high-power use, resistance (and how well the cell holds voltage under load) matters more — and you still have to accept the runtime hit if you choose a lower-capacity “power” cell.

What Does It All Mean?

This highlights two practical truths:

1. Higher resistance = more wasted heat and more voltage sag, especially as current climbs. That heat contributes to faster cell aging over time.
2. Mid-range flashlights don’t pull enough current for resistance differences to matter much, so you can optimize mainly for capacity and runtime.

So basically, if you're shopping for battery cells for a flashlight and it's for a standard mid-range, high-brightness flashlight, then you want to shop based on capacity — you want as much runtime as you can reasonably get. But if you are shopping for a battery for one of those extremely high-powered flashlights, then low resistance and strong high-discharge performance should take priority over absolute capacity.

So, What Is The Best Battery For High Power Flashlights?

It depends on what you mean by high power.

If you're talking about your standard issue high-brightness flashlight that can be used to effectively light up a completely dark house, then that will be the EVE INR21700/50PL, with its ≤ 7 mΩ of resistance and 5000 mAh of capacity (4800 mAh minimum). It has plenty of discharge capability for this class of light, and it gives you excellent runtime.

In contrast, if you're talking about a very high-end powerful flashlight that you would use to light up a massive empty field in the middle of the night (and especially one that pulls very high current on turbo), then the Reliance INR21700-RS40 is a much better option. Even though it’s only 4000 mAh, its ≤ 4 mΩ internal resistance and very strong continuous discharge rating help it waste less power as heat, hold voltage better under load, and maintain higher real-world brightness at the top end.