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21700 vs 18650 Battery! Same Size? Replace One For Other?

Some flashlight brands have picked a new darling battery to make lights around, the 21700 li-ion battery.

So is there a reason for this? 21700 vs 18650 batteries.

We think there is a pretty good reason…

For electronic device manufacturers if they usually use 18650 batteries then it might be worthwhile for them to use 27100 instead as the power source.

Just like with 18650 the brands that are making high-quality batteries are Panasonic, Samsung, Sony, 

21700 Battery Vs 18650 Match Up

21700 Battery Basics

Using the term 21700 battery is pretty specific.

The number is referencing a rechargeable lithium ion battery which are the core of most rechargeable flashlights. There are lots of different li-ion chemistries but the basic thing is that 

All 21700 batteries are li-ion batteries but not all lithium ion batteries are 21700, this is just one of the sizes.

Size

The battery number 21700 actually talking about the dimensions of the battery in millimeters.

21mm in diameter and 70mm tall.

Electricity Profile

21700 batteries have a cell voltage of 3.6-3.7V which is the average voltage during a complete discharge. During the discharge the range is from 4.2V to 2.5V but in general you should not go below 3V for the health of the battery.

Capacity and Discharge Rate

Battery Capacity is measured in milliamp hours (mAh). 21700 batteries currently range between 3000 mAh to 5000 mAh.

Discharge Rate is measured in amperes (A). 21700 batteries currently range between 1.6 A to 6.5 A

Capacity and Discharge rate play different sides of the same coin, if the discharge rate is high then capacity is low and vise versa. For flashlight enthusiasts that like super high brightness levels high amps are better. These would be IMR batteries.

But, if you are looking for long battery life then something with high capacity and low amps would be good.

One thing to keep in mind with li-ion batteries is that they have a self-discharge rate of around 10% per month. So, these are not batteries that you want to store or put in devices that just sip power since the battery will just die in a couple of months either way.

One of the biggest benefits of li-ion rechargeables is that they can be recharged over 1000 times and they do not have a memory issue. You should be able to recharge them to the highest capacity they whole time.

18650 Battery Basics

18650 batteries have been the darling of the flashlight world and of the Li-ion rechargeable battery world for quite a while.

Everything from electric bikes, electric skateboards, laptops, Teslas, and other electric vehicles, to high powered flashlights.

But, things might be changing.

Size

The battery number 18650 actually talking about the dimensions of the battery in millimeters.

18mm in diameter and 65mm tall.

Electricty Profile

18650 batteries have a cell voltage of 3.6-3.7V which is the average voltage during a complete discharge. During the discharge the range is from 4.2V to 2.5V but in general you should not go below 3V for the health of the battery.

Capacity and Discharge Rate

18650 capacity is measured in milliamp hours (mAh). 18650 batteries currently range between 2000 mAh to 3600 mAh.

Discharge Rate is measured in amperes (A). 18650 batteries currently range between 4 A to 20 A

These two things play against each other a little. Usually batteries with high amps will have lower capacity. For flashlight enthusiasts that like super high brightness levels high amps are better. These would be IMR batteries.

But, if you are looking for long battery life then something with higher capacity and low amps would be good.

One thing to keep in mind with li-ion batteries is that they have a self discharge rate of around 10% per month. So, these are not batteries that you want to store or put in devices that just sip power since the battery will just die in a couple of months either way.

One of the biggest benefits of li-ion rechargeables is that they can be recharged over 1000 times and they do not have a memory issue. You should be able to recharge them to the highest capacity they whole time.

Difference Between 21700 and 18650 Batteries

These are both li-ion rechargeable batteries so they share lots of the same characteristics.

The biggest difference in these batteries has to do with their size and because of that their capacity. Since the capacity is pretty different so is the runtime.

But these are still small enough to handheld.

If we use the cylindrical volume formula of V=2(pie)(radius)^2 x height then a 21700 battery has a volume of 24245mm^2 and a 18650 battery has a volume of 18429mm^2. An increase of in volume of 31.5% which means quite a bit more capacity.

If we look around at the max capacity of these batteries. 3600mAh for 18650 and 5000mAh for 21700 there is actually a 38.8% increase.

The cool thing is that the size difference is so small that in most applications like a flashlight or something else you would not even notice the difference.

Can I Use a 18650 Battery Instead Of a 21700?

If a device is asking for a li-ion battery then it is expecting somewhere between 3 to 4.5 Volts to run.

Just because of the size issue usually lights that are designed for 18650 batteries will not fit a 21700 battery. But, many 21700 flashlights will have an extra sleeve that you can put an 18650 battery in to fill up the gaps.

So yes you can use a 18650 battery instead of a 21700 battery often.

Conclusion

So can you use one in place of the other?

The answer is, many times yes.

Since both of these batteries run on the same technology they have most of the same assets with the only difference being size and thus capacity.Hope you found what you are looking for on our site. If you are looking for a flashlight when you found us check out our overall flashlight buyers guide where you can get down to what you really want.