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Everything You Need to Know About the 21700 Battery

This post tells you everything you need to know about the 21700 battery. We'll talk about different types, features, charging, lifespans, and our recommendations for batteries and chargers.

21700 battery

What is a 21700 battery?

A 21700 battery is a high capacity lithium ion rechargeable battery. Their proper name is a “21700 cell”. The 21700 cell has voltage of 3.7v and has between 3000 mAh and 5100 mAh (mili-amp-hours). The 21700 is physically larger than an 18650 battery, and AA/AAA batteries.

2170s may have a voltage range between 2.5 volts and 4.2 volts, or a charging voltage of 4.2 volts, but the nominal voltage of a standard 21700 is 3.7 volts, just like the 18650s batteries.

There are two types; protected and unprotected. We absolutely recommend protected 21700 batteries. Protected 21700 batteries generally have a button top. Unprotected 21700s typically have a flat top.

The average 21700 battery charge time is about 4 hours. Charge time can vary with amperage and voltage of the charger and the battery type.

battery and flashlight size comparison

Basic 21700 Specifications

  • Voltage: 3.6V to 3.7v
  • Cost: varies by battery
  • Amperage (mAh): varies by battery – 3000mAh to 5100mAh
  • Max Continuous Discharge: Varies by battery
  • Size: 74.8mm x 21.49mm (+/- 0.3mm)
  • Weight: varies 60g to 80g
  • Battery Type (Chemistry):
    • LiNiMnCoO2 INR / NMC
    • LiCoO2 ICR
    • LiMn2O4 IMR
    • LiPo – we are not aware of any lithium-poly 21700 batteries
  • Protected Mode: is optional but we recommend ONLY protected mode 21700 batteries

Protected vs Unprotected 21700 Batteries?

21700 protected batteries have an electronic circuit. The circuit is embedded in the cell packaging (battery casing) that protects the cell from “over charge”, heat or “over discharge”, over current and short circuit. A 21700 protected battery is safer than an 21700 unprotected battery (less likely to overheat, burst or start on fire).

Unprotected 21700 batteries are cheaper, but we do not recommend their use. Unprotected batteries should only be used where the load/draw and charging is externally monitored and controlled. The protected batteries normally have a “button top”, but check the specs to make sure.

If a battery is damaged or looks corroded or appears to be leaking, get rid of it at a battery recycling center. Be safe.

See “Battery Recycling – How to Recycle Different Battery Types and Corroded Batteries Safely”.

How much power does an 21700 have?

A 3.7v a 5100mAh 21700 stores about 3 aH to max of 5 aH. It can store about 15 to 20 watt hours. A small air conditioning unit that can cool about 9000 BTU uses about 1100 watts per hour. So it would take more than 74 of these 21700 batteries to run the air conditioner for 1 hour.

In comparison you would need three 12v 40 amp car batteries. But 74 21700 are smaller than three car batteries.

How many times can you recharge an 21700 or other battery?

Recharge cycles vary and are limited. Think of it like a bucket. The trick is that the bucket also gets filled with other junk over time, so there is less room. As the battery is reused (recharged), the battery degrades due to oxidation and electro-chemical degradation.

This happens to any rechargeable battery such as an 18650, 21700, 26650, 14500, AA, AAA or even a car battery. They can only be recharged a limited number of times.

You want to select rechargeable batteries that can be recharged many times. We specifically recommend 21700's because they have the ability to be recharged 300 to as many as 2000 times.

How frequently should I recharge my 21700?

The way you recharge your battery impacts the life of the battery. If you can measure it, you want to deplete from 3.7v down it to about 3v before you recharge. If you are not sure, use the device until it indicates a battery needs to be replaced. For a flashlight, run it till the light is dim or goes out.

A good charger will tell you the voltage of the battery so you can eventually get a sense of the life of the battery in various devices. If you recharge too frequently you “use up” the life without a return.

Some people don't let it dip below 3.3v (or even higher). Each brand and model of 21700 has different maximum cycles. So this is really a process of matching your device and usage to the life cycle of the battery.

Be aware that an 21700 battery that drops below 2.5v may “lock” the device so it can't be used. The “lock” function happens in devices such as vaping devices.

How do I know my 21700 is dying?

Here is a list of 6 ways you can tell if you need to get rid of an 21700 (or other rechargeable battery). Look through these to determine if your 21700 is nearing the end of its life and needs to be retired:

  1. The battery will lose a charge on the shelf must faster than normal. As an example, it loses it's charge after a couple of days or even worse overnight.
  2. The battery gets hot when charging or discharging, warmer than normal.
  3. You have used the battery frequently over 2 to 3 years.
  4. The battery can hold less than 80% of its original capacity.
  5. Recharge time gets abnormally long.
  6. If there is ANY cracking or deformation in the battery.

These are the 6 signs your 21700 is dead and it is time to get a new one. If you ignore these warning signs you risk fire or even having the battery explode while being recharged.

How can I measure the quality of an 21700 if I am unsure of the age of a battery?

A trick is to buy one or two similar 21700s and mark them “new” with a Sharpe (or label them A, B, C, etc). Then use them and compare their voltage and discharge rates with the questionable 21700s.

Basically you are comparing good vs unknown this way.

You can also gauge temperature this way. Charge both the new and unknown one to see how hot the new one compares to the older one you are unsure of.

21700 Chemistry

There are a number of different chemical combinations for 21700 batteries. Nearly all 21700 batteries are Li-ion (Lithium Ion) or sub-variant such as LiNiMnCoO2 INR, LiCoO2 ICR or LiMn2O4 IMR. I have not seen Ni-Zn or Li-Po versions of a 21700. If you come across either, please leave a link in the comments.

21700 Features

A battery might advertise: protected mode 3.7v 21700 5100 mAh low self discharge. What does that all mean?

  • “protected mode” as noted above means it has an overcharge and overdraw circuit protection built in. It has a “PCB” (protection circuit board)
  • “3.7v” or “3.6v” – is the optimal or peak voltage. It will drop as you use the battery.
  • “5100 mAh” measures the amp hours the battery can provide. A higher number is better. The highest realistically available on an 21700 today is about 5100 mAh, anything higher than that is probably marketing hype or an outright scam.
  • “Low self discharge” is a good thing. That means it will hold a charge in storage. The less it loses in storage the more charge will be left for you to run your flashlight or other device.

What are 21700 batteries used for?

Flashlights, electronics, laptops, vaping, power tools and even some electric vehicles use 21700s. Some high lumen flashlights such as the 21700.

Laptops and other electronic devices use one or more 21700’s and have recharging electronics built in. 21700's are also used in vaping (smoking) devices.

21700s are are generally Lithium Ion batteries. If you are familiar with electronics you can change out some battery packs manually, but be careful – using the wrong type of 21700 or using it incorrectly can cause a fire.

What is the best travel 21700 battery?

Nitecore NL2150RX available directly from Nitecore. This is an 21700 3.7v 5000mAh protected cell with a built-in USB-C charger.

It is a few dollars more, but it allows you to charge it on the go and not have to carry a dedicated charger.

21700 with built in micro USB charger

What 21700 Brands are Best?

The Orbtronic, Samsung, LG, Panasonic and Nitecore are good reliable 21700 rechargeable cells (batteries). Be sure to buy them from a reputable source such as Amazon, Battery Junction or direct from the manufacturer such as Nitecore.

We don't use the lower voltage and amperage 21700s, because they have lower amp hours and low peak wattage and lower sustained wattage. If you use a lower amperage 21700, you might as well drop to a cheaper 18650 version of the device/battery.

We are willing to pay a few more dollars for the longer life, higher capacity and better quality.

Best 21700 Battery Charger

The best 18650 battery charger is the Nitecore UMS4 Battery Charger because it can charge pretty much anything (it will be a bit slower charging up to 3amp but will charge just about anything.

Specifically, it supports: lithium ion 26650, 22650, 21700, 18650, 17670, 18490, 17500, 18350, 16340 (the 16340 is also known as RCR123), 14500, 10440 and Ni-MH and Ni-Cd AA, AAA, AAAA, C rechargeable batteries. This is our favorite charger for the 21700s, 18650s and AA/AAA.

Note the UMS4 will NOT charge 1.6v/1.5v Ni-Zn AA batteries. We are testing the Xtar 1.5v rechargeable AA batteries and so far, we like them but they require a matching custom Xtar 1.5v AA charger.

21700 flashlight and battery

Best 21700 Flashlight

The best 21700 flashlight is the Fenix PD36R. It is not cheap but it is durable and very bright, and has a pocket clip. The light level is 1600 lumens (the 18650 version is 1000 lumens) and it is water resistant to IPX68. There is no quick access to the strobe mode. Modes are listed below

  • Turbo: 1600 Lumens / 2 Hours 50 Minutes (Light level drops down for temperature protection)
  • High: 800 Lumens / 5 Hours 49 Minutes
  • Medium: 350 Lumens / 8 Hours 24 Minutes
  • Low: 150 Lumens / 26 Hours
  • Eco: 30 Lumens / 115 Hours

The 2nd choice is the more expensive Olight Seeker 3 Pro 4200. It is crazy bright. USB charging. The modes info is listed below.

  • 4200 lumen / 2 min (Light level drops down for temperature protection)
  • 1200 lumen / 30 min (Light level drops down for temperature protection)
  • 300 lumen / 10.5 h
  • 50 lumen / 55 h
  • 5 lumen / 15 d

We normally like Thrunite and Nitecore flashlights. The Thrunite T15 and Nitecore MH12s both support USB-C charging and both are VERY BRIGHT but both get HOT on highest brightness. Consider them if you want a moderately priced flashlight that uses 21700 batteries.

size comparison of 18650 and 21700 batteries

21700 Questions and Answers

Are 21700 batteries better than 18650? A 21700 is basically a physically larger version of an 18650. The voltage is the same, the wattage should be generally higher (but not always).

Best 21700 battery? We like the Nitecore NL2150RX with a runner ups being the Fenix, LEDLenser, and Samsung 21700, 3.7v protected mode high drain (20amp) batteries are all decent alternatives.

Are 21700 batteries good? Depending on the manufacturer, yes. Stick with name brands from good suppliers.

Are all 21700 batteries the same?
Definitely not. The power (mAh) varies. They can be button top meaning “Protected”. Protected 21700s have a tiny PCB – protection circuit board. The non-button top are Unprotected meaning no PCB and should only be used in devices that have protection circuits.

Some vary in SIZE also. A 21700 should be 21mm by 70mm but some vary and the worst ones vary a lot. Some have integrated USB chargers, some do not. There is also some variation in chemistry (materials) used to make the batteries.

Best 21700 battery charger? Many of the 18650 chargers will also charge the 21700. You need to read the fine print to confirm.

Nitecore Ci4 Four Slot Universal Battery Charger
Nitecore UMS4 Four Slot Universal Battery Charger

We like the USB powered Nitecore UMS4 Battery Charger. It can charge: AAA, AA, C, D, 14500, 18650 and 21700 rechargeable batteries using nearly any USB power source to charge your batteries. The UMS4 can use a USB crank, USB solar charger or the regular A/C USB adapters.

If you are ok with an AC power supply the OPUS BT C3400 is outstanding. Providing high end features at a decent price.

Where can I buy 21700 batteries?
There are a number of places. Right now Amazon does not have a lot. Battery Junction and Nitecore have inventory.

21700 Buying TIPS

Capacity: If you see anything above 5100mah for a 21700 battery be suspicious. It is likely false advertising.

Protected vs Unprotected: Make sure you are comparing PROTECTED or UNPROTECTED. The electronics of the PROTECTED batteries adds cost. If it is really cheap it might be UNPROTECTED and not say it.

Weight Tip: If a 21700 weighs less than 60g (2.11oz) it is likely not good quality or is fake.

Source: Buy from reputable source and name brand manufacturer.

Shop Around: Always compare costs before you purchase. A good LG 18650 or Nitecore 21700 could be on sale, in high demand or out of stock somewhere and make a big difference in price.

Test One: When buying 21700 batteries – buy ONE before you buy multiple in case the size is not 100% correct. Some cheaper manufacturers don’t entirely follow the 21mm x 700mm specification. Even reputable manufacturers batteries may be consistently larger or smaller.

Be Suspicious: If it looks too good to be true, be very suspicious.

21700 battery review

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  1. Inregards to these 21700 protected and unprotected batteries: they don’t all come in the same length! You could buy a battery and discover that it’s too long for your charger. In addition it may be too long or stressful on the battery compartment of your flashlight.

    1. True. Luckily I havent found any (yet). If you find some manufactures that are OUT OF SPEC please note them. The spec is a “cylindrical cell defined by its 21mm x 70mm dimensions”. If it isnt that then it isnt technically a 21700 and should likely be avoided.

  2. It’s my understanding that I would need a 5 amp charger to charge a protected 21700 5000 mA/hr battery. Who makes a charger for these larger capacity batteries? My 2 amp chargers won’t charge over 2 amps.

    1. Good question. No the amperage is like water pressure, it is still flowing, just not as fast. The charger can still charge even if its a 2amp or 3amp (fast charge). You can charge a 21700 with a lower amperage charger that supports 21700 BUT it will take more time. We recommend the Nitecore CI4 in the article. The Nitecore SC4 is a 6amp charger. But even with the Nitecore SC4 you are dividing the amperage across the slots. So multiple batteries causes charge time to increase. If you are really needing fast charge, the SC4 is the better choice. We recommended the CI4 because its simple, less expensive, and can run using pretty much any USB source. And my personal (non confirmed opinion) is that the slow charge is easier on the batteries.