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.
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.
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.
See “Battery Recycling – How to Recycle Different Battery Types and Corroded Batteries Safely”.
A 3.7v a 5100mAh 21700 stores about 3 aHto 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.
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.
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.
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:
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.
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.
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.
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.
Nitecore NL2150RX available directly from Nitecore. This is an 21700 3.7v 5000mAh protected cell with a built-in USB-C charger.
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.
The best 18650 battery charger is theNitecore UMS4 Battery Chargerbecause 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.
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.
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.
About 21700 rechargeable
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