Everything You Need to Know About the 18650 Battery

This post tells you everything you need to know about 18650 batteries. We'll talk about different types, features, charging, lifespans, and our recommendations for batteries and chargers.

What is an 18650?

An 18650 is a lithium ion rechargeable battery. Their proper name is “18650 cell”. The 18650 cell has voltage of 3.7v and has between 1800mAh and 3500mAh (mili-amp-hours).

18650s 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 18650 is 3.7 volts.

There are two types; protected and unprotected. We absolutely recommend protected cell 18650 batteries. Protected cells include a protection circuit that stops the cell from being overcharged.

Unprotected cells can be overcharged and burst or potentially cause a fire unless there are specific electronics to protect the battery. The popular LG HG2 and Samsung 25r are both UNPROTECTED batteries, only use them in a device designed to use unprotected 18650s

We also recommend you stick with high quality brand name 18650s. Knock offs may lie about high mAh.

The average 18650 battery charge time is about 4 hours. Charge time can vary with amperage and voltage of the charger and the battery type.

Various Battery Sizes

Comparing sizes in the picture above the 18650 is 1170 cubic mm, the 14500 and AA are 700 cubic mm, the AAA is 467 cubic mm. Note the 14500's cannot be used in all AA devices unless they support both 3.7 and 1.5 volt batteries. The 21700 at 1550 cubic mm, is larger than the 18650 battery – the 21700 and 18650 is not interchangeable.

Protected vs Unprotected 18650 Batteries?

18650 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 18650 protected battery is safer than an 18650 unprotected battery (less likely to overheat, burst or start on fire).

Unprotected 18650 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 specifications to make sure. Generally 18650 flat top batteries do not include the protection circuit.

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

How much power does an 18650 have?

A 3.7v a 3400mAh 18650 stores about 2 aH to max of 3.5 aH. It can store about 10 to 13 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 110 of the 18650 batteries to run the air conditioner for 1 hour.

In comparison you would need three 12v 40 amp car batteries. But 110 18650s are smaller than three car batteries.

How many times can you recharge an 18650 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 18650's because they have the ability to be recharged 300 to as many as 2000 times.

How frequently should I recharge my 18650?

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 18650 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 18650 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 18650 is Dying?

Here is a list of 7 ways you can tell if you need to get rid of an 18650 (or other rechargeable battery). Look through these to determine if your 18560 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. 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 18650 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 18650 if I am unsure of the age of a battery?

A trick is to buy one or two similar 18650s 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 18650s.

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 is compared to the one you are unsure of.

18650 Battery Chemistry

There are a number of different chemical combinations for 18650 batteries. We recommend that you focus on protected mode, the chemistry can change and isn't always reported. Many simply say Li-ION (meaning Lithium Ion). There are actually a number of Li-Ion batteries. Here are some of the current “types”. Depending on your device type one might be better than the others.

1. LiFePO4 which is Lithium iron phosphate
   • also known as IFR or LFP or Li-phosphate
2. LiMn2O4which is Lithium manganese oxide
   • also known as IMR or LMO or Li-manganese (high amp draw)
3. LiNiMnCoO2which is Lithium manganese nickel
   • also known as INR or NMC (high amp draw)
4. LiNiCoAlO2which is Lithium nickel cobalt aluminum oxide
   • also known as NCA or Li-aluminum
5. LiNiCoO2 which is Lithium nickel cobalt oxide
   • also known as NCO
6. LiCoO2 which is Lithium cobalt oxide
   • also known as ICR LCO Li-coball

18650 Features

A battery might say protected mode 3.7v 18650 3000 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.
• “3.7v” – is the optimal or peak voltage. It will drop as you use the battery.
• “3000 mAh” measures the amp hours the battery can provide. A higher number is better. The highest realistically availble on an 18650 today is about 4000 mAh, anything higher than that is marketing hype.
• “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 18650 batteries used for?

Flashlights, electronics, laptops, vaping and even some electric vehicles use 18650s. The Tesla uses 7180 of these batteries. Many high lumen flashlights such as the Thrunite TN14 or Fenix PD35 use the 18650 or the even larger 21700.

Laptops and other electronic devices use one or more 18650’s and have recharging electronics built in. 18650's are also used in vaping (smoking) devices.

18650s 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 18650 or using it incorrectly can cause a fire.