A Guide to Lithium Ion Cells in Flashlights

First and foremost, lithium-ion chemistries (specifically Lithium Cobalt Oxide or “LiCo”) can be dangerous in the hands of an unprepared user. If used incorrectly they can rupture or explode with enough force to cause fire, permanent damage to surrounding objects, injury, and even death to bystanders. Now, because of this possibility, it is also important to realize that when you put this volatile cell in an air-tight metal tube (a flashlight), you essentially have a potential pipe bomb in your hand. LiCo cells should be used and handled with caution, care, and knowledge. In reading this guide, you assume all risks involved with li-ion cell chemistries, and agree not to hold RigCast responsible for any accidents, injuries, or affects to you or your property otherwise.

Ok, now that I’ve got you running scared I should point out that when li-ion cells are used properly the risk can be controlled substantially, and the benefits can be substantial. Trading up to Li-ion cells in your flashlight can be gratifying, economical, fun, and safe if you practice and enforce basic knowledge and care for the cells. The biggest reason ‘flashaholics’ take their alkaline training wheels off and start running li-ion cells is because rechargeable cells allow the user to use their flashlights virtually guilt-free. Buying primary cells (non-rechargeable “batteries”) for high-end flashlights can get extremely expensive, so users will often restrain and limit the usage of their lights for fear of having to purchase more primary cells. Li-ion cells eliminate this lingering guilt and let the user use their lights whenever they want.

I created this guide to help beginners learn the proper way to transition from primary cells to li-ion rechargeables for use in LED flashlights. Although all of this content is original, I would like to cite CandlePowerForums.com as my main resource for learning all of this information. It’s a lot of information, but it is all relevant if you are serious about making the switch. Be responsible and arm yourself with this knowledge.

First I’d like to set the scope of this guide. Although there are several popular li-ion cell chemistries, this guide will be focusing on Lithium Cobalt Oxide (LiCoO2 or “LiCo”), a 3.7V cell with ~85% efficiency and a low ~1% self-discharge per month. This guide is limited to LiCo because it is the most popular cell chemistry to switch to for flashlights, so much so that it is commonplace for LiCo to be generalized as “lithium-ion” in conversation.

Why LiCo Is Great

  1. LiCo holds rank for the highest energy density available today in rechargeable cells.
  2. They are very efficient at exchanging energy.
  3. They hold their charge even after long periods of storage.
  4. They can be charged and discharged hundreds of times.
  5. They save us money!

The thing is, LiCo is great chemistry. It’s more popular than you realize. If you are reading this on a laptop or mobile device, chances are you have LiCo under or in your hand right this second. Ever use wireless power tools? That’s LiCo. Have an iPod? LiCo! Cell phone? LiCo again. All these applications are safe because they are highly regulated within the device and charger. There has been a lot of money, R&D, and fail-safes put into those applications to ensure that the idiot consumer has a hard time killing himself with it.

So, if all of those popular devices are used everyday without any mention of accidents, then how can using LiCo cells in flashlights be so dangerous?

Why LiCo Isn’t Great

First of all, a battery is not a cell. A battery is made up of many cells, but cells can be removed from the battery (in theory) and used individually. Now, imagine your laptop battery is made up of many AA-sized LiCo cells, all linked together to power your computer. Now take a single one of those LiCo cells out of your laptop (and away from its failsafes) and cram it in your flashlight. That is essentially what we are doing here, folks. These cells are generally not manufactured for individual applications in the way we are applying them. When that single LiCo cell isn’t used in its intended laptop battery, you remove it from all of its R&D and fail-safes that were keeping it (and you) safe. Granted, that’s more how things used to be than they are today. Today, it’s not so primitive, and individual LiCo cell applications are gaining more and more popularity, especially among R/C hobbyists and flashaholics. Thankfully, this newfound popularity for LiCos has sprung an uprising for cells to be safer on an individual standard, which we will get to in a minute. The point is, despite best efforts, LiCo cells still have their drawbacks.

  1. They like to live a comfortable life. If you treat them badly, they will treat you badly, by catching fire or exploding.
  2. Because of their temperamental nature, they shouldn’t be pushed to limits. This means slower charging rates and cautious discharging. You don’t want to push them over 4.2 volts while recharging, or below 3 volts under load.
  3. They can’t take physical abuse very well either, so take care not to drop LiCo cells.
  4. They can be expensive.

The C Rules

Before learning how LiCo is dangerous, it will be important to understand why. The reason is called “C.” This isn’t complicated. C is just the relation of the charge/discharge to the capacity of your cell in an hour of time. Just think of C as a variable equal to your cell’s capacity. For instance, I have a 2900 mAh LiCo cell. If 2900 is the capacity (variable) that equals C, then 1C = 2900 mA (2.9 A), while 2C = 5,800 mA (5.8 A). Easy, right?

Lesson time. It is generally recommended to not charge any LiCo cell at a rate beyond 1C. So, if your cell is 500 mAh, then it should be charged with no more than 500 mA. Less than 500 mA (0.5 A) is fine, but anything above 500 mA is not ok for that specific cell. For this reason, you should verify the charge rate of your li-ion charger before putting any LiCo cell in it.

Lesson two. It is generally recommended to not discharge any LiCo cell at a rate beyond 2C. I know, I know, the amperage that an LED consumes on every given power level of every given flashlight isn’t exactly information on hand. Don’t get caught up in the discharge numbers so much. The important thing to note here is that you should be sure that the flashlight you are using a LiCo in, is compatible with your LiCo cell. For this reason, I have created the LED Torch Shootout to help the average consumer verify which lights are compatible with LiCo cells. There are more compatible torches than are on that list, so let me know if you would like me to add a new torch to the shootout.

The Charger Rules

The most dangerous time to be around a LiCo cell is when it is charging. The danger exists because LiCo chemistry can become very unstable if it becomes overcharged. Remember, LiCo cells should never be charged past 4.2V, otherwise there is a serious risk of “thermal runaway,” the point at which the chemistry excites itself to combustion. Avoiding this is also simple, and there is no math involved. All you have to do is commit to a hobby-grade LiCo charger, and babysit your cells while they are charging. A hobby-grade charger will terminate the charging process the moment 4.2V is reached, while lower-grade chargers will “trickle charge” and slowly continue pushing the charge in the chemistry. I personally recommend the Pila IBC charger, as it is only $40, it is one of the safest chargers on the market, and it is compatible with most cells used for flashlights. On a side note, the Pila IBC charges at 600 mA, so apply your new-found safety knowledge when using the Pila IBC.

Also, while you are using your charger, be sure you stay close by. If you are following all the LiCo safety rules then you should be fine, but it is responsible to be present when charging LiCo cells just in case. This is the same reason we don’t turn the oven on and leave the house. On that note, some people also dedicate a small fire-proof box to charging LiCo cells in, although I don’t personally do this because of heat build-up. Your call.

The Digital Multimeter Rules

A digital multimeter (DMM) is a great way of testing your cell’s health and charge. I’m not going to get into how to use one, but here’s a voltage cheat sheet you can use as a basic reference for determining a cell’s charge:

> 4.2 V = Over-charged. Dangerous cell, recycle it immediately.
4.2 V = 100%
4.1 V = 90%
4.0 V = 80%
3.9 V = 60%
3.8 V = 40%
3.7 V = 20%
3.5 V = 0%
< 3.5 V = Over-discharged. If less than 3V, it is too dangerous to recharge. Recycle it immediately.

If your DMM is reporting a LiCo cell is out of that range, recycle it responsibly and buy a new cell.

The Cell Manufacturer Rules

This isn’t the time to be buying cheap. If you are going to use LiCo cells, you should commit to going all the way or not at all. That definitely applies to the brand of LiCo you are purchasing. I, personally, exclusively recommend AW brand LiCo cells with protection circuits. I won’t buy anything else, because I know that they are the best of the best, and my safety is worth it. There are other respectable LiCo brands available, but I will not endorse anyone else. If you want to find alternatives, feel free.

As the reader, it would be appropriate right now to wonder how much variance there could be between two different brand’s cells when the chemistry is the same. Which brings us to…

The Protection Circuit Rules

Because of the inherent risk and recent uprising in popularity, it is now common for LiCo cells to come with a small protection circuit built into the cell itself. This protection circuit will cut the cell off and let the chemistry relax, making up for some of the lost ‘R&D and fail-safes’ that exist in the consumer products these cells were intended for. However, the protection circuit should be considered an extreme safeguard for the outer voltage limits of the cell, not a guarantee that you can use as an excuse to push your cell. It is the quality of these protection circuits that are the main difference between cell brands.

Regardless of the brand you pick, as a precautionary measure, you should always make sure you are buying protected LiCo cells in order to give you that extra layer of protection. I repeat, always buy protected LiCo cells, never buy unprotected LiCo cells.

The Cell-Torch Compatibility Rules

LiCo chemistry delivers a nominal 3.7V under load, which can be more than twice the voltage that a typical flashlight is built to control. If you put a LiCo into a flashlight that isn’t capable of handling that kind of voltage, you may destroy your flashlight, your cells, or yourself. Keep in mind that voltages stack when used in series, which can exponentially increase the danger.

An easy way around this is to initially purchase lights for your LiCo cells and not the other way around. If you are buying LiCo for a torch you already own, always make sure the flashlight OEM recommends LiCo cells in that flashlight before assuming. Also, this varies by flashlight. Just because a flashlight manufacturer makes one torch that runs on LiCo, does not mean all of their models run on LiCo. It’s simple really, do your OEM research, use the shootout, participate at CPF, or ask someone who knows what they are doing. Just be sure.

Safety Check

That’s the safety briefing. You made it! Following those guidelines can keep you safe, so I want to make sure you get it. Here are the cliff notes of what you should have just learned:

  1. LiCos are dangerous if abused or used without care.
  2. Do not charge a LiCo at a rate beyond 1C
  3. Do not discharge a LiCo at a rate beyond 2C
  4. A LiCo reading more than 4.2V should be recycled immediately
  5. A LiCo reading less than 3V should be recycled immediately
  6. Buy a quality hobby-grade charger
  7. Babysit your LiCo cells when they are charging
  8. Only buy high-quality protected cells
  9. Always ensure your flashlight is OEM compatible with LiCo before trying LiCo in the torch

The Fun Part

If you have absorbed all of that, and have vowed to respect the LiCo and enforce the safety precautions, then you are ready to start playing! This is a whole new world. Where do you start? Well, let’s learn a little bit about LiCo cell sizes. You’ve heard of AA and AAA “batteries,” right? Well, you’re not in alkaline land anymore, and we have different names in LiCo land.

First of all, the LiCo cell nomenclature generally describes the diameter (mm), length (mm), and shape of the cell in numbers. An example is 16340, a short and stubby little LiCo. The 16340 is 16mm wide, 34mm long, and the 0 means it is cylindrical. For the purpose of this guide, we will only be focusing on the most common LiCo cell types, cylindrical ones.

Before getting into the varying sizes, it is important to know that these are rough estimates. A 16340 cell is rounded to those dimensions, which means the cell could technically be wider and longer. Manufacturers aren’t building cells to exact dimension standards, so there is some variance here. I’ve even seen cases of a flashlight only accepting one of five different 17670 cells. Now that you are aware of that, here’s a general compatibility cheatsheet (with no guarantees). LiCo on left, primary equivalent on right:

(1) 16340 or RCR123A or RCR123 cell = (1) CR123A or CR123 cell
(1) 17670 cell = (2) CR123A cells in 17mm bore torch body
(1) 18650 cell = (2) CR123A cells in 18mm bore torch body
(1) 14500 cell = (1) AA cell
(1) 10440 cell = (1) AAA cell

Now that you know the safety rules, and know the compatibility of cells, you should have the confidence to jump into LiCo waters. I think you’ll find out (like I did) that the water is a bit too cold at first. Of course when I say ‘cold,’ I mean uncomfortable. Damn me and my metaphors. I remember when I got my first LiCo cell. I treated that thing like it was a live grenade with the pin pulled. I couldn’t be too careful, because I knew that this cell had an energy density that rivals TNT (not exaggerating). This paranoia made me develop a lot of new questions that I didn’t even think of asking before I got my cells. Let’s review these post-purchase FAQs, so you are on your A-game.

Common Post-Purchase Questions

How do I know when a LiCo cell should be recycled under its normal lifespan? All things must pass, and sadly, our LiCo cells must retire eventually as well. You might baby your cells and ensure that you never break any rules, but when is it time for LiCo cells to retire naturally? You should be checking your LiCo cells with a DMM often so this won’t be anything out of the ordinary. Generally, a LiCo cell is considered “End of Life” (EOL) when it isn’t capable of retaining at least 80% of its original charge capacity. To test a LiCo cell’s health, either use a designated battery tester or do the following:

  1. Charge it fully.
  2. When it is fully charged, remove it from the charger and test it with your DMM. Note the voltage.
  3. After 30 minutes, test it on the DMM again and compare the two voltages to the chart below.
  • No drop in voltage = 100% condition
  • 0.05 drop in voltage = 75% condition
  • 0.10 drop in voltage = 50% condition
  • 0.15 drop in voltage = 25% condition
  • 0.20 drop in voltage = EOL, recycle the cell

Basically, if you have a fully charged cell, and it is reading 4.0V or below, it has lived its life and you need to replace it.

Can I remove the cells from the charger in the middle of a charge? Fortunately for you, you found this RigCast article and are now a responsible LiCo operator. You know better than leaving LiCo cells unattended while charging. Since charging can take up to 5 hours, an occasion might arise where you must absolutely leave the house mid-charge. What should you do? If you have a newer smart charger, don’t worry. The smart chargers (like the Pila IBC) make it completely safe for your cells, your charger, and you to remove the cells in the middle of a charge. Some older chargers had problems charging cells that had an existing charge of 50% or more, so you might want to research your individual charger beforehand. Generally, newer smart chargers are going to be fine with a mid-charge interruption.

Do I have to stay right next to the charger every second during the charge? Just treat the charging cycle like you would a newborn baby. You don’t have to hover over it for hours, but you should check in on it every 10-20 minutes or so. What should I be looking for? If you start to smell something unusual, if the cells are too hot to touch (~140 degrees Fahrenheit), or if you hear a hissing sound, then it’s time to unplug and assess. I have never encountered these symptoms, but I follow all the rules.

When I am using the LiCo cells in my torch, how do I know when to take the cell out? What a great question! By now you should know that over-discharging LiCo cells is one of the big rules we never break. When you are using your LiCo cells in your torch, you’ve got to use good judgment. You definitely do not want to rely on the protection circuit to tell you when it’s time to switch cells. If you have been using your torch for an extended period of time, and it dims rapidly and then turns off completely, you have pushed the cell into protection mode. Please try to avoid this, as protection circuits aren’t intended to be used as a notification system. If you start to notice your light dimming relatively quickly (exponentially within minutes), then it’s time to turn it off and put in some fresh cells. Also, if you ever do deplete a cell to the point that it is dimming rapidly, be sure to check its voltage with a DMM before you put it back on the charger. Remember, below 3V means you have to toss it.

Can I leave the LiCo cells in my torch for long periods of time if it is turned off? Technically, there is nothing wrong with leaving a LiCo cell in an enclosed container for extended periods of time. However, if you do decide to leave it in a flashlight, you will want to ensure that the flashlight is not draining the LiCo. Even while turned off, a flashlight can perform “parasitic drain” on a cell if the cell is connected to the electronics of the flashlight. If you want to leave your cell in the flashlight, ensure there is a way to cut off this parasitic drain, like loosening the tail cap. You can also check the current being drawn with the flashlight off (tail cap off) to ensure that there is no transfer. You can learn how to do this at CandlePowerForums. However, if you aren’t going to be using your light enough to take advantage of LiCo chemistry, you may want to consider just using primaries.

What’s the ideal way to store LiCo cells? To extend the life of your stored LiCo cells, you should store them at a 40% charge, which is usually around 3.8V. Room temperature is ok, but you may store them in a ziplock at ~38 degrees (your refrigerator) to stabilize them further. Freezing LiCo cells is not recommended. Please read this article at Battery University for more information.

I dropped one of my cells. Is it toast? This can vary on height dropped, what it impacted, quality of cell, and a post-drop assessment. Is it leaking anything or hissing? Is it dented anywhere? Check it on a DMM and verify the voltage is within spec. If everything checks out, it is your call. If you think it’s fine, see if it will work in a torch. If it works normally, then see if it will take a charge. If it takes a charge, perform a li-ion health test on the cell with a DMM. After indicating a full charge, is it staying above 4V?

16340 doesn’t fit in my charger. How do I get them to charge? Can I use them in a series in the charger? Since there are a lot of chargers on the market, I can only speak based on the one I have experience with, the Pila IBC. 16340 cells can fit by using a 16340 dummy cell. A dummy cell is just a chunk of aluminum that allows the current to pass through it. This dummy cell acts as a spacer and will allow the cells to fit normally in the charger. Technically, charging two live cells in a series can be done, but I recommend against it. If the two cells don’t have the same voltage, you can encounter some violent results. I recommend avoiding the situation, and just using one cell per charging bay (dummies/spacers excluded). Additionally, it is never acceptable to use only dummies to occupy a charger bay by themselves. Never ever!

My cell exploded/I inhaled vapors/I got some LiCo innards on me. What should I do? Go to the ER! If you inhaled the gas from a LiCo or came into contact with a fluid, or the thing exploded on you, it doesn’t matter. All these cases warrant a trip to the ER. For future reference, it wouldn’t hurt to keep a small amount of calcium gluconate around the house as an immediate treatment for coming into contact with the innards of a LiCo chemistry.

Final words

You have just completed a crash course in LiCo safety, compatibility, and common practice. Before going out and professing your expert knowledge on the subject, I would like to reiterate that I am a hobbyist, not a chemist or an expert. Although the information in this article has been reviewed for validity, I cannot guarantee anything if you chose to employ this volatile chemistry in your electronics. If you have any doubts about your capability or LiCo’s capability, I highly recommend you do more extensive research. You should be well equipped and comfortable before making this switch, so please contact RigCast if you have any questions.

For your own research purposes, I have compiled a Google Doc that consolidates a lot of the resources I used to learn most of the information you have just read. If you have additional concerns or a thirst for more knowledge, I encourage you to read through the compiled resource.

Stay safe out there, and have fun guilt-free torching!

3 thoughts on “A Guide to Lithium Ion Cells in Flashlights”

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