Lithium-ion battery safety 101

Lithium-ion battery safety


Many of the flashlights discussed/reviewed/modded in Budget Light Forum (BLF) use Lithium-ion rechargeable batteries. Use of Lithium-ion batteries does introduce some risks, and requires more precautions than alkaline batteries. But like any tool, when used properly, the risks can be managed and reduced.

Nederlandse vertaling / Dutch Translation

Islandic þýðing / Islandic Translation

traduction française / French translation

Deutsche Übersetzung / German Translation

русский перевод / Russian Translation

Traducción al español / Spanish Translation

Tradução português / Portuguese translation

Превод на български / Bulgarian translation

Target audience:

This post outlines safety precautions for users that are new to Lithium-ion batteries. It does not attempt to provide a complete tutorial on Lithium-ion batteries nor the technical reasons for Li-ion dangers. It is intended as a starting point.

There will always be some disagreement about safety rules. Some of these safety rules may not apply to those of you who were lab assistants for Dr. Goodenough, but hopefully they are appropriate for newer users. You have to be aware of your knowledge and experience level.

Lithium-ion rechargeable batteries can be safely used when used properly.
Lithium-ion rechargeable batteries can be dangerous when improperly used.

  • Buy quality batteries.
  • Buy a quality battery charger.
    • This is another place not to go the cheap route.
    • An under-performing cell can be safe in a good charger but no cell is safe in a poor charger. (SawMaster)
    • Read the reviews. Buy a brand name with a good review.
    • Features to consider in a battery charger:
      • Overcharge protection.
      • Reverse polarity protection.
      • Independent channels so you can charge different battery types or discharge levels at the same time.
      • User interface that gives you what you like. Indicator lights vs. LCD display.
    • The charger should charge at 4.2V +/- 0.05V, but the battery may come off at a different voltage.
    • Charger reviews.
  • Buy and use a multi-meter.
    • Many like Digital Multi-Meters (DMM).
    • Learn to check your Lithium-ion batteries voltage.
    • Note: Not everyone agrees you need a DMM, but many people do recommend them.
  • Charging / Discharging
    • Batteries can be charged at: 0*C - 45*C (32*F - 113*F)
    • Batteries can be discharged at: -20*C - 60*C (-4*F - 140*F)
  • Don’t over charge your batteries.
    • Max voltage should be 4.2V.
    • Only charging to 4.1V may give you more recharge cycles.
    • A good battery charger helps prevent over charging.
    • Battery chargers may charge +/- 0.05V and still be in tolerance.
    • After charging check your battery's voltage with your DMM after a short (~5 min) rest.
  • Don’t over discharge your batteries.
    • Many people choose to recharge when their batteries reach 3.5- 3.6V
    • Batteries are depleted when they reaches 2.7-3.0v. Recharge.
    • Batteries begin to suffer damage if discharged to 2.5 volts.
    • Batteries should be discarded if they discharge to 2.0 volts.
  • Don’t charge your batteries when temperature is below freezing. (0*C / 32*F)
  • Don’t let your batteries get too hot.
    • If you are going to store your flashlight in your car in the summer, primary batteries are recommended over Lithium-Ion rechargeable batteries.
    • High temperatures degrade cells faster, but as long as the temp is less than 60*C (140*F) there is probably not a safety issue.
  • For long term storage (months) it is best to store at 40-50% capacity. (3.77-3.87V)
  • When not in a flashlight, batteries should always be in a protective carrier.
    • This protects from shorting and physical damage.
    • Never carry loose batteries in your pocket or bag.
    • <==
  • Don’t use batteries with physical damage.
  • Protected batteries are generally safer than unprotected batteries.
    • Protected batteries have a circuit built in which protects from over-charging and over-discharging.
    • Protection circuits increase the length and width of the battery slightly. Protected batteries won’t fit in all flashlights. Know your flashlight.
    • Some flashlights have low voltage protection and will cut off at low voltage. Know your flashlight.
    • Some high draw flashlights will trip the protection circuit in a protected battery and require unprotected batteries. Know your flashlight.
  • Multi-battery flashlights have higher risk than single-battery flashlights.
    • If just starting out you may want to limit yourself to single-battery flash lights.
  • When using multi-battery lights, to reduce the chance of uneven discharge between batteries, it is important that all the batteries:
    • are the same type,
    • are the same manufacture,
    • have the same capacity (Ah) ,
    • are charged to the same voltage.
    • You may want to mark the batteries so you can use the same set together over time.
  • Charge batteries in a location where there are not flammable materials.
    • Put your charger on a counter, not on your sofa.
  • Never leave charging batteries unattended.
    • Check them periodically to make sure they are not getting hot. Warm is ok.
    • Note: there are different opinions on how often you need to check.
  • Damaged batteries can cause fires!

"Abuse conditions such as overcharge, over-discharge and internal short-circuits can lead to battery temperatures far beyond the manufacturer ratings. At a critical temperature, a chain of exothermic reactions can be triggered. The reactions lead to a further temperature increase, which in turn accelerates the reaction kinetics. This catastrophic self-accelerated degradation of the Li-ion battery is called thermal runaway.2"

"During thermal runaway, temperatures as high as 900 °C can be reached,3 and the battery can release a significant amount of burnable and (if inhaled in high concentrations) toxic gas.4"

Royal Society of Chemistry

  • In the event of a Lithium-ion battery fire seek medical attention immediately.
    • Burning or venting Lithium-ion batteries produce hydrogen fluoride.Symptoms from hydrogen fluoride injuries are not immediately apparent.
    • "the battery can release a significant amount of burnable and (if inhaled in high concentrations) toxic gas.4"
      • Royal Society of Chemistry
    • Seek medical attention immediately. Don’t wait until you experience symptoms.
  • There are mixed opinions on using magnets to create button-tops from flat-top batteries.
  • Don’t get batteries wet.
  • Don’t burn batteries.
  • Dispose of batteries properly.
  • Keep batteries out of the reach of children and pets.

Further reading about Lithium-Ion Batteries:

Please let me know what I've missed or what I've gotten wrong.


Battery technology changes rapidly. Please check the edit date below.

1 Thank

The length and width of a protected cell is bigger

Thank you Jack! Post has been updated.

I’d recommend putting citations to sources behind the statements.
These batteries are “evolving” rapidly and the age, and source, for each belief is important to have on record.
I don’t mean you have to be responsible for pointing to the best source — just point to where you found what you felt reliable information, so people can go from there.

Regarding storage of batteries in a car during summer, there is some disagreement as to the actual risk:

I’d suggest rewording the sentence to read:
“If you are going to store your flashlight in your car in the summer, primary batteries are recommended over Lithium-Ion rechargeable batteries.”

And for some reason, the statement, “Protected batteries are safer than unprotected batteries,” irks me. Just a style issue. I would have phrased it, “Protected batteries are generally safer than unprotected batteries.” That’s because there are so many exceptions where unprotected batteries are as safe or safer than protected batteries. For example, PCB’s have been known to fail in multi-cell applications. Or the most simple example of a high quality unprotected cell versus a low quality protected cell. But I’m getting picky. :wink:

All I want to say is: Thanks for putting this thread together! :GRADE:

Edit: Oh wait, as long as we’re allowed to be ‘picky’ here, it should be cell/cells rather than battery/batteries. :wink:

Is it normal for liIon cells to get a bit warm whist charging?
I’ve never felt any of mine go above ambient.

And not to be a knob,but what’s the difference? That’s one thing I’ve never seen explained.

Battery vs Cell I beat David to the punch :wink:

Cheers, makes sense now.

Thanks! beautiful thread!

Great info, looks pretty complete and easy to read.

One thing that you may want to expound upon is;

“Don’t over charge your batteries.
Max voltage should be 4.2V”

Maybe explain that most chargers have a tolerance of 1%, so for instance 4.22 volts is not a problem.
Hate to see new members obsess over .02-.04 volts.
Also all voltages should be measured after a short “rest period” and not instantly after coming off charger.

This also does not cover LifePo 3.2 volt or 4.3, 4.35 batteries, but that is another show as they say

Thanks for putting this list together Mr. Scott


Thanks for the comment. References are good. Unfortunately I’ve been copy-and-pasting things into a word file as I came across them and haven’t been saving the references. I will attempt to find as many citations as I can.

Done and done. Thank you! Picky is fine.

> copy-and-pasting thing into a word file

If you still have the original text you copied and pasted, just paste a chunk of it back into a Google search as a quoted string; likely it’ll take you right to where it first appeared.

If not, at least, going forward, it’s easy to capture the source URL as well as the words you want, which will help as this evolved.

This should absolutely be a sticky.

What is “short” with respect to “a short rest period”? As a newbie that is one of my first thoughts.

I thought I’d read that tolerances were +/- 0.05v for chargers. Of course I can’t find the citation right now. >:-/

A few minutes should be fine for accurate voltage measuring. Wait until the cells cool down to use them in their respective device.

You’re right. I was thinking Short being 5 minutes. Sorry I did not include an actual time frame.
1% tolerance in this case would be .042 volts so yes near .05 volts would be a reasonable variance.

Members should also realize that the health and even size of a cell can also effect voltage termination values.
My experience has shown that many times a smaller cell such as a 16340 may end charging at 4.17- 4.18 where as a normal 18650 on the same charger would come off at 4.20-4.22.
So if the user only were to try the 16340 cell they may think there is something wrong with the charger when this is not the case.
As in many things in life you are looking for a suitable safe range and not an absolute value on each and every cell.

I agree with your recommendation that the user chose a good name brand charger and not just an unmarked charger.


Well done, I am in awe that you took the time to do this, and I think that it has the potential to save an unknown number of li-ion battery users a whole deal of time/grief/injury.

I have some thoughts. What I am going to say comes from having worked with many people who are new to the field I work in and trying to be more effectively informative without wasting time or coming across as condescending.

1. Delete the first two paragraphs. You do not need to excuse yourself for trying to save people bodily injury. Acutely, the first paragraph could be a few pictures of people that have suffered such injuries due to the improper care of li-ion cells, although that may come across as offensive or heavy-handed to many. I think a quick explanation of the dangers of li-ion cells followed by a brief explanation of what is contained in the guide would suffice as a more pleasant alternative to that, but getting to the point quickly and concisely, especially in a thread designed on promoting safety, is important.

2. The section on a lithium battery fire is not substantially alarming. It needs to be much more of a noticeable bullet point. I’ve seen several “venting with flame” claims by manufacturers that seem to have the intention of assuring the customer that no real danger is present. The chemicals released during a li-ion fire are bad enough that medical attention is necessary immediately instead of the next day to prevent chronic problems.

I think you did a bang up job on this and I appreciate how much work it is to put something like this together, and I thank you for doing it. Kudos to you Mr. Scott.

Couple of concerns.

This thread is posted in the “Rechargeable Batteries” category not the “Rechargeable Cells” Category.
HKJ only seems to review batteries, not cells.
That is a lot of editing for this post.