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Li-Ion Safety while charging and using these cells

Discussion in 'Flashlights & Other Illumination Devices' started by SilverFox, Mar 23, 2007.

  1. SilverFox

    SilverFox Empty Pockets

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    Jon asked me to drop by and see if I could answer some questions about Li-Ion safety.

    Li-Ion cells are very simple to take care of. As long as you follow the basic rules, they give wonderful performance.

    However, step outside the rules and they have the possibility of rapid disassembly, more commonly known as rapid venting with flame.

    When using the Li-Ion cells that we have access to, limit the current draw to 2C. I prefer to use the actual capacity. Others go from the labeled capacity. I have run into situations where if you go by the labeled capacity, the cell will be good for around 10 cycles until it drops to 80% of its original capacity and is considered used up.

    If you use a cell beyond its capabilities, it heats up. When the cell temperature exceeds 140 F, damage is done to the cell. If you continue to push the cell, thermal runaway is possible. The best indicator to tell if you are pushing things to hard is to check the temperature of your cells after using them.

    Li-Ion cells have a usable voltage range from 4.2 volts down to around 2.5 - 3.0 volts. At high current draws (1C - 2C) you can run them down to 2.5 volts. At lower current draws, it is better to terminate the discharge at 3 volts. The Li-Po high current cells are terminated at 3.3 volts, and the voltage will quickly rebound to 3.7 volts open circuit resting. A cell at 3.7 volts is considered discharged. Our Li-Ion cells usually will rebound to above 3.0 volts.

    If you over discharge a Li-Ion cell, damage can happen. This won't be apparent until the next charge cycle. If dendrites and crystals pierce the separator, the cell can short out during the next charge cycle. Often, an over discharged cell can be recovered, but it is risky. Many times the cell will have increased internal resistance, and have less capacity. The proper way to recover a cell that has been over discharged is to immediately charge it at a slow rate (around 100 mA) to gradually bring the voltage up to over 3.2 volts. Once you are above 3.2 volts, you can do a normal charge. The amount of damage is proportional to the length of time the cell remains in the discharged state. All cells are not created equal, and some handle over discharge better than others.

    It is best to recharge often and never get into a situation where you are over discharging your cells.

    Some cells have a fall back protection circuit. This is designed as a last stand against over discharge your cells. If your circuit fails to shut off at the right voltage, the protection circuit is a last stand against deep discharge. Please note that the protection circuit should not be thought of as a safe level to discharge to every time. Recharge often and don't run your cells into the ground.

    Charging is where there is the most potential for explosive action. Never charge Li-Ion cells unattended.

    What was that again? NEVER CHARGE LI-ION CELLS UNATTENDED!!!

    Set up your charging area to eliminate combustible materials. Charge outside if you have to.

    When a Li-Ion cell explodes, it reminds me of a road flare. It doesn't last as long, but there are flames coming out and sparks flying everywhere. Lots of smoke and fumes too. I am not interested in lighting a flare in my house, and so I take a lot of precautions to make sure that if something does go wrong it is contained in a reasonable manor.

    The charging algorithm for Li-Ion cells is refereed to as Constant Current/Constant Voltage. The cell is charged at CC until it reaches 4.2 volts. Then the current is tapered off as a CV of 4.2 volts is maintained. When the current drops to around 50-100 mA, it is shut off. Constant trickle charging is dangerous for a Li-Ion cell. The precision required from a Li-Ion charger and the CC/CV method with charge current shut off at the end, make the charger a little complex and the price of a charger can reflect this.

    Charger manufacturers realized that it is a lot cheaper if they eliminate the CC phase and just let the charge taper off using the internal resistance of the battery to limit the charge voltage. These chargers generally charge at lower rates, but can charge cells to voltages higher than they should.

    Charging a cell to 4.200 volts should give you around 300 cycles. If you increase the charge to 4.3 volts, you reduce that to around 30 cycles. If you lower it to 4.1 volts, your good for around 1500 cycles. It is far better to slightly undercharge than to overcharge.

    Trickle charging is bad for the cell. The battery manufacturers recommend a charge rate of between 0.5C and 1.0C. Faster charging rates can damage the cell and may cause it to rapidly vent with flame.

    A 1C charge rate will take around 1.25 - 2.0 hours to charge. If your cell takes longer than this, it is damaged. When the cells capacity drops to below 80% of its initial capacity, it should be recycled.

    The RC people discovered that cells can go out of balance when charging cells in series. There have been fires that have resulted in damage to houses, garages, trailers, and cars. To avoid this problem, they have taken two approaches. One is to use a device that balances the cells during the charge. The other is to charge in parallel.

    Taping the cells to monitor each cells voltage insures that no cells will be overcharged. This is a good way to go, but you need to wire taps to each cell and hook up a balancer to your charger. Some chargers have balancers built in, and some need an external balancing circuit.

    If the cells are within 0.5 volts of each other, they can be paralleled together and charged as a large single cell. This limits the voltage to 4.2 volts, and the charge current depends on the smallest capacity cell in the group you have paralleled together.

    There is an instantaneous transfer of energy when hooking cells up in parallel. The highest we have been able to measure is a very brief pulse of around 5 amps. This was with a fully charged battery pack hooked up to a fully discharged battery pack. If you can keep the voltage differences to 0.5 volts difference, or lower, that surge is greatly reduced. I have been using this method for over two years now, without problems. The disadvantage is that if you hook a number of cells in parallel, you need a charger capable of high currents to charge them. I generally do a pair of cells at a time just to keep things simple. My Schulze is capable of charging at 5 amps, and that is about right for 2 18650 2600 mAh cells. I also have a 10 amp regulated power supply that would allow me to charge 4 cells in parallel if needed.

    I might add that it is very important to never charge Li-Ion cells unattended.

    These are the basics. To pick a charger, look at the capacities of the cells you have and find one that allows you to charge at a proper charging current. Test the charger to make sure it does not overcharge your cells. If charging in series, keep in mind that cells can go out of balance and cause problems. If charging in parallel, be mindful of the transfer of energy when you hook the cells up. Try to only parallel cells that are close in voltage to each other. Clear your charging area of combustibles, and always charge your cells attended. If things go south, a bucket of sand is handy for suppressing the fire at the cell, and a fire extinguisher works well for putting out the fires started by the flying sparks.

    If you follow all the rules, chances are very good that you will never have a problem. I caution people to not be afraid of Li-Ion cells, but to treat them with they respect they deserve.

    Tom
     
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  2. tvodrd

    tvodrd Empty Pockets

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    :bow: :bow: Tom, for that!! I've been fortunate to not having an "adverse event" (Except for a few tossed on the coals of the BBQ out in the desert. ;D ) Bunch of Newton-Meters in a small package!

    Larry
     
  3. greenLED

    greenLED Empty Pockets

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    Excellent post, SilverFox. I'm very glad to have you around EDCF.

    I've read of a couple of DSD Charger-related failures. In your opinion, is that a reliable charger to use if you have a couple of different sizes of cells? I understand I shouldn't mix and match cell sizes/capacities while charging. I also have a new-gen Pila charger (thanks, JSB!) that I've been using. Is this better than the DSD?

    Thank you.
     
  4. SilverFox

    SilverFox Empty Pockets

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    Hello greenLED,

    I don't have a DSD charger, so I can't comment on it.

    I do have the Pila ICB charger. I use it a lot and highly recommend it.

    Of course, the Schulze greatly reduces the need for other chargers... :)

    Tom
     
  5. Luxbright

    Luxbright Empty Pockets

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    Hats off to you, Tom. O0
     
  6. petebroccolo

    petebroccolo Empty Pockets

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    Thanks for posting this thread, SilverFox - I have made a copy and will send it along with my other files when my partners ask my advice on illumination tools.
     
  7. WolfAmmoMan
    • +4 Supporter

    WolfAmmoMan Banned

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    Thank you for this wealth of information ! O0
     
  8. Gryphon

    Gryphon Loaded Pockets

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    I've heard of rechartgeable LIon cells over the last year or so, but always wondered if it was worth it. TBH, based on your excellent writeup, it really seems like it's not. Aside from the reusability aspect, which of course has a certain appeal for cheapness and pollution reasons, is there a benefit to these cells? Are they higher-output than the equivalent disposables, for example, assuming nothing goes wrong?
     
  9. phoenix.stu

    phoenix.stu Loaded Pockets

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    Very well put. I have several R/C vehicles and I have a couple that I am thinking about converting to Li-Ion. I have read several articles however, not quite so in depth and precise. This is very useful information. Thank you for sharing. O0
     
  10. SilverFox

    SilverFox Empty Pockets

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    Hello Gryphon,

    The advantage of Li-Ion cells is that one Li-Ion cell replaces 3 NiMh cells as far as voltage goes. This allows for small powerful packages.

    It's kind of hard to stay away from Li-Ion cells because they are used in cell phones, computers, cameras, and so on. The key to safely using them in flashlights is to understand their strengths and weaknesses.

    Since most flashlights don't have the same protection circuits that cell phones and computers have, you have to check things yourself. Having and understanding how to use a voltmeter are important, and having a quality charger eliminates a lot of the safety issues.

    Anytime you store energy there is some risk. You would be surprised what a NiMh cell can do when it gets overcharged, but with some of the Li-Ion chemistries you have the flamability of the electrolyte issue. I fully understand your reluctance to use Li-Ion cells, but it is possible to safely use them, and safely charge them.

    Tom
     
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  11. deusexaethera

    deusexaethera Loaded Pockets

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    Yes, the rechargeables have significantly higher output than primaries. The really high-output flashlights all use rechargeables for this reason, or else they have to use multiple primaries instead of a single rechargeable.

    The real-world effect of all the technobabble in the first post is: you need to buy a good charger, which you'll have to do anyway since an AA charger won't work, and you should periodically swap-out a battery that's in-use for a fresh one and charge the old one, even if it didn't appear to be run-down. If you use your flashlight every day, just swap-out the battery when you get home in the evening -- it only takes a few seconds to do it.

    Also, as I pointed out on CPF back when I was a member there, as long as the battery is inside a flashlight, it's very unlikely to do any damage even if it does flare-up. The worst-case scenario is the light turns off and suddenly gets really hot, so you drop it and kick it away from you, then wait for it to cool off. Even so, I've never actually seen anything of the sort happen, and neither have most of the people here. That should give you some idea of how unlikely a catastrophic failure really is. You'll probably deal with more car accidents than exploding batteries in your lifetime.
     
  12. Valerian

    Valerian Tea-powered admin

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    Not necessarily, it depends on what kind of regulation the light has. If it's a buck-boost thingy, that means it will lower too high voltage down to spec and boost a too low voltage. Therefore, in some cases, li-ion vs. lithium primary makes no difference whatsoever to brightness. On some lights, it does, though.

    I'm not an expert, but I gathered from CPF that when a cell vents, it's likely to release some nasty chemicals. So be careful when you retrieve and open that cooled off light.
     
  13. deusexaethera

    deusexaethera Loaded Pockets

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    That's not what I meant. I was talking about the battery's output, not the light's output. Rechargeable lithium cells can output substantially higher amperage than primaries can -- sometimes at a rate that's a multiple of their total capacity (i.e. 2 amps from a 1 amp-hour battery, etc.) -- though that's generally not advised, because it shortens their lifetime. The step-up converters you mentioned actually require that kind of amperage, because like any transformer, higher output voltage requires higher input amperage, and vice-versa.

    To be sure. My point was only that a reasonably well-made metal light isn't going to explode in your hand just because the battery flares up.
     
  14. Valerian

    Valerian Tea-powered admin

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    Ah, so it seems. I guess I should have read that more carefully.
     
  15. JBorneu

    JBorneu Loaded Pockets

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  16. JPHing

    JPHing Loaded Pockets

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    Just curious.

    If you are not able to attend to your batteries being charged, is it ok to pull then out and continue later?


    Sent using Tapatalk
     
  17. scaru

    scaru Loaded Pockets

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    Yep,perfectly fine. While it isn't the best treatment for them It will work fine. (This is because the charger will start with a CC curve when you put them in again) But it is perfectly safe to do so.
     
  18. JPHing

    JPHing Loaded Pockets

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    Ah I see. Thanks :)
    I know the approx charge time now, so will time it better next time.
     
  19. david57strat

    david57strat EDC Junkie!!!

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    Excellent, thorough, useful information, here. Thanks for sharing it with us, Tom!
     
  20. Rodewaryer

    Rodewaryer Empty Pockets

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    I guess it only takes once for an incident to make one regret leaving a battery unattended. I do it all the time but only use good chargers (a nitecore Intellicharger and an Xtar VC2), I stopped using that generic junk some time ago. I will keep your description of what happens when they...let go, in mind from here on out.