Lithium Polymer Batteries 101Tips


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  1. PGR

    PGR Well-Known Member This Topic's Starter

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    Every phone I've owned after my Treo 300 has been powered by a Lithium Polymer battery, and I've seen the same misconceptions and incomplete or bad information about those batteries in the forums for every one of those phones including the Evo. Well I'm home sick and can't do much else besides cough, so I'm going to try to educate y'all a little.

    And why do I think I'm qualified to do that? Well I just turned 60 and I've been actively involved with electronics, both as a hobby and professionally, since I was 10. I've also been a model (R/C) aviator for many years and rechargable batteries have played a huge role in both of those pursuits. I began using LiPo cells in my projects and planes long before they started appearing in consumer electronics. Not much was known about them back then and we were basically on our own to figure out what worked and what didn't and we also had to design and build our own chargers. Those were exciting times given LiPo cells' tendency to "vent with flame" when they aren't happy.

    So without getting too technical, here are some things you should know about LiPo battery packs:

    The single most important thing you need to know is LiPo batteries can explode. They actually "vent with flame" with a big woosh, but "explode" seems to be the most popular description. Perhaps the surest and quickest way to make that happen is charge them beyond 4.2V per cell. Modern consumer electronics have plenty of built-in safeguards in place to prevent that from happening, but some "budget" battery packs and chargers don't have all the safeguards. The other common reasons LiPo packs explode is excessive heat and physical damage. I've personally seen two cars which had phones explode on the dashboard and it wasn't pretty. I also saw an executive's desk after a phone exploded in a drawer nearly 12 hours after he rolled over it with his car. The technical name for the phenomenon is "thermal runaway" and the actual chances of it happening to you are pretty slim if you stick to name brand batteries and chargers and use some common sense.

    And you never want to forget or ignore this simple rule: If a battery pack ever starts to puff up like a little pillow or change size or shape in any other way, treat it like a firebomb with the timer ticking. Take it outside and put it in your barbecue or a steel pail or something.

    While I'm on the "vent with flame" rant I'd like to add this: Almost all modern cell phones (including the Evo) use a single-cell (3.7V) battery pack. The charging circuit in the phone will prevent you from overcharging it, but all bets are off if you remove the pack from the phone and charge it in an external charger. One very common way model aviators accidently blow up LiPo packs is by charging them with a charger set to a higher cell count, and this could happen to a cell phone battery if you were to try to charge it with a charger made for a camera, for example. The other way model aviators blow up packs is by charging them at a higher current rating than they're designed for. I mention this because it may not be a good idea to charge a stock 1500mAh battery with an external charger designed for a 3000 mAh battery, but I can't say for sure without knowing the charging current of the charger. It would be both safe and acceptable to charge a 3000mAh battery with a charger designed for a 1500mAh battery, but it would take twice as long.

    Next, you never want to discharge a LiPo cell below 3V per cell. 3V is completely discharged for a LiPo cell and if you go below that voltage you'll do unrecoverable damage to the cell chemistry. Once again, modern consumer electronics have plenty of built-in safeguards in place to prevent that from happening, but it's not hard to do by accident or by design once the battery is out of the device. Put the battery in an external charger and unplug it, for example ... No, don't.

    There is no practical reason to unnecessarily discharge or cycle a LiPo pack. All you'll accomplish is reduce the number of charge-discharge cycles the pack will be capable of before it begins losing capacity. LiPo packs don't develop a "memory" like the old Ni-Cad cells did and they'll last significantly longer if subjected to partial charge-discharge cycles than they will with full charge-discharge cycles. In layman's terms, your battery will last longer if you charge it whenever you can rather than wait until you have to all the time. Also, the capacity of new LiPo pack will usually improve after it's been through a few charge-discharge cycles, but the best way to do that is through normal use.

    LiPo cells have a shelf-life and they basically begin to degrade on the day they're assembled. But they degrade faster if they're stored at full charge, so if you won't use a battery pack for a significant length of time (~2 weeks or more), discharge it about half way before you store it. You don't need to get real technical about this because close counts. Just use the battery until the gauge shows about half green.

    And finally this, for those of you who are compelled to play with volt meters:

    A fully-charged healthy LiPo cell will have a resting voltage of 4.2V, but the nominal working voltage is only 3.7V. As I mentioned above, these cells are completely depleated when the resting voltage reaches 3.0V so the entire working voltage range for a LiPo cell is only 0.7V nominal. In low-current devices like a cell phone the actual working voltage range is more like 3.85V to 3V, but that additional 0.15V doesn't really amount to much. So what happens to that voltage range between 4.2 - 3.7V? In the simplest of terms, it's just a surface charge of sorts. Even at low current loads the voltage will quickly drop to the nominal working voltage where it will hold pretty close to steady until the cell is about 80% depleated.

    [​IMG]

    The graph above is a typical 6C discharge voltage curve for an average LiPo cell. 6C means the cell was discharged at a rate (in mA) that was 6 times the rated capacity of the cell (in mAh). In other words, if the cell was a stock 1500mAh cell for the Evo, then the discharge rate would be 6 x 1500 or 9000mA (9A). Needless to say, a cell phone will only draw a tiny fraction of that current which would affect the curve in the following way: The starting voltage would be more like 4.1V and it would quickly drop to around 3.8V instead of 3.6V. Then the voltage would gradually drop until it hit about 3.4V and drop from there relatively quickly.

    And that is just about everything you need to know to get along with Lithium Polymer battery packs and then some. To wrap this up, here's a short video of what can happen if you mistreat a LiPo battery. Judging by the visual evidence, my guess is it's a single 2100 to 2500mAh cell:

    http://www.youtube.com/watch?v=4OsBc8RqSKU

    Enjoy your phones, people! ;-)

    Pete
     

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  2. Mr. Ed

    Mr. Ed Well-Known Member

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    Thanks pgr! I don't have time read it now but it looks like a good read!
     
  3. krypticide

    krypticide Member

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    Darn cobalt and its unstable oxidation states. ;-)
     
  4. Palmetto Fellow

    Palmetto Fellow Well-Known Member

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    I don't understand the need for this step.

    If the rate of discharge is faster for full cells, then that means it will discharge faster while full. But after several weeks of rapid discharge, the battery may be only HALF FULL. At which time, it will begin to discharge more slowly. Seems to me, the more you start with, the more you
     
  5. PGR

    PGR Well-Known Member This Topic's Starter

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  6. Palmetto Fellow

    Palmetto Fellow Well-Known Member

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    Degrade, discharge...they both begin with a D.

    I understand what you mean now. So given my case where I have a brand new EVO battery in the wrapper, should I just leave it alone for a year, and then start using it? I am not one to remove my phone case, then the battery cover, and swap batteries regularly, so using them in tandem isn't a good idea for me. I want to use up the best life of the first battery, and then move on to the second one.
     
  7. PGR

    PGR Well-Known Member This Topic's Starter

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    My suggestion is you sell it and buy another when you're ready to start using it. As I already stated, the cell chemistry will wear out even if you don't use the battery. All you can possibly do is slow down the process by storing it under ideal conditions. If that's the route you have to take then don't charge the battery until you're ready to put it into service. It will also help if you store it in a refrigerator (NOT a freezer).

    I've done a few informal studies** on new R/C lipo packs and found as much as a 20% difference in capacity between packs with recent manufacture dates and packs that have been loitering in the supply chain for a year or more. The big packs cost several hundred dollars so I'll only buy fresh packs with recent manufacture dates unless the vendor is willing to discount the older ones.

    And I won't buy brands without an understandable date code on 'em, but that doesn't seem to be an option with cell phone batteries 'cause I haven't seen one yet with a date code.

    Pete

    ** The studies were informal but the test instruments and methods were professional
     
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  8. Palmetto Fellow

    Palmetto Fellow Well-Known Member

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    I don't think it's worth it to sell this battery. They go for $20 on eBay shipped...and that's from people with 50K positive ratings.

    After my overhead, I'll be keeping $10 or less...

    That being said, the battery in the wrapper will still be better off than the one I've been using for a whole year right?

    This could all be moot however. Once Seidio releases the Rugged case for their extended battery, I will be taking a long hard look at upgrading to their extended battery. With the Rugged case, that's over $120 to make the switch. We'll see...

    Thanks for the insight!
     
  9. wilfried

    wilfried Member

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    I can perhaps predict your answer, but I'll ask anyway. What are your thoughts on the cheep no-brand eBay batteries? I realize you get what you pay for, and they may not be reliable or have the capacity promised. But are they actually dangerous? Are they much more likely to explode, or "vent to flame" as you say?

    Thanks for your article. I already know much of it, but you explained the ins and outs very clearly.
     
  10. PGR

    PGR Well-Known Member This Topic's Starter

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    The overwhelming majority of my personal experience and knowledge about LiPo batteries involves packs for R/C airplanes and helicopters. Cell phone LiPos aren't really any different, though. In fact, most of the bare cells for cell phone batteries come from the same manufacturing facilities as the bare cells for R/C battery packs. Believe it or not, the majority of the worlds LiPo cells are made by a surprisingly small number of companies.

    That said, my experience with budget-priced "no-brand" LiPo packs hasn't been good. I've bought a few that were acceptable, but even those didn't perform or last like my favorite brand-name packs do. Most budget packs are made with cells which didn't pass somebody's quality control standards or are near that proverbial "use before" date for one reason or another. As often as not, those cells are auctioned off in bulk lots to the highest bidder instead of being destroyed, and guess who buys 'em? :rolleyes:

    As for your safety concerns, budget cells used for model aviation do seem to burn more often than the name brands do, but I'm not sure the same would apply to budget cell phone batteries. My gut feeling is the budget batteries don't tolerate abuse as well as name brand batteries do and cell phones make it pretty hard to abuse the batteries.

    For what it's worth, you can probably find at least a dozen answers for any question you could possibly ask about LiPo batteries here:

    www.rcgroups.com/batteries-and-chargers-129/

    The only trick will be separating the wheat from the chaff, but that's no different than a cell phone forum. :D

    Pete
     
  11. PGR

    PGR Well-Known Member This Topic's Starter

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    By the way, last night I stumbled across this excellent database of cell phone battery test results:

    BatteryBoss Calls Out False Capacity Claims!

    It doesn't include batteries for the Evo but it does provide a good general comparison of OEM batteries versus the aftermarket wonders. I found this graph particularly interesting:

    http://batteryboss.org/results/FreshOEMvs6mosOEM_both1500mAh_at_250mA.png

    It's a discharge curve for a new OEM battery and the same battery after 6-months of daily use. Trust me, people: The 7.7% loss of capacity is nothing compared to the loss my packs typically suffer in planes and helicopters, but us model aviators have a habit of pushing battery packs to maximum limits and beyond.

    I happen to own a CBA-III like he used to run all those tests and I also have several programmable LiPo cycler/chargers with data-logging capabilities. Maybe I'll make a charging jig for Evo batteries and run some tests of my own. Truth be known, I hope Doug beats me to it because I really don't have the time for that right now. In fact, it looks like I'll have to work on the Friday after Thanksgiving, but I'll be financially rewarded for it. ;)

    Pete
     
  12. PGR

    PGR Well-Known Member This Topic's Starter

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    This is how I treat LiPo battery packs:

    The attached PDF file contains in-flight data from a 6-minute aerobatic routine.

    The plane was a 3D Hobby Shop 47" Velox with a ~550 Watt power system.
    The battery pack was a 2200mAh 3S (11.1V) LiPo.

    The red line is the cumulative mAh used.
    The purple line is the Amp draw.
    The blue line is battery pack voltage.

    I landed at 6 minutes and the rest is ground taxi back to the pits.

    ~2200 mAh in six minutes. Now that's cookin'. :D

    Pete
     

    Attached Files:

  13. Palmetto Fellow

    Palmetto Fellow Well-Known Member

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    If you scroll down, there are a handful of EVO batteries tested. I wish there were more though...
     
  14. yuanlestat

    yuanlestat New Member

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    Sir, I truly respect your experience and I've been looking around the web of who would really be trusted on this and so far you're the only guy I found reliable on this basing on your experience both on RC aviation and gadgets.

    If possible, can you please explain these in layman's term? especially the part of charge-discharge cycles, normal use, difference with partial and full charge-discharge.

    And on your experience, it would be very helpful if you could tell us some tips on gadgets using Li-Po batteries on how to extend the battery lifespan about the following:


    * 8 hours first time charging: needed?
    * effects of full vs. partial charging and which is a better routine?
    * effects of full/deep discharge and is it needed to be done always?
    * when to do discharging and charging?
    * effects of using the gadget while charging?
    * effects of overcharging?
     

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