Safely Creating A Li-Ion Pack From Phone Cells

[Glen], at Maker Space Newcastle Upon Tyne, is refreshingly honest. As he puts it, he’s too cheap to buy a proper battery.

He needed a 1AH battery pack to power his quadcopter controller and FPV headset, and since inadequate discharge warnings had led him to damage lithium polymer cells with these devices, he wanted his pack to use lithium-ion cells. His requirements were that the cells be as cheap, lightweight, and small as possible, so to satisfy them he turned to a stack of mobile phone cells. Nokia BL-4U cells could be had for under a pound ($1.46) including delivery, so they certainly satisfied his requirement for cheapness.

It might seem a simple procedure, to put together a battery pack, and in terms of physical wiring it certainly is. But lithium-ion cells are not simply connected together in the way dry cells are, to avoid a significant fire risk they need to have the voltage of each individual cell monitored with a special balanced charger. Thus each cell junction needs to be brought out to another connector to the charger.

[Glen]’s write-up takes the reader through all the requirements of safe lithium-ion pack construction and charging, and is a useful read for any lithium-ion newbies. If nothing else it serves as a useful reminder that mobile phone cells can be surprisingly cheap.

Lithium cells have captured our attention before here at Hackaday. Our recent Hackaday Dictionary piece provides a comprehensive primer, we’ve featured another multi-cell build, and an interesting app note from Maxim for a battery manager chip.

47 thoughts on “Safely Creating A Li-Ion Pack From Phone Cells

  1. 1) Love this. I got given a load of older Blackberry batteries and did much the same. Great for powering misc projects.

    2) Soldering directly to the battery’s terminals isn’t really a hazard, b/c you’re soldering to the output/voltage regulation circuit rather than to the battery. See attached photo. (I only took 1 apart. The rest I use in the plastic case to prevent shorts.)

      1. I suppose a good public-shaming will help me to be a better person in the future…

        Tack-soldered onto the spring contacts. Yeah. (In my defense, it’s served me well for eight years now. Or is that just more embarrasing?)

        But note the clever use of surface-mount straight pins! :) The extra bit of vertical just clears the lip of the plastic casing. And they were in my parts pile at the time…

        1. I see nothing wrong here, however you should solder the wires internally and bring them out of the case, leaving the contacts alone. This way you can still use the charger for any packs that aren’t modified.

        2. Done the same for an old phone with broken and unrepairable charge connector a friend brought to me to recover her files. Battery was dead, but still could be recharged, so I soldered some wires to the exposed battery tabs and successfully charged it with an old charger not that different from the one you used. I chose that charger, among others good for single lithium cells, because it was a bit old and designed for lower current cells so there were less risks of overcharging the battery. Most, probably all, cellphone batteries are protected against over discharge and short circuit, but I’m not sure about overcharging, so I wanted to be cautious.

    1. A big problem with this is that many of the protection circuits only handle about 8V of reverse voltage, because they are not designed for series-connected packs. So when one cell in a full pack decides to engage its protection, the two other cells will promptly apply 8.4V in reverse over it, possibly burning it out.

      Mostly a non-issue for 2S packs, somewhat of an issue for 3S packs. For 4S and more, you can consider the per-cell protection circuit pretty useless.

    2. I have a stack of old laptop batteries so I made a universal connector / adapter so they can be connected to the power management port of laptop I currently use. I then downloaded some software that will query the batteries onboard controller and tell me what condition they’re in.

      It turns out that most of them are well over 85% and quite (re)usable. Now I have to find a way to safely manage them for charge / discharge.

      Any ideas? for a management circuit. I have about 30 cells. I want to use some for a battery backed clock.

  2. Seems kinda odd when you can find proper packs for almost that cheap anyway. Also, is it really that Lithium Ion cells can be run dead, or is it just the circuitry that comes on phone batteries that prevents this?

    1. There SHOULD be a protection PCB in the pack, which will prevent discharge below a certain cell voltage. It does this by inserting a FET between the cells and the pack output, and a control IC which opens the FET when the cell voltage is too low, or when charge or discharge current is over a preset value.

      This protection circuitry may be in the phone itself, but for maximum safety, and to comply with the requirements for shipment by air, it should be part of the pack. I’d suggest sacrificing a dead pack to know for certain, because LiIon cells can be killed if discharged below their minimum voltage, which is *around* 2.75V

        1. The phone will be able to discharge all the way until the MOSFET cuts off. From that point, the battery drain will be quiescent current of the battery management circuits, self discharge and leakages.

          I always remove the batteries for long term storage (except for things that can’t be removed.)

      1. The protection circuit is a last resort. The cut-off voltage is way below what is healthy for the cell. It is just to prevent the cell from ballooning if the phone is left off for a year or so.

    2. Lithium ion and LI-Po are actually very similar chemistries and neither should be run flat. He must have encountered some Li-Po cells that lacked the protection circuitry.

  3. Just make sure they’re genuine packs. Some of the cheap Chinese replacement packs are made with inferior cells…the kind that tend to self-incinerate. Packs assembled from top quality cells won’t do that.

      1. You have no guarantee that the parts comes from the same supplier or that the capacity varies between packs/batches.

        In applications like this where you want matching capacities is going to be much harder. You can do capacity checking with a good battery analyzer.

        1. Capacity checking can be important, especially when mixing cells from different batches (or with different usage histories). As they age, lithium cells get higher “rock content” (material that will no longer hold a charge). And older cell behaves as a smaller capacity cell. Even lithium batteries bought as spares (and stored new in box) age as badly as their twins that are put into service. An old lithium battery functions the same as a new small capacity battery. Some battery chargers have the ability to test the remaining useful capacity of unknown lithium batteries, using charge/discharge cycles. Some older lithium battery technoligies only had a few hundred charge cycles, so I would be wary of using such test — perhaps better to keep charge and usage logs (and some devices actually do that for you, if you have a history of dmesg logs or equivalent, to browse for such information).

          1. “Even lithium batteries bought as spares (and stored new in box) age as badly as their twins that are put into service.”

            That is a gross exaggeration, at least it is for LiIon batteries made in the last decade or so.

            I’ve had a number of pulls from 4-5 year old new-old-stock packs that tested at the original nominal capacity (which is probably about 4% below the actual capacity of the cells when they were brand new).

      2. Sorry, poor phrasing. There are good Chinese cell manufacturers and bad ones. Stick to name brand, higher quality cells and you should be fine. You get what you pay for and with LiIon cells, especially, it’s worth buying top quality.

        1. there are NO good chinese cell makers when it comes to lipo. sorry if that paints with too broad a brush, but there is no ‘teeth’ to doing harm when you sell across from china to the US. the chinese can make bombs and sell to us and they are shielded from lawsuits.

          this means that they could care less what happens after they sell us these smal grenades that we think are batteries.

          read enough amazon reviews of lipo copters and you’ll see that house fires and such are a non-zero occurrence. and go ahead and try to sue amazon for selling them; they are protected and then try to sue the china seller or the china maker. you won’t be able to; and this motivates them to take the cheapest way in making the batteries. even if they blow up, they have your money.

          the notion of ‘good cells’ is a fallacy. everyone makes them ‘to a price’ and the press simply ignores the inherent danger is buying goods that are POWER BASED and yet made to a pennies-level price point.

          1. First of, “lipo” doesn’t really mean shit. Assuming you are referring to pouch cells though, there are in fact reputable Chinese manufacturers. Sourcing cells from them on their own, well, that may be difficult, but consumer electronics makers do it all the time.

            You seem to believe that just because you can create an argument you convince yourself with (or repeat one you found persuasive), that you have described reality. You haven’t.

          2. Sorry, but all Lipo / Lion cells are made in China. They’ve got almost all the world’s resources of raw materials for them, and their export laws prevent exporting raw materials, they must be turned into products.
            So the perfectly safe batteries in the phones we all carry were also made in China.

      3. > Would you be surprised if I told you that some of the “top quality” stuff you are talking about came from China as well?

        He probably meant coming from Chinese fabs with no quality control. Every mass produced electronic product is made in China nowadays, quality control is what makes top notch technology different from rubbish.

  4. Time ago I tried to find cheap source of LiIon cells, so I bought like dozen of different battery replacements for mobile phones of various types and from various sellers from ebay.
    All of them were crap, with lower than declared capacity. Some had 50% of capacity, some had only 25%. Genuine batteries from reputed local source (albeit more expensive) had around 100% of declared capacity, often more. So, be careful with cheap battery suppliers.
    YMMV, though.

    1. Similar experience, but in my case I bought a bunch of AA “high capacity” NiMH batteries. A large amount of them were feather-weight (obviously fakes). I cut one upen — it had a tiny button cell inside the shell — just enough to charge and pass a quick battery test, but not enough capacity for any useful purpose (much like the fake flash devices that report dozens of GB but only contain 2GB of slow unreliable flash memory, so the work for a quick test only).

      A lot of cheap stuff is over-rated, literally. You often get what you pay for, but from disreputable sources you get ripped off, and now and again you get real bargains (depending on your needs).

      LiPo battery packs are the same — R/C groups discuss which brands are great value and which brands to avoid (with both safety and value taken into account).

      1. I bought a bunch of 2500 mAh Tenergy AA cells from their ebay store years ago. The best ones tested at 1700 mAh.
        With that kind of luck direct from the manufacturer, I’d hate to think what the knockoffs would be like!

      2. Still got any of the fakes? Do another teardown, blog about it with decent pics, and submit it to the Hackaday tips line. I think we’d all be interested to see.

        1. You may want to take a look at the photos here.

          Plenty of examples around, just search for “fake lithium cells” etc.
          Faking batteries and flash memories, along with electronic components, mainly chips and mosfets is very common among shady far east sellers. The point is: if something can be counterfeited just by relabeling it, then it will be done in massive scale.

  5. He’s wrong when he states that you can’t kill LiIon cells by deep discharge. He’s also wrong in stating they’re not a fire risk.
    Reference this datasheet for a typical LiIon 18650 cell which gives a minimum discharge voltage of 3.0V and warns of fire or explosion if mistreated:

    LiPolymer cells are more vulnerable to damage because of their construction, but LiIon (cylindrical) cells are just as dangerous if mistreated.

    1. Liion will not explode/fire if you just discharge it to 0.

      It will do only if you start charging it, after discharging under 2V (at this point some chemical reactions creates internal short circuit). But it’s not 100% risk, even for cheapo batteries.

      1. I just recycled 8 Panasonic CGR18650H 1500mAh Li-ion cells from a very old laptop battery, using a cheap USB power bank circuit & holder from Aliexpress. They were all around1.75V when I started, and now each battery charges my Nexus 7 tablet with more than enough juice to be useful, e.g. a quarter of the tablet’s battery. No fires, no drama.

        The charger BMS knows what to do with cells on the brink like these, and can save them.

        1. Genuine 18650 have a pressure ninja-fuse and it’s quite common that this kind of overdischarged batteries in one moment reports 4,2V and after 20 minutes it reports 0,0V, guess what would happen with “chinese replacements”.

          The more time/discharge, the worser parameters are. Usually they aro not worth of salvaging.

          1. The Nexus 7 (2013 version) has a battery capacity of almost 4000mAh. These batteries seem to transfer 1000mAh or more to it, after losses. I have tested multiple cycles on only one of these batteries. Two which I have left fully charged on the shelf both dropped from 4.20V to 4.14V over a few days. I’m measuring with the battery test function of my multimeter.

            The chip in the power bank is called FM6316FE.

      1. My understanding is that actual “li-poly” was pretty much a dead end. It referred to a polymer electrolyte. Had it worked out, it would have worked for pouch cells, prismatic, and cylindrical cells. But it didn’t…

        Now, it doesn’t mean anything. People use it to refer to pouch cells, but maybe not all of them.

    2. Thanks for the heads up re li-ion batteries being just as prone to damage when drained. I guess my kit must be shutting down before the pack gets drained too far

      However nowhere do I say that these packs are not a fire risk

  6. Not sure I like the title of this article. I don’t feel that construction of your own Lithium-based battery pack from old packs is ever truly safe.
    This is one of the few things I won’t mess with. I have seen (caused, actually) a lithium battery fire before. I don’t want to chance it happening again.
    That said, thanks for the information especially the dictionary link.

  7. This article contains some false claims and unsafe practice. First, already mentioned crap about Li-ion’s ability to be completely discharged without damaging them. That is not true, they are same as LiPo in that aspect, overdischarge them, and you damaged them. Second, crap about multiple pins on laptop battery because of balance charging. No, pins are SMBUS communication pins. Laptop doesn’t have access to individual cells and laptop doesn’t do balancing. PCB inside battery does that.
    Making battery pack without first testing individual cells for their capacity and internal resistance is not very smart move. If you have batteries with different capacities (happens with eBay stuff) you’ll discharge one cell before other and that can result in cell damage if it doesn’t have protection or decreased capacity of complete pack if it has protection. General rule is to avoid connecting any cells in pack if they’re not matched.

    1. Or connect them in parallel but always charge the pack like there was just one cell connected, namely the weakest one. That would be a safety measure in case, say, you have 5 protected cells in parallel and for some reason the protection switches off 3 of them so the remaining 2 would be charged with too much current.

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