LiFePO4 charger

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Markus Gritsch shared his LiFePO4 charger project in the forum:

Since I really like using LiFePO4 AA and AAA batteries in some of my projects, I finally gave in and built a dedicated charger for them.
Previously I used a lab power supply to mimic the constant current/constant voltage charging curve, which worked also fine. But after seeing Patrick Van Oosterwijck nifty LiFePO4wered/USB™, I thought it would be a bit more convenient to charge these batteries using USB.

Via the project log forum.

 

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12 Comments

  1. Pix on project page looks like a Soshine LiFePO4 AA/14500, pretty useful cells, one cell in a low-end Nikon camera lasted ~350 shots without flash. I use a Powerlion dual cell charger though. They are pretty good batteries to use in projects, but unless you properly measure Vbatt, the voltage/power fall-off near depletion can be pretty quick and disconcerting.

    1. “voltage/power fall-off near depletion can be pretty quick ” -but in many applications, a flat I/V discharge curve is more of a “feature” than a “bug”.

      1. Oh yes, I realized how weird that sounded after I hit the button…
        Reason being I was mentally comparing them to alkalines only. Since I skipped Li-ion packs completely in favour of LiFePO4 cells for my portable projects, I forget that the fall-off is normal for lithium chemistry. Heh…

      2. Implementing a fuel gauge function on one’s project running LiFePO4 is also tricky, a zero-pin method as in measuring internal Vref then Vdd won’t be much accurate. So following the “lazy programmer” philosophy, I just recharge periodically…

  2. In my MSP430 based bedside table alarm clocks I have a state where the display shows the battery voltage. Due to the flat discharge voltage curve, the displayed voltage stays at 3.3 to 3.2 Volts for several month. And once the voltage really starts to drop, I know it’s time to recharge soon, but due to the low power consumption of these devices, there is no need to hurry and I still can wait a few days/weeks :)

  3. What’s the big deal with this LiFeSO4 chemistry anyway? So I get a single AA battery that runs at 3.2V full open-circuit and has a fairly flat discharge I/V curve. That’s fine. But the cell is only a crummy 700mAh and the recharge life is a 1000 cycles. Plus to use this in your typical 2XAA consumer device I have to buy a AA dummy cell, and those are a rip-off cost-wise.

    The MSP430 clock Markus Gritsch mentions with one of these batteries is a good application for a single AA LiFePO4 cell. But that’s a pretty narrow range of applications.

    So the only real advantage is the single cell and 3.2V right? If that’s it, I’ll pass for most applications. If not, please explain.

    Thanks, David

    P.S., For other readers, the articles linked by this post (and the post itself) do not seem to come out and say what charge controller is being used. So to save you a bunch of digging, it looks to me like it is a Microchip MCP73123.

    1. Battery choice is just a choice, all types of batteries have their pros and cons. To each his own.

      It’s a safer chemistry, for example it does not blow up when a nail is driven through it, do read some safety tests, they are interesting. So for lower amperage stuff, I definitely prefer this over more standard 4.2V Lithium cells or Li-ion packs. Of course the average cell is unlikely to blow up, but I for one would still rather engineer my projects conservatively…

      These LiFePO4 AAs can’t really do much beyond 1C discharge (you’d need A123’s nanoparticle tech for that) but that’s a Good Thing to me, more internal impedance and it will just get hot instead of blowing up. A good match for low-ish power projects.

      Here’s one data point on costs: I have more than made back the cost of the AA cells and the dummy cells just using them for my cheap Nikon camera alone. The pair of Fujitsu alkalines that came with my camera did 499 shots. A single LiFePO4 AA cell (plus a dummy) did 421 shots on one charge. Obviously if I didn’t pick a camera that uses AAs, costs would be different. But I know their strengths and limitations and so I am happy with LiFePO4 AAs.

    2. Actually, online reports of testing of LiFePO4 AA cells give capacities mostly in the 550mAh range. So, the manufacturer’s claim should be read as “Up to 700mAh”. *cough*
      LiFePO4 is not terribly popular as a rechargeable cell, but there is some demand, the manufacturers have not been abandoning the chemistry.
      It is always wise to do some research and study before choosing something like LiFePO4. Get your use cases in order, then buy, not the other way around.
      After years of using crappy NiCd and NiMH cells, I am plenty happy with LiFePO4…

      1. > “Up to 700mAh”

        Yes, the fine print on my Soshine 700 mAh AA cell reads: min. 550 mAh :) LiFePO4 energy density is not as high as for LiPo cells, but that’s ok for me.

        > AAs can’t really do much beyond 1C discharge

        The small printing on this specific battery here says: “Max Continuous discharge 1.2 A”.

      2. I think there are charts at candlepowerforums where you can see the voltage sag with different discharge loads, it’s much better than parsing manufacturers’ info. AFAIR for these AAs their performance wasn’t particularly good at high loads. 2C or 1.2A won’t be running at 3.2-3.3V. You can get something, but the sag is a performance killer…
        LiFePO4 isn’t very conductive (hence the high impedance of the AA cells), but the elements are non-toxic and plentiful. AFAIK, the best cells are by A123, their tech is unique vs the rest, and these can really give you 10-20A.

    3. Well, it’s all there in the attached schematic, so it’s not really a secret :) The complete part number is MCP73123-22SI/MF, and the corresponding code printed on the IC is 77HI (as can be seen on the picture).

  4. I can tell you that the far east vendors we worked with on a product using LiFePO4 cells had several variants, the high current type could do 2.6 V at 100 A from a 1S2P 18650 pack. Resting voltage was 3.33 V. They were not A123. Market is/was mostly ebike and scooter, I think. Less power density and less explodey than other Li ion.

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