Possible Group Buy - Check Interest - Lithium Titanate cells

Discussion in 'Hobby Engineering' started by rainwulf, Jul 15, 2018.

  1. Sunder

    Sunder Member

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    3S @ 12.6V = 4.2V per cell.

    Thats standard LiPoly, not LTO.

    3S @ 9V = 3.0v per cell. Standard cutoff for LiPoly.

    And at that weight, I'd have a hard time believing that to be a genuine 100Ah.

    12.6v @ 100Ah = 1260wh.

    According to Wikipedia, LiPo cells are 100-265wh/kg.

    Cells alone on top quality stuff would be 4.75kg. The control circuitry and case would bring it well over 5kg.

    Too many lies for this to be a trustworthy seller. You might get a 50Ah LiPo pack, you might get a box full of bricks and 3 x 18650s from an old laptop. Or you might get a fireball. My bet's on the fireball.
     
  2. bryn

    bryn Member

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    i was thinking that- the weight & low cycle count is very suggestive isn't it
     
  3. OP
    OP
    rainwulf

    rainwulf Member

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    Yea pass. Giveaway is the cycles. 1500? thats lipo territory.

    LTO is at LEAST 10 times better then that.
     
  4. Sorenad

    Sorenad New Member

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    I finally received 10x Altairnano 60Ah cells.

    Remarkably, Aus Post still delivered the boxes despite the corners being blown out and clearly labelled as containing lithium batteries.
    All they (or someone else) did was put some blue strapping around the boxes to stop them completely collapsing.

    Luckily each cell appears to be physically ok and a quick check with a multimeter revealed 9x to be 2.28 +/- 0.005V and one measuring 2.19V.
    Although I already knew how much these things weighed (~2kg each), it's still a little surprising when you actually have them in your hands.

    I'll try charging them up over the next few weeks and conduct a discharge test with a load bank (used to test generators and big LiFePO4 batteries) at work.

    The pack of 10 measures ~300H x 260W x 140D, so should end up making a reasonably compact aux battery for my car with decent capacity......weighing 20 something kg.
    TBD whether I'll need to get an extra 2 batteries for this application.

    Should be rippers for solar though.......

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    Last edited: Feb 16, 2019
  5. aXis

    aXis Member

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    OK, so I'm finally making progress on assembling my LTO pack.

    I've made battery pack holders out of MDF, they look a lot like a wine rack. I couldn't get a hole saw quite the right size so have had to line the holes with foam strips, not quite as neat as I'd like but they seem to work well and are probably gentler on the batteries.

    Click to view full size!


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    I've also made up enough busbars from 25x3mm aluminium bar, I've designed them around being under 1mOhm resitance. I also purchased some wave spring washers to help hold it all together, they were about $1 each in Australia, cheaper from china but slower shipping.

    Click to view full size!
     
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  6. OP
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    rainwulf

    rainwulf Member

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    Nice work!
     
  7. aXis

    aXis Member

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    OK, basic pack assembled now but no balancer yet. It's a bit scary thinking about it's short circuit capabilities, I removed my wristwatch and used an insulated spanner when putting it together. Really needs some terminal covers ASAP, I'll 3D print some.


    Click to view full size!


    Click to view full size!


    Pack is showing 26.5V, approx 2.21v per cell. They are currently all close to balance and I'm considering running them on a gentle charge/discharge cycle (eg 20% to 80%) without individual cell protection. The balancer circuit will be a fair bit of work still.
     
  8. aXis

    aXis Member

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    LTO battery pack is in and running naked (no BMS), though I have used conservative endpoints for charge and discharge - cycling between 20% to 80% capacity. Forgot to take photos sorry.

    Yesterday my solar UPS captured 820Wh and delivered 780Wh. That's 95% round trip efficiency including the charge controller and a partial battery cycle, pretty stoked!

    About half of the incoming solar power was absorbed by the LTO pack for time-shift release later in the evening, reaching around 60% capacity before the sun went down and it started discharging again. There seems to be very little change in pack voltage switching from charge to discharge which really helps the round trip efficiency. I know with lead acid this is not the case and you need much higher charge voltages to drive the chemical reactions well.

    Only concern is my solar panel is not performing that well, I thought it was 200W but seems to be maxing out at 120W.

    edit: got the delivered Wh wrong, the efficiency was even higher :)
     
    Last edited: Mar 12, 2019
  9. aXis

    aXis Member

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    Did a quick test on the LTO pack after a couple of cycles and the worst case imbalance has crept out slightly to 40mV - 2.17V low versus 2.21V high. Nothing too concerning and I'll check it within another week. There did seem to be some thermal effects, batteries at the floor level were higher voltage than the ones at the top of the pack.

    I also did some math on the pack impedance at low amps - currently showing 25mOhms total which is consistent with 1mOhm rated impedance per cell x 12 cells, plus 14mOhm from 1.5 metres of copper cabling.
     
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  10. aXis

    aXis Member

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    Boring status update.

    I've cycled these for another week and then taken them out for testing at a pretty similar charge level, the imbalance has improved to 30mV - 2.15V low vs 2.18V high. I've also had several days with as high as 97% efficiency for the whole system.

    I would still like to build the balancing/monitoring system and it will probably draw about 10Wh per day, or 1% efficiency overhead on the current solar output.
     

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