Possible Group Buy - Check Interest - Lithium Titanate cells

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

  1. Sunder

    Sunder Member

    Joined:
    Apr 26, 2012
    Messages:
    3,765
    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

    Joined:
    Feb 13, 2003
    Messages:
    1,719
    Location:
    Sydney
    i was thinking that- the weight & low cycle count is very suggestive isn't it
     
  3. OP
    OP
    rainwulf

    rainwulf Member

    Joined:
    Jan 20, 2002
    Messages:
    4,227
    Location:
    bris.qld.aus
    Yea pass. Giveaway is the cycles. 1500? thats lipo territory.

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

    Sorenad Member

    Joined:
    Dec 16, 2018
    Messages:
    23
    Location:
    Moreton Bay area
    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.......

    [​IMG]
    [​IMG]
    [​IMG]
     
    Last edited: Feb 16, 2019
  5. aXis

    aXis Member

    Joined:
    Jun 27, 2001
    Messages:
    5,447
    Location:
    Kalgoorlie, WA
    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!


    Click to view full size!


    Click to view full size!


    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!
     
    Sorenad and shredder like this.
  6. OP
    OP
    rainwulf

    rainwulf Member

    Joined:
    Jan 20, 2002
    Messages:
    4,227
    Location:
    bris.qld.aus
    Nice work!
     
  7. aXis

    aXis Member

    Joined:
    Jun 27, 2001
    Messages:
    5,447
    Location:
    Kalgoorlie, WA
    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

    Joined:
    Jun 27, 2001
    Messages:
    5,447
    Location:
    Kalgoorlie, WA
    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

    Joined:
    Jun 27, 2001
    Messages:
    5,447
    Location:
    Kalgoorlie, WA
    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.
     
    shredder likes this.
  10. aXis

    aXis Member

    Joined:
    Jun 27, 2001
    Messages:
    5,447
    Location:
    Kalgoorlie, WA
    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.
     
  11. OP
    OP
    rainwulf

    rainwulf Member

    Joined:
    Jan 20, 2002
    Messages:
    4,227
    Location:
    bris.qld.aus
    Yea they seem to be extremely resilient, their amazing charge/discharge characteristics mean they don't drift. These things truly are amazing devices. Not only that, but it seems that they internally self discharge at high voltages causing a bit of heat, thus actually doing their own kind of internal balancing. Self balancing lipos... used to be only lead acid had that quality. If that really is the case, you could actually gently "GENTLY" equalise charge them to 2.8 volts a cell!

    I don't have my pack anymore as its actually inplace and working on a site, so Axis, time for some experimentation!
    Also the charge efficiency.. its insane. That's supercap territory.
     
    Last edited: Mar 22, 2019
  12. Mathuisella

    Mathuisella Member

    Joined:
    Dec 3, 2002
    Messages:
    7,149
    Location:
    in your gearbox...grindin
    Wow, i want some of these haha.
     
  13. Mathuisella

    Mathuisella Member

    Joined:
    Dec 3, 2002
    Messages:
    7,149
    Location:
    in your gearbox...grindin
    So, the pack you have there how many AH is it ?
     
  14. aXis

    aXis Member

    Joined:
    Jun 27, 2001
    Messages:
    5,447
    Location:
    Kalgoorlie, WA
    It's nominally 40Ah @ 27.6V, around 1.1kWh total. Until I get the balancer finished I'd be quite happy cycling it between 20% and 80%, so approx 660Wh available.

    In other news I started routing and assembling the balancer a couple of days ago and am progressively testing each each MOSFET leg, going OK so far and the on-state resistance is lower than my meter can measure.
     
  15. aXis

    aXis Member

    Joined:
    Jun 27, 2001
    Messages:
    5,447
    Location:
    Kalgoorlie, WA
    Show and tell time again.


    Click to view full size!


    Click to view full size!


    That's 4 modules of the battery balancer built, I still need to build another 8 module (2 boards worth), the main arduino controller board, a supercapacitor bank and then all of the fly wiring.

    Each module is effectively an opto-isolated DPST solid state relay that will connect each cell to an offboard flying 10F supercapacitor. The MOSFET's have about 5mOhm resistance so the complete loop is about 20mOhm, plus another 20mOhm ESR from the capacitor. Since I'm only balancing voltages in the 10's or maybe 100's of millivolts that should only be a few amps of inrush current and heat wont be an issue.

    It's about this point I'm wishing I had solder paste and a reflow oven .... I probably made my pads a bit small and it was pretty tricky hand soldering them last night!
     
    Last edited: Apr 5, 2019
  16. Sorenad

    Sorenad Member

    Joined:
    Dec 16, 2018
    Messages:
    23
    Location:
    Moreton Bay area
    I finally got around to starting to build a pack with the 60Ah Altairnano cells.

    I'm using a iCharger X6 30A charger as a built in charger/balancer/BMS.
    One good thing about the X6 is that it has an LTO program that charges and balances each cell to 2.85V with 2A balancing. It's also able to support and save custom charge profiles.
    Another bonus is that it also supports regenerative discharging at 30A, so could potentially use that if ever I need to top up the start battery......or discharge to another battery if required.

    The box is built from 8mm polycarbonate bonded with methylene chloride and stainless M4 x 20mm screws

    I just have to add a 50A anderson plug for charge and a 175A anderson for discharge, put a top on and create a substantial bracket for the pack so it can be safely mounted in the back of the car and easily removed if needed. The whole pack weighs in at around 21kg so far.
    Although the cells are tightly held in the box, I'll add a 8mm polycabonate rib across the middle above the PCB to prevent things bouncing around inside

    The wiring and crimps pictured are only temporary to try out the charger. It ended up charging and balancing all cell 'pairs' to within 7mV of each other with the upper 'pair' charged to 2.850V. The PSU (DPS5020) I was using to test the charger unfortunately tops out at 20A.

    IMG_20190506_162726.jpg IMG_20190506_162740.jpg
     
    Last edited: May 9, 2019
  17. Mathuisella

    Mathuisella Member

    Joined:
    Dec 3, 2002
    Messages:
    7,149
    Location:
    in your gearbox...grindin
    Super jeleous :p
     
  18. Sunder

    Sunder Member

    Joined:
    Apr 26, 2012
    Messages:
    3,765
    Not too different from how I'm building mine :) I'm using fold and crimp technique to bond though, and a ABS bracket to hold the polycarbonate sheets together.

    I heard they do expand somewhat when being charged/discharged, but hopefully not enough to crack the polycarbonate in your case, or the ABS in mine.
     
  19. Sorenad

    Sorenad Member

    Joined:
    Dec 16, 2018
    Messages:
    23
    Location:
    Moreton Bay area
    The hardest part is just getting the batteries. I'm sure that once you have them you'll do a better job than me :)
     
  20. Sorenad

    Sorenad Member

    Joined:
    Dec 16, 2018
    Messages:
    23
    Location:
    Moreton Bay area
    The next time I build I'll definitely try the fold and crimp method.
    It's a bit daunting trimming down the tabs and punching holes or notches, as a few mistakes can potentially mean a ruined battery.
    Getting the tabs, tab holes, plates, PCB holes and soldered nuts to align properly is a huge pain in the arse without jigs or templates. Especially going 'freehand' with the hole punch.

    I had my cells sitting at around 2.4V when I put the front panel on the box. I didn't notice any flex or change to the polycarbonate when fully charged or discharged, however did notice that the cells appear to look a little more 'wrinkly' when discharged despite still being a tight fit in the box. I'll have to grab my verniers and dial gauge and measure for any dimensional difference at different states of charge.
     
    Last edited: May 10, 2019

Share This Page

Advertisement: