Electric Porsche 944

18650 Li-ion range extender

by on Mar.22, 2014, under Electric Car

For daily use my pack  provides more than enough range for what I need and want the car to do.  However there are situations where it would be nice if the car had 2 or 3 times the current range.  The Tesla Roadster and Model S have the highest range in the EV industry for production cars and they use massive packs of 18650 cells.  They benefit from the economy of scale  of the laptop and power tool industry as these are the most common cells used.  In the coming years this may reverse and the laptop and power tool industries may benefit from high quality cells developed for the EV industry.

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Jehu Garcia is currently working towards an 18650 based pack for his VW Samba Bus using recycled laptop batteries.  I already have a few hundred 18650 cells on hand so I thought it just might be a worthwhile project to build a removable high capacity battery.  Although I have a few hundred cells, what I need to make this work is a few thousand cells.  If any of you guys have a stash of “dead” or unused laptop batteries and just want to get rid of them please feel free to send them my way to speed up this project.

The IT department at my day job was happy to give me the dead laptop packs since there wasn’t an established recycling process in place yet, I’m willing to bet the same exists in many companies large and small.

To put all of these cells together there are some inexpensive plastic holders available from Fasttech.com.

18650 Battery Holder

 

The initial plan is to use small  fuse wires just like Jehu, and I may use a copper clad board as the current collector, it provides stability and is easy to solder to.  The currents won’t be that high and with a massive width of copper sheet on the PCB it  actually has a fairly high current carrying capacity.

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The main purpose of this pack is not to drive the car on it’s own, but to supplement the existing pack in the car for longer range so the maximum current output doesn’t have to be as high.  I would like to build a pack with between 60 and 100Ah at ~86 cells which is a  voltage that would match quite closely the nominal and charged voltage of the LiFePO4 main pack in the car.  Assuming roughly 2Ah per cell on average from used packs I will need 30-50 cells per module and 86 of those in series, totalling 2580 to 4300 cells for the desired capacity.  This  pack would weight around 275 to 450lbs which although significant is worthwhile for 2 – 2.7x the range.


2 Comments for this entry

  • Alex

    Hi there, I am also looking to build one of these 18650 packs using old laptop batteries. Aiming for about 80-100 cells per pack. Have also been in touch with Jehu who has been very helpful and has lots of knowledge.

    I am trying to put lots of thought into the assembly pack and avoid the use of solder altogether. For fusing of each cell there is a guy on endless sphere who is just looking at manufacturing pre-cut fuse plates…very handy.

    I am interested in what you are saying about using a copper coated plate to assemble the cells on. I am thinking of a system where the cells could be spaced apart using the cell holders but modified so they can slide down to the middle of the cells rather than the ends. then envisaging pressing the cells and fuse plates together between two pressure plates that would effectively be screwed together to create the pressure contact (so no solder needed). That way, if s cell dies or shorts, I can simply uncscrew the pressure plates, take the failed cell out, reposition new fuse plate and pressure plate and screw back together….quick and easy and no soldering.

    That’s the theory and thinking process anyway, but there is no doubt this is new territory for me…and I am looking to join thoughts with everyone working on similar projects.

    What are your thoughts on the above? Would you have any suggestions/comments?

    Would be great to hear back from you.

    thanks.

    Alex

  • admin

    Hi Alex,

    It’s not a bad idea, be careful with tension or use very rigid plates so that you have good contact between all cells. If you are using a copper coated plate you could easily incorporate the fusing (mill away some copper to leave fuse width traces to carry the current). I wouldn’t use the off the shelf holders (not what Jehu and I are using anyway), I would have some plastic sheet/plate milled with the desired pattern with a tight fit to the cells and slide the cells in so the sheet is centered.

    Do some testing, create a cell or two and test it under load, make sure the cells don’t heat up and the connections are reliable.

    Good luck!
    Robin

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