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  1.  
    I just watched a basic you tube vid on making a DIY tesla type power pack to dump excess PV generated electricity into to use outside generation period.
    It uses 2nd hand 18650 3.7V lipo cells found in old laptop batteries and power tools etc.
    I've about 60 odd from knacker Makita 18V batteries I've been collecting, most are individual good as the problem is usually only with the first cell in the 10 pack.
    Anyone got any experience of building up pack with these cells and or useful links
    I think 50 odd would give me 500W for an hours if my very basic calcs are correct
    • CommentAuthorringi
    • CommentTimeJan 3rd 2016
     
    I hope you have good fire insurance....
  2.  
    I'll stick it outside :wink:
    found some good info on ebiker.com re pack building
    • CommentAuthortorrent99
    • CommentTimeJan 4th 2016
     
    I recently built an e-bike battery out of 40 ***TABBED*** brand new 16850 cells.
    That took about 7 evenings and lots of nervous soldering....

    Doing it with unknown, possibly untabbed cells is an order of magnitude more difficult.
    Think very carefully before attempting it.

    There's a lot of info on building packs of this sort on the Endless Sphere electric bike forum (google it). There's also LOTS of warnings!!!

    In short, not for the faint hearted....
  3.  
    found an nice solderless pack design that's worth more thought
    https://endless-sphere.com/forums/viewtopic.php?f=14&t=57810

    Ive never been the faint hearted type :)
  4.  
    torrent what was the final spec of your 40 cells pack ?
    • CommentAuthorEd Davies
    • CommentTimeJan 4th 2016
     
    If you have a heap of these cells and can separate out the good ones then giving them another life seems like a good scheme. I particularly like the idea of recycling old (crashed or partially dead) electric car battery packs. Seems to be a lot of it about - in Norway.

    However, I still don't see the point of using this small form factor (18650) if you're starting with new cells, whether as a one-off DIY pack or if you're Nissan or Tesla. Particularly for stationary applications where weight and volume isn't quite so critical, bigger LiFePOâ‚„ cells seem a lot more convenient (less fiddly connection) and safer (the chemistry is intrinsically more stable). It's difficult to compare like with like but the reduced amount of packaging for bigger cells must help, I think.

    With the huge numbers of 18650s made for laptops, etc, it's easy to understand why Tesla started off with them - being able to play manufacturers off against each other must be nice. But it'll be interesting to see if the change when they've got their own production facilities running. E.g., double the length and diameter while sticking with the same chemistry.
  5.  
    I was under the impression the 18650 lipo had good weight to power ratio and was slightly safer design
    though this info is just via youtubers not from and real study of the area
    • CommentAuthorEd Davies
    • CommentTimeJan 4th 2016
     
    I can't see how fewer bigger cells with the same chemistry wouldn't have a better power to weight ratio. But you might well be right - it's plausible they'd be more vulnerable in an accident.

    Whatever, the trade-offs on chemistry and form factor for portable/mobile vs stationary applications seem different to me.

    James, what are your plans for managing the balance of the pack?
    • CommentAuthortorrent99
    • CommentTimeJan 4th 2016
     
    Posted By: jamesingramtorrent what was the final spec of your 40 cells pack ?


    It was a 10S4P pack so a 36V 10ah pack basically.

    RE: Reusing the laptop cells. These are probably a good match for the purposes you are looking for. These cells tend to be optimised for high capacity LOW drain, in comparison to the HIGH drain medium capacity cells needed for e-vehicles.

    The guys on endless sphere have a wealth of experience in processing/testing old laptop battery packs to identify the good cells, match the cells up etc. They also have designs for BMS systems. (BMS systems for e-vehicle voltages are relatively cheap on ebay e.g. 24v/36V/48V) A long look around that site will pay dividends.

    What sort of voltage/capacity are you looking to build?

    RE: Safety, the 18650 is believed to be fairly safe (the good cells have a number of safety mechanisms built in), though I wouldn't want to overcharge or puncture them!!! (check out the many LiPO fire vids on youtube)

    RE: Chemistry. Yes LiFePO4 is probably the best chemistry for this purpose (very high number of charge cycles, however low drain). However, I've not seen it in 18650, and anyway if your source is reclaimed tool/laptop packs then you are probably limited to the LiCO and LiMN chemistries. (Although I believe DeWalt use high spec A123 LiFePO3s, which have been suggested as an alternative to e-bike batteries!)
    • CommentAuthorSprocket
    • CommentTimeJan 4th 2016 edited
     
    Torrent is right... not for the faint hearted, especially not with used cells, and not without tabs. You can weld your own tabs with a simple-ish homebrew setup but if you are buying new cells anyway it's not worth the trouble.

    I've soldered quite a few packs of up to sixteen tabbed cells, including adding the balancing circuitry for 2S, 3S, 4S (available cheaply on ebay) into those packs. Using new cells like this is fine for ebike and laptop batteries but not worth the trouble of using used cells IMHO. A single cell failure (made much more likely with used cells) is pretty inevitable and will likely be loads of trouble to detect and fix even if it does not take the rest of the pack with it. Large packs also suffer from local heating at poor cells that reduces the capacity of the cells around it.

    I know that the original Tesla Roadster used 7000 of these things but their pack design balancing and cut-out circuitry was a bit fancier and they still suffered a lot of problems with reduced capacity and short life; hence all the interest in battery upgrades for them.

    If you buy new 18650s then I would stay away from Ultrafire - they are already recycled and repackaged laptop cells and their performance is very variable.
    But if I wanted 500Wh or more I would be looking at new prismatic cells instead of 18650.

    There are some interesting LiFePO4 cells and batteries starting to appear (including motorcycle and marine battery formats - see SHorai LFX) but they are still very pricey from what I have seen and I think they still don't match the capacity per volume/weight of older Lion cells.
    • CommentAuthorjamesingram
    • CommentTimeJan 5th 2016 edited
     
    thanks for the info . I'm uneducated in this area, just like the idea of playing with stuff.
    Ed I was thinking of balancing charge prior to building up pack with something like
    http://www.skyrc.com/index.php?route=product/product&product_id=185
    cheaper options out there
    pack spec would be 12 0r 24V I guess to suit a basic inverter. I could dump in via iboost+ using the second circuit to charger . Not sure how I'd get power out. I can think of a few dodgy options.
    Perhaps I should start with a ebike first :)
    The example I've seen on youtube the guy was surprised by the quality of the apparently dead laptop batteries after conditioning.
    I know with the 18V Makita packs it's only the first battery that goes the rest are fine.
    Be good to come up with a modular non soldered design that would make replacing duds easier.
    • CommentAuthortorrent99
    • CommentTimeJan 5th 2016
     
    It's not a dead idea, but requires a lot of thought. Only worth it if you can get the tool pack/laptop cells for practically free and have a lot of time to devote to it!

    Some ideas off the top of my head:

    1) You need some mechanism to grade the cells. The guys on endless sphere have numerous threads on cell testing involving artificial loads, magnets etc.

    2) You should make it as modular as possible. Rather than one big battery I think you should use an architecture that lets you add extra batteries as time goes on. They need to be connected to each other via diodes (to prevent a charged battery trying to dump 100's of Amps into a flatter one!), Schottky diodes at a minimum, better to use an "ideal diode" circuit. (Tiberius on endless sphere or pedelecs forum has a good design).

    3) Use something like that no-solder assembly to allow you to build up and tear down the packs easily.

    4) With old and mixed cells like this you really are going to need BMS circuits to actively keep the packs in balance. You probably won't get away with just charging and then assembling and expecting them to stay in balance.
    • CommentAuthorEd Davies
    • CommentTimeJan 5th 2016
     
    I'm not convinced of the need for active balancing. From what I've read it's positively harmful in that it can leave the leading cells on trickle charge which isn't what they like at all.

    It's also more power electronics operating. A BMS has caused a least one (I don't doubt more) car fires.

    Much better, I think, is to balance as well as possible to start with then let the cells do their own thing. Lithium is pretty tolerant so long as you don't over charge or over discharge individual cells so it's important to monitor each cell (or each parallel row) to make sure its voltage stays in range. Where things can go horribly wrong is if you take the overall battery voltage, divide by the number of rows, and assume all cells are at the voltage. The low-voltage disconnect on the inverter (whether it's manual or automatic) has to look at cell voltages, not just the overall battery voltage.

    Imbalance probably will accumulate over time as the leading row limits charge and the trailing row limits discharge so that'll reduce the available capacity. Discharging the leading row(s) a bit or charging the trailing row(s) a bit will then increase the available capacity. With an appropriately isolated power supply this can be done with the battery operational.
    • CommentAuthortorrent99
    • CommentTimeJan 5th 2016
     
    Well if not active balancing, at least some row by row voltage monitoring and low voltage cutoff (as described above) is essential, as well as some means of isolating a row to balance charge it off line. Basically with the kind of cells suggested i.e. lots of different types/ages/capacities etc there's more chance of things getting out of whack, so building it with monitoring and maintenance in mind is a good idea.

    James if you haven't found it yet here's the gold mine for info on recycled battery pack building:

    https://endless-sphere.com/forums/viewtopic.php?t=26383
    • CommentAuthorEd Davies
    • CommentTimeJan 5th 2016
     
    Irrelevant nonsense but I'd suggest not trying this trick with 18650s:

    http://www.kjmagnetics.com/blog.asp?p=AA-motor

    Slightly lower internal resistance than the typical alkaline cell, I suspect.
    • CommentAuthortorrent99
    • CommentTimeJan 7th 2016
     
    Well it'd be fun to watch if you tried (outside with a bucket of sand to hand!)
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