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Optimal 13S2P Battery Pack Wiring & BMS Setup/Configuration?

  1. Mar 27, 2017 #1
    Like the title says, I'm Building a 13S2P battery pack. I'm trying to figure out how to wire it most safely and efficiently.

    It's been a long time since I've build a custom pack of this size & as I'm using $220 worth of 60A 3.7V 26650's, I'd like it to not explode, short, or go bad anytime soon.

    Due to placement size constraints, I've had to do sort of a weird pack design. My design and current progress is as follows.

    The 2 +'s and -'s are also cells and are the series + and -'s. They are on opposite sides of the pack from each other.

    S1C3 S1C2 (+1S1C1) (+2S2C1) S2C2
    S1C4 S2C6 S2C5 S2C4 S2C3
    S1C5 S2C7 S2C8 S2C9 S2C10 S2C11 S2C12 (-2S2C13)
    S1C6 S1C7 S1C8 S1C9 S1C10 S1C11 S1C12 (-1S1C13)

    I currently have both series, 1 & 2, wired up just as shown & have three nickle strips soldered on each | > < connection, but I don't have them wired parallel yet, and I'd also like to further strengthen the series connections.

    So, my questions are.

    Where should I add the parallel connections? Just connect every + to every + and every - to every - from each series? Series 1 cell one to series 2 cell 1, series 1 cell 2 to series 2 cell 2 etc? Where can I add extra connections to increase handling of current and amps?

    When I connect the 2 13s BMS's, I just connect BMS - to cell one series - that the first + is attached to and BMS +'s to each positive terminal for cells in the series, right?

    If I choose to install some 70amp fuses where would I install those to best protect the system but not pop random fuses & strand myself?

    I'm planning to add some solar cells to the system also. Where would I connect those so that power can be drawn from the solar cells first instead of the battery, but also charge the batteries when not using the bike or more than the solar cells generate, while not damaging the solar cells?

    Are they any other recommendations or additions you can think of?

    For those wondering, this will be hooked up to a 48V 40A motor speed controller & I also have a 48V 60A speed controller.
    Last edited: Mar 27, 2017
  2. jcsd
  3. Mar 28, 2017 #2


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    Staff: Mentor

    I only skimmed your post so far, but can you say more about what you have read about paralleled batteries? I know it's done in some well-matched situations, but I'm still reluctant to do it.
  4. Mar 28, 2017 #3
    I am interested, but confused. I'm pretty sure that "13S2P" refers to 13 series cells, with two strings of 13 series cells in parallel? So 26 cells in total? I'm not really following your configuration - could you provide it in standard schematic format?


    Sounds like this battery pack can provide current for an ~ 2 HP motor?

    I concur with berkeman, I'm reluctant to parallel lithium cells w/o a lot of reading first. And perhaps some matching of the strings, and monitoring of that match as they age? Can you use a half-sized controller on each string? Co-incidentally, I've been toying with a similar series/parallel arrangement of 1W LEDS on my lab bench, and rapidly concluding the cure is worse than the disease. LEDs as a sink have a lot in common with Li-ion as a source.

    With regards to the solar panel charging versus powering the bike, I think you need to better characterize your system. I can't imagine carrying a solar panel on a 2 HP bike that would provide significant propulsion power. A search shows that ~ 40 pound panel, 40" x 65" would provide ~ 8 amps @ ~ 30 Volts in full sun and direct orientation. I guess that could help , since I assume the 40 amps is more for acceleration so 8 amps might keep you going, but you would need to keep the panel oriented to the sun, not easy while mobile. Plus the extra 40# and air drag of a big panel (a side wind might knock you down, and break your panel - or you!).
  5. Mar 29, 2017 #4
    Paralleled batteries are batteries that will add Ah instead of volts as series batteries do.

    For instance:

    If you wire in series 3.7 + to 3.7 - to 3.7 + 8Ah cells, you get 11.1V 8Ah.

    If you wire cells parallel 3.7 + to 3.7 + to 3.7 + to - to - to - 8Ah cells, you get 3.7V 24Ah.

    My setup is 13 in series + to - to + to - 48V 8Ah. 2 of the 48V 13 series in parallel + to + to - to - 16Ah.

    You're following correctly. 26 total cells, 13 per current cell in series. 2 13 series + to - batteries I'm going to wire parallel + to +, - to -. Each 48V 8Ah separate pack should remain the same, but now the total pack will add up to 1x 48V 16Ah pack instead of 2x 48V 8Ah packs. The motor will be around 2HP-3HP total.

    Its important to make sure all cells are the same voltage when wiring them together because they will try to equalize across the series & you can cause shorts & overheating if you don't.

    I'm trying to stay in the low to middle end of the batteries/pack's amp rating 40A-60A to ensure the batteries last a year or more.

    As for the solar panels I'll be adding to the bike, I'm not buying consumer grade panels. Those are much too heavy for my need, plus they are stupid expensive. It's cheaper to make them & you can customize the design for your specific needs then.

    I'm making mine out of 3.2W .5V solar cells I've purchased. Each cell weighs about .5g. So I'm looking at around 15g in panels, 20g in tabs & solder & probably 5-10 pounds in the Aluminum sheet & Plexiglas or similar enclosure/mount I'll make.

    I'll have enough room for around 30 over the basket & behind the seat in the enclosure/mount I'll make at some point in the next few months. The bike will be battery alone at first, I'm just planning ahead.

    Since the panels will only be able to make around 15-20V max I'll have to have it wired up such that whatever V & A the cells don't produce it can take from the battery while running.

    It will charge cells in groups of 4 when parked. I'll have it set to jump to a different group every 30 seconds or minute or something. The balance system should keep the cells balanced between this & using the bike.

    I can also just simplify & have it hooked up to not charge cells but still draw power from solar panel when driving. But charging batteries is just a cool extra feature I'd like to have on it at some point, because why not.

    Hopefully you're following & I'm answering questions so you can understand them.

    Let me know if you all need pictures, further explanation or more information.
    Last edited: Mar 29, 2017
  6. Mar 30, 2017 #5
    Here's a https://www.instagram.com/p/BSRe6bzBbLd/ [Broken] of my current pack build/wiring. I'm out of silver solder & flux right now, so waiting for more to arrive to finish it all up, add balancing systems & power/charger XT90 plug. It's only wired 13S1P X 2 right now, all reading correct voltages in both separate series.

    Another question, when I wire it in parallel obviously I connect each series respective cells to each others +'s and -'s. Is it important to have all of the nickel strips in both series and parallel connections same length to ensure one set of nickel strips isn't receiving too much volt/amp draw, no preferred pathways in other-words, or is that mostly just on series connections?
    Last edited by a moderator: May 8, 2017
  7. Mar 30, 2017 #6
    Cool. Where do you source your parts? I've never seen a plastic holder like that (though I never looked either!).
  8. Mar 30, 2017 #7
    I either fabricate them or research & find them. Most things you think of have already been thought of.

    A few years back when I was building these I'd built a 3D printer and was making much more of my own plastic/metal parts. But I don't have one currently so I'm having to source elsewhere.

    These are called https://www.amazon.com/gp/product/B01CKUUVHY/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1. The ones I'm using are 26650 cells, a more normal cell (inside most laptops, etc) is 18650's. They can be found all over Amazon, eBay and the web otherwise. Same for nickel strips, solder, batteries, solder gun, etc.
    Last edited by a moderator: May 8, 2017
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