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Paralleling BMS protected Li-ion packs.

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  • Diggs Ut
    commented on 's reply
    hoggdoc when paralleling two packs of the same both the Ah and the current carrying capabilities double.

  • Diggs Ut
    replied
    Well, with the price of 56/70 cell packs being what it is, they are not exactly disposable that I can just throw a pack away and build or buy a 126 cell pack to replace it so adding to what I already own makes the most sense. I am a mechanical engineer (in another life) and me and my electrical engineering friends see no issues with a parallel setup as I am running as long as 100% charging is not being used and the BMSs are not balancing. (Sure, they'd like to see a fuse or two thrown in also, but....) (I see calfee20 earlier in this thread is doing the same with parallel uneven packs.) Pack voltage needs to be the same but uneven Ah packs can be used with no issues also. Again - I'm just saying this is what I've done to add to the knowledge in this thread. Everyone needs to make their own decisions......

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  • AZguy
    commented on 's reply
    I agree it's much better if packs are identical and new, hopefully with cells from the same lot. But it's still a compromise from an engineering perspective and better to go with a properly sized/constructed battery

    Golf carts and the like are typically lead-acid and the individual batteries going into the "composite" battery are 1P - entirely different animals than what we're using here... they can/do explode (from hydrogen release) but that potential is generally much smaller and the electrolytes while highly corrosive aren't flammable

  • AZguy
    commented on 's reply
    I don't think I'm confused at all

    If the only thing someone is looking for is range than there's far less compelling reasons to parallel instead of a switchover or at least some or'ing electronics. Granted if everything is perfect the batteries can share the load and only have to run at ~half output which should give a bit more life but if you aren't stressing them in the first place (they are properly sized) it's not likely to make a heck of a lot of difference and IMO still much better off to start off with a single larger battery

    If the goal is to increase output capability then it's not just a little better to go with a properly sized battery in the first place

  • hoggdoc
    commented on 's reply
    I think you are confusing the increase aH capacity running batteries in parallel with being able to provide more current (amps). To increase the current capacity the individual batteries have to be designed to output more current.

  • 73Eldo
    replied
    I think if you had a pretty well matched set and both had the 2 wire BMSs (not a separate charge port) you would be alright without anything in between them because the BMSs should be able to at least do their jobs. Having them not very well or at all matched just seems like its hard on everything and I don't think the BMS in the Wolf can do much if you are possibly charging it though the discharge port. Big boom probably not but is it really the best use of resources? If they are pretty well matched and balanced then ya you should be splitting the load but not matched I don't think you will have an even split.

    I'm no expert on these sort of packs. Most of my series parallel experience comes from things like golf carts and diesel equipment that often uses more than one battery. In all that stuff if there is one that isn't a good match either because of age or that its just different the rest seem to suffer too. If you do decide to toss used stuff together your best results come when you load test all your options and put the closest matches together. You also should periodically load test them just to make sure they are all staying close.

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  • AZguy
    replied
    I'm of the notion that making a Li-ion battery from two batteries (paralleling them essentially is making one big battery from two) is a bad idea (at least not without proper electronics in between them), especially with dissimilar batteries and especially if it's to get higher current handling. There are very few competent engineers worth their salt that would say it's a good idea and if being a competent engineer that's worth their salt means over thinking things than so be it - engineers "over think" things as a matter of course to ensure it's been thunk enough Click image for larger version  Name:	i-VQKdnjW.gif Views:	0 Size:	316 Bytes ID:	129575

    I'm of the notion that if you want a larger battery for more current handling then ideally get/build a single larger battery - it's the ideal engineering approach and entirely practical with electric bikes... if there's a compelling reason to parallel batteries then provide the proper well-engineered electronics to manage that - also the proper engineering approach


    Can you get away using things that aren't well engineered or "under thought"? Of course - doesn't mean it's "optimal" or even safe... I probably wouldn't cross a high bridge with a heavy load that was designed by a guy that puts Li-ion batteries in parallel... just sayin'

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  • Diggs Ut
    replied
    My post is to try and keep these like examples in one place on this forum as I originally had quite a few questions about running parallel that I couldn't find answers to. I run a BBHSD from Luna with a 52v Wolf battery. It's a country cruiser with a 52T Leckie ring up front and about 2,500 miles on the conversion. I usually charge to 80%. But, even at 100% I out-rode the Wolf battery a few times. Just when I'm most tired, it cuts out. So I built a second battery from used medical cells 52v but 5x14 instead of 4x14. (I've built packs before.) It has a 40 amp BMS wired in and it's output port is it's charge port. It is plugged directly into the second Wolf pack XT90 port. It splits the load with the Wolf pack and charges with the Wolf pack when the Wolf is charging through it's charge port. It's strapped into a trunk bag on the rear rack and I figure the two give me about 27 ah. I've never charged them together to 100%. I did that separately and then rode them individually and recharged to 80% before reconnecting. (I've only done it once as it's not quite been a year with the dual battery set up.) I'm not saying everyone should do this that wants to run a parallel setup. But I also think (as someone stated before) that many are over-thinking the issue bit. I don't like the idea of running individual packs and switching back and forth. Parallel packs reduce the load to 50% on each pack and I rarely discharge the pair below 40% (and don't charge past 80%) even on long rides. Should make for a long happy battery life not to mention the performance gains of little to no voltage sags when pulling hard on the packs.
    Last edited by Diggs Ut; 06-06-2021, 06:28 AM.

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  • 73Eldo
    commented on 's reply
    That switch could be made to work but it looks fairly large and not even slightly water resistant. It looks like its really intended to be a reversing switch for a 3 phase AC industrial motor.which ironically is what most e bike motors are but we are controlling them quite differently.

  • AZguy
    commented on 's reply
    Since they're The "S" versions they manage the arcs too - I think that might be hard on a switch

  • Growbur
    commented on 's reply
    No resistance loss there. Very simple switch. Simple is good.

  • AZguy
    replied
    I just use connectors to switch between two batteries - inexpensive, reliable, quick, easy

    Click image for larger version  Name:	sw_2639.jpg Views:	0 Size:	227.7 KB ID:	128607

    Click image for larger version  Name:	sw_2641.jpg Views:	0 Size:	257.5 KB ID:	128608

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  • Growbur
    replied
    I had a motorcycle that had a reserve fuel valve and had a pickup truck with two fuel tanks that had a tank switch. Rather than add electronics to stabilize the discharge from multiple batteries it seems to me that a DPDT and maybe a center off switch in the pigtail between the batteries and the motor controller isolates each battery so dissimilar charges or even dissimilar batteries could be added. There would be minimal resistance loss, minor cost and something most people could build on their bench. The downside would be that you would need to switch manually.
    Would something like this work? https://www.amazon.com/Baomain-Unive...72668131&psc=1

    Leave a comment:


  • AZguy
    replied
    It all looks scary to me but then again anytime any batteries are paralleled I'm uncomfortable and the only way I'd even consider it is if the batteries were identical and purchased at the same time

    There are ways to run multiple dissimilar batteries but I wouldn't do it with out very smart management electronics

    Best is to obtain a single suitable battery

    I guess I'm not the right guy to comment

    Leave a comment:


  • paxtana
    commented on 's reply
    IDK about the overall design but I would make sure that's a fast blow fuse
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