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Paralleling BMS protected Li-ion packs.

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  • OneBadWolf
    replied
    Originally posted by Growbur View Post
    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-Universal-Changeover-SZW26-63-Position/dp/B01IZ5ZFYC/ref=asc_df_B01IZ5ZFYC/?tag=hyprod-20&linkCode=df0&hvadid=198107824285&hvpos=&hvnetw = g&hvrand=4361311871238822143&hvpone=&hvptwo=&hvq mt =&hvdev=t&hvdvcmdl=&hvlocint=&hvlocphy=9029719&h vt argid=pla-349572668131&psc=1



    I use an PVC electrical box for my electric control. On my older controllers, they required an external switch, or breaker to energize the system. I have two guarded toggle switches, one for each batt, a keyswitch, a waterproof toggle for my 12V subsystem, and a rotary contactor, that I salvaged from an external car booster box. I run 2 identical 52V 14S4P batterys, Panasonic GA cells, and 25 Amp, 4o Amp peak BMS's, for a combined output, of 50 Amps continuous, 80 peak. Combined capacity of both batts, is 26 Ah.

    The start sequence, is both toggles on, keyswitch on, subsystem on, contactor on. Shutdown is the reverse. This, eliminates any concern about the toggles burning out, as there is no voltage going through them, when they are switched on, or off. Also, it allows for isolation of a battery, in the event one or the other develops a fault. I have 2 52V chargers onboard, in one of the 2 DIY hard, and locking panniers, with a long auto retractable cord mechanism, salvaged from a vacuum cleaner that supplys power to both chargers, and can charge each battery off each charger, or, I can use only one charger, and switch the master toggles both to on, and charge both batterys, at half the rate. I use that, if I have time to charge. My batterys, are just now, after over 7 years of hard service, starting to fail. By using the two masters, once powered up, I can fail one, or the other, and verify that the voltages are equal. After 7 years, they are a max of 0.6 V apart.

    I no longer require the contactor, as my controller, a Nucular Micro Nuke, has a solid state switch, but I have left it in place as a kill switch, as it will instantly cut all power if switched off. My controller, also has a very precise charger built in to it, I can feed approx 58V DC into it, and it then will put an optimal charge into each, much more capably than the chargers that came with the battery. I have not babied my batts, I have the controller set right to the threshold of where the BMS will cutoff. That is at about 4.4Kw. I credit the longevity, to each batt working half as hard, as what they would alone, most of the time. With field weakening, my top speed is 64 Kph, and I have sufficient torque, to pull my friends half ton. I also am a huge believer in ferrofluid, the motor temp sensors, are about half of what they were before the ferrofluid. I used an industrial fluid, that cost 10% of what Ebike shops charge, and after four years, the motor is just fine.

    The bike was built from a Giant Stance, full suspension, and the rear shock, was ok up to about 30KPH. I weigh well over 250 Lbs, so I upgraded the fork, and changed the geometry of the rear suspension, by making some different linkages, and bushings, and installed a coil motorcycle shock, with preload, and variable dampening. It's great, except, the minimal space for a batt inside the triangle, is less. I have a quarter inch clearance between the batt case, and the shock at full compression. I cannot remove the batt from it's base, as there is no space to slide out, so I permanently mounted it, using large hose clamps, covered with heat shrink.

    Anticipating the replacement of the batterys, a year ago,I bought 3 new looking 52V 20 Ah batts, 14S6P (84 18650s each) in aluminum cases at an online auction. I believe they are the same make as the ones Lime uses on their Ebikes. There is no way that I can fit 2 of them, they are way too big. My plan is to tear them down, and use the cells to rebuild my current batts. I have enough, to build 4 batterys, the same as the ones on the bike now, and have some left over. So, I'm on the hunt for a batt tab spot welder, that will handle .2 pure Nickle strips, and does not cost an arm and a leg, if anybody has any ideas. Anyway, my point is, I can't imagine what it would be like with a single battery, at least one that would fit on a bicycle frame. Two batts in parallel, not only provides more performance, and range, and longevity, but also provides redundancy, allowing one to limp it home, if a BMS dies, or a pack goes bad, or wiring is damaged in a crash. I ride all winter, in Canada, and made some electric heated gloves, from bundled strands of carbon fibre sewed into the glove liner, and the excess capacity powers them nicely. The wind chill at -20C @ 50 Kph, is about -35C. And if you are riding into a 30Kph head wind, it's really cold. Colder than my X's heart. Nothing that isn't heated will keep hands warm in those temps, the wind comes through the seams, and the aluminum handle bars do a great job of conducting the cold into hands. I wouldn't ride in the Winter without a second batt. On the bike trails here, I'll sometimes go 15K each way, and not see a single person, I try to charge at my destination, but it's bad for the batterys to charge in cold temps, and I can't always find a place to take it inside, there are a bunch of things that can not easily be predicted, that could put you in a situation, where the extra capacity, can make a huge difference. The only down side, with the way I have mine mounted, is that the batt on the top tube, results in a higher centre of gravity, than I'd like.


    Last edited by OneBadWolf; 05-12-2024, 06:31 AM. Reason: Pic added

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  • stts
    commented on 's reply
    waste of time
    Last edited by stts; 03-24-2023, 03:25 PM.

  • AZguy
    commented on 's reply
    Yeah I gave up on Ali - they wanted my passport and seemed like everything short of an anal probe

    I do a lot of small circuits but have all the tools and usually can quickly identify the limitations of what should be attempted at the small scale - I don't mind spending some time/money on these "hobby" electronics but I have limited time for it

    Whatever you choose to do good luck.. if looking for advice I've got plenty of background with this type of stuff... do it just about every day ;-}

  • stts
    commented on 's reply
    waste of time
    Last edited by stts; 03-24-2023, 03:24 PM.

  • AZguy
    commented on 's reply
    Not sure what you mean by the battery dying but P-ch MOSFET's in the above are disconnecting the bat+ and leaving the bat-'s connected together as a common ground which everything can use as a negative reference

    When N-ch's are used typically they are disconnecting the bat- so no longer a common negative reference... not a deal killer but it complicates things to have to use a common positive reference (need to sense both batteries), especially at electric bike voltages, and the above can't handle that

    It is possible with charge pumps to use N-ch's in the high side but that's an entirely different drive than what you all have in that circuit (again start from scratch) and will complicate things, especially at electric bike voltages


    But hey I'm just an old guy that's been around the block for a long time and know there's plenty of new tricks this old dog can learn

  • stts
    replied
    [I got banned for being a pro-fascist jackass]
    Last edited by paxtana; 06-02-2023, 11:11 AM.

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  • stts
    commented on 's reply
    waste of time
    Last edited by stts; 03-24-2023, 03:28 PM.

  • AZguy
    commented on 's reply
    There are very good reasons you *never* (and I mean never) see PMOS in electric bike current paths

    Breaking the negative side is not as trivial as it may seem since now you lose your reference for voltage sensing with anything remotely resembling that circuit above

    I guess I'm just saying that I'm of the notion the circuit above brings next to nothing for electric bike applications... that trying to make it work will take more time than just starting from scratch...

    But go ahead and prove me wrong! I'd truly enjoy it

  • stts
    commented on 's reply
    waste of time
    Last edited by stts; 03-24-2023, 03:27 PM.

  • AZguy
    commented on 's reply
    I think the greatest issue is the P-channel MOSFET's doing the heavy lifting which isn't practical for bike battery voltages and currents

    Once you replace them with N-channel the whole rest of the circuit goes out the window for the most part since now the negative side is what's getting broken which is entirely non-trivial

    Oddly enough I've got a station with battery back up in my shop right now (just finishing this failure analysis project today ironically) that was toasting batteries and the problem turned out to the the LVCO disconnecting the negative (which is what N-channel's will do) and another connection to battery negative causing the failure of the LVCO to disconnect the battery and over-discharging the battery...


    Regardless, these are reasonably simple circuits at the fundamental level... it's just the details that make them challenging...

  • stts
    commented on 's reply
    waste of time
    Last edited by stts; 03-24-2023, 03:27 PM.

  • AZguy
    commented on 's reply
    I don't think it is terribly useful for bikes

    The back to back MOSFET's are P-channels and would need to be changed to N-channels (at much higher Vds) to get reasonable Rds(on) and everything on the left would need to get swapped around (inverted) to deal with that

    The logic is far too low of voltage and looks made to detect only whether not a battery is present and above 4.38V... it would be possible to regulate a lower voltage for the NAND gates and use dividers on the threshold detectors I suppose but there is still the issue that now the logic needs to drive the MOSFET's relative to bat+ and that will throw the voltage detect all out of kilter

    I suppose it could be done but personally, I'd just start from scratch and create something optimized for bike batteries


    One of the really big challenges with doing something like this for bike batteries is going to be getting all the heat out of the back-to-back MOSFET's

  • stts
    commented on 's reply
    waste of time
    Last edited by stts; 03-24-2023, 03:27 PM.

  • AZguy
    commented on 's reply
    FWIW that circuit doesn't appear to be designed for electric bike batteries

    It's for low voltages (even 12V batteries would likely be too much) and would get very hot with just a few amps

  • stts
    replied
    waste of time
    Last edited by stts; 03-24-2023, 03:24 PM.

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