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Paralleling BMS protected Li-ion packs.

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  • paxtana
    replied
    Charging the battery resets the bms. Meaning that if you are charging the battery while using it, if anything happens on the discharge side or internally like a short, it may not be able to provide short circuit protection.

    I can not say for sure as it may vary depending on the battery, but in the event of a short it may attempt to shut down, then immediately wake up from the charging, and short out again and just keep repeating this dangerous cycle.

    The only battery I would feel safe doing this on would be a Luna Wolf because you would be stripping it of some protections. You can fuse on discharge but you can't fuse at the cell level unless the battery was designed that way, so with any other battery you are losing some protections.

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  • jGecko
    replied
    Edit: The underlying fundamental question is...can the charging port of an ebike battery pack be used while the battery pack is being used to ride about?

    What do you think of this idea? I make a small current limiting circuit and put this current limiter on the output of 36V pack#1 (say...limit to 2 to 3 Ampa), and then use freshly charged Pack#1 to recharge 36V Pack#2 through it's normal coaxial 2Amp charging port WHILE it is in use on the ebike. This is not putting the packs in parallel...this is using one pack to charge the other while it is in use.
    The problem is that I am just beginning to study ebike BMS/charging/operation circuits and don't know if they are designed to prevent charging while a battery pack is in use.
    I also don't know what (if anything) REGEN designs disable while charging the battery pack with the bike in motion. So would the viability of what I'm suggesting change depending on whether or not the bike uses regen?

    Or, I suppose I could even use a higher voltage battery for Pack#1 and just use a current AND voltage limiter set to 2A/42V.


    Edit to add: Explaining the whole reason for thinking about this, is that the existing battery pack uses an integrated output connector (no harness) that slots into a mating connector integrated directly into the bicycle frame. This is an attempt to avoid having to cut open the original battery pack to substantially modify it in a way to allow paralleling the outputs of the packs.
    Last edited by jGecko; 09-06-2019, 05:25 AM.

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  • Gadgets4grls
    replied
    This fits into this thread so thought I'd post it.


    I just finish swapping a failed bionx system to a Bafang BBSHD system. I was previously running two Bionx batteries with one charging from solar while the other was powering the trike, then swap, Bionx's BMS wouldn't allow charging and high current discharge at the same time so that was the solution.
    Since I had to replace the system when the motor failed I decided to try something different. I went with a Bafang BBSHD and three Luna Cycle Wolf Panasonic GA 13.5 batteries that I'm now running/charging in parallel with a Cycle Satiator.
    Attached are a couple of pictures of the wire harness that I made. I forgot to get a picture of it completed with heat shrink before I installed it on the trike. :-(
    Couple of the construction details for those interested

    10ga wire along all of the discharger lines. Charge lines are only 14ga but I'm not going to normally run more than 7amps on the charge side
    Hi-temp ring terminals, crimped with a hydraulic wire crimper and then soldered before marine grade heat shinking.
    Brass screws and flex head lock nuts on the connections.

    Schottky 80V 200amp diodes, each leg is rated for 100amps so I can run the motor off of any single leg without worrying about overloading the diode. Never plan on doing this but it's an option if needed. On the plus side I'm only seeing .1 V drop across the diodes.
    I have the fourth leg pig tailed out for future expansion to either a fourth battery or feeding from the solar charging system if the charging while riding doesn't work out with the Wolf's BMSs

    Just thought I'd post this up for all of those running parallel batteries

    Gadgets

    mod edit: See this link for the full thread on how this was done
    Last edited by Gadgets4grls; 04-03-2019, 11:43 AM.

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  • Sebz
    commented on 's reply
    You either get 72V 20ah and 20amps discharge in series
    Or you get 36V 40ah and 40amps discharge in parallel

    Taken you take the right precautions or course.

  • calfee20
    commented on 's reply
    "''''''''No......

  • paxtana
    commented on 's reply
    As far as the discharge rate of the combined packs when in parallel, yes it would double the discharge rate and the capacity.

    It would NOT double the voltage. To to that you would need to put it in series, which we would not recommend.

  • Ebikeidiot14
    replied
    If you have two 36v 20ah batteries with 20amp bms's. Do the amps add together? Like do you get 72v 20ah 40 amp or 72v 20ah 20 and?

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  • paxtana
    commented on 's reply
    If the packs are not identical then it is not a good idea.

  • Lazeebiker
    replied
    Hi, I've aquired a 9ah 36v 10INR19/66-4 333WH li-ion battery pack and would like to know if I can run it in parallel with my 10 AH 36V 6068135/5000MaH 2P10S 360wh Li-ion silver fish battery pack which is 2 years old?
    The 9ah pack has very little, if any use but has been left dormant for a year or so, though it seems to have charged up ok.
    I would be charging them seperately.

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  • New Mariner
    replied
    Sebz how about a video on how to make a triple parallel connector for the charging ports (xt60s) on mini batteries

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  • kenandmal
    replied
    Thanks very much, I won't be doing it that way then.

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  • paxtana
    replied
    kenandmal I understand you would not have them both switched on at the same time.

    Most switches on ebike batteries are BMS based and do not isolate the battery from being charged through output, in other words just because it won't allow power to be pulled from the battery does not mean it blocks being charged through the discharge wiring. Most BMS are specifically designed this way as forced charging through discharge is part of common BMS reset procedure used by many BMS boards. So the only thing keeping power from flowing backwards into the "off" battery would be if the "on" battery has a lower voltage.

    So if you use the integrated on/off switch on the batteries there is a good chance while depleting the first battery it would indeed be isolated, once you turn it off and turn on the second pack that it would result in essentially unregulated charging of the first pack since it then has a lower voltage than the second battery. This would be bypassing all BMS protections and potentially flowing in at a rate much higher than the charge rate recommended by the cell manufacturer.

    The imbalance would not be the sort of small imbalance we were covering on the initial post on this thread which describes something more or less self-limiting. A small imbalance is somewhat safe. What you are describing is NOT safe, there is a possibility your battery could catch fire.

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  • kenandmal
    replied
    Hi paxtana I would not have them both switched on at the same time. I suppose I really want to know is does the battery on off switch isolate the battery

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  • paxtana
    commented on 's reply
    If you have it wired in parallel anything you do to cause a large voltage imbalance between those 2 batteries is potentially unsafe. It would not be an issue of the battery that is turned on draining into the battery that is turned off, it would be an issue of when you turn on the battery that was previously turned off it will be much higher voltage and it will flow into the lower voltage battery all at once potentially damaging it.

    It sounds like what you want is some sort of switch that isolates each battery and allows you to switch between them as a power source for the bike without actually being in parallel. Not really the same thing.

  • kenandmal
    replied
    Can you help, I want to add another 36v battery and would wire it in parallel, I do not want them working together as that defeats the object ( for me anyway ) I would like to know if I only switch one battery on would it still discharge to the other.

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