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Paralleling BMS protected Li-ion packs.

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  • paxtana
    commented on 's reply
    The comments section of this knowledge base article is not really the place to get in-depth with info on how to build a battery. I would recommend to start a new thread asking those questions. Generally you do want a bms and there's even a parallel adapter you can use to connect up the battery as well as pics showing how this works right in this same thread

  • philipgaines
    replied
    Thank you, paxtana. Will the second battery require its own BMS? If so, how do I choose one, price being no object? And here’s a super-naive question: Again with all the appropriate caveats, is this basically just a matter of connecting the black and red wires leading to the first battery to the same wires of the new battery? I’ve watched Micah’s videos, and I’m confident I can build the 3P13S myself; what are the really important things I have to keep in mind?

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  • paxtana
    commented on 's reply
    Due to liability we can't say yes, there are obviously some risks. But even some of our own guys, myself included, do put batteries in parallel, sometimes even batteries that do not have the same chemistry and so on. It is important to understand the risks though.

    Don't charge them when they are connected in parallel, that's a lot more risky

  • philipgaines
    replied
    Sebz: I've read everything you've written about paralleling batteries, and it's pretty overwhelming for someone who's really handy but new to this. Let me ask a bottom-line question: If all the planets are aligned in terms of battery chemistry, voltage, amp hours, BMS priorities, connection details, etc., would I be able to 1) add a second battery to my current 48v 8.7ah battery and run the whole thing as one battery which will discharge equally between the two units and 2) charge both units at the same time? (I would be making a 3P x 13S out of 2.9ah 18650s.)
    Last edited by philipgaines; 06-11-2020, 11:09 AM.

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  • tklop
    commented on 's reply
    Thank you, AZguy for the expanded explanation! That helps :-)

  • AZguy
    commented on 's reply
    If connected in parallel and charged in parallel their inernal voltage will remain the same but the lesser battery will have more internal resistance during discharge so during discharge less will come from it and their voltage will in effect remain the same... to a point... If the smaller capacity battery has a lower internal resistance due to being made up of lower internal resistance cells or being new then after being discharged for some time it's very likely current will flow from the larger capacity battery to the lower capacity one after the load is removed. If the load stays constant (which never happens in e-bikes) then things should work out until the load is removed and at that point there would be the case of one battery discharging into the other

    I understand folks run different batteries in parallel and anecdotally don't run into trouble

    Me? No way. I might consider two equal *new* batteries with cells preferably from the same lot but only with a *very* compelling need to do so... Otherwise much, much better to have a battery built to the size needed... way lower down would be to have two equal batteries with a battery management unit taking care of pulling power from them to achieve higher currents which would be the only use case I can see since getting more range it would be much better to just switch between batteries... at the bottom is just connecting two unequal batteries in parallel and expecting it to work well...

    YMMV
    Last edited by AZguy; 02-11-2020, 07:40 AM.

  • tklop
    commented on 's reply
    Thanks, AZguy :-)

    I'm not real certain about all the various complexities--as I said... "my ideas might be utter garbage" --and I stand by that assertion!

    But I'm also not so sure that the voltage can't sag on one battery in this scenario...

    What you said about resistance makes sense though... Still, I guess I just don't understand.

    I'm thinking that the two batteries' cells themselves are not actually in paralell; only their BMS's are. It's not just "a bigger battery" that Sebz has created--is it? Seems to me, that it's more like two separate and complete power-systems, which are attempting to work in cooperation... And that seems to be what it is, that makes things complicated... Or at least that seems to be the concern...

    Sebz said he observed current-flow between the two--which (if I'm not totally oblivious to Ohm's Law) implies that indeed imbalances in voltage can in fact exist... Were measured... Again--maybe that's my ignorance speaking--but seems to me that otherwise, Sebz could not have observed current-flow between his batteries in his test-setup...

    When connecting individual battery-cells in parallel with each other to build up series-groups, the moment you've connected their tabs, the voltage-readout for all the cells in that particular series-group will instantaneously be equal. Their tabs are bonded--so of course the meter cannot possibly read anything else. But internally, between the individual cells in that group which you've just bonded together--there are still differences, and there is indeed current-flow--it's just not happening where your meter can read it. It takes a little while for those cells themselves to finish equalizing. In the mean-time, if you take them back apart again, their "different" voltages will again be readable on a meter. The cells in each series-group of course do eventually equalize, and remain that way (though they'd drift apart again if the series-groups were separated back out into individual cells again)--but from what I understand, that's also a function of the fact that they're all of equal capacity, chemistry, etc. It seems matching battery-components are happy battery components...

    Now: With two different batteries--once their two BMS's outputs are bonded, their voltage-readouts too will match--instantaneously. But also--just like when bonding cells in a series-group, there would also be some current flow--no matter how closely matched--when you first connected the two batteries (assuming a non-discharge state). I expect too, because each BMS might have its own balancing program, there might be still more back-and-forth with current-flow during charging, or even when static--however minimal that might be.

    But it seems to me, that once you began to discharge--because the two different batteries would have different internal-resistances, they'd be expected to discharge at different rates--and then what were subtle differences might become amplified. At any rate, any current-flow between the two batteries which resulted (as the one battery attempted to equalize the other) would have to be evidence of an actual difference in voltage... Wouldn't it? Ohm's law seems pretty inflexible on this one: You cannot have any measurable current flow, unless you also have a measurable voltage... Can you?

    Gosh... I think I think too much--and ultimately, I'm afraid I'm only really showing my ignorance-level here. I'm pretty-much your run-of-the-mill basic-electricity education-level-type-guy when it comes to this stuff. My background in aerospace maintenance didn't require much more than that. In any case, I'm obviously not a battery-chemist! ;-)

    Anyways... I remain intrigued by the topic--and the ins and outs of it. I've got potential to take advantage of the concept--esp. if there's a simple, safe and reliable way to get it done.

    I'll certainly keep following the thread!
    Last edited by tklop; 02-11-2020, 07:55 AM.

  • AZguy
    commented on 's reply
    One can't "sag" more than the other since they are in parallel

    Under heavy load the one with the higher internal resistance (source of "sag") will just stop delivering as much power and the one with the lower internal resistance will do the lion's share

  • tklop
    commented on 's reply
    I'm trying to reason this out a bit...

    Suppose the smaller battery sagged during your discharge--perhaps even to the point of shut-down... Meanwhile, your larger battery was busy trying to both power your "load" as well as "recharge" the smaller one. If that was happening, and the larger battery's BMS was near its max-discharge--you might not have had a lot of amperage flowing between the two--you might not have been frying the smaller BMS yet, because you still had that 30A "Load" stealing away some of that potential charging in-rush (and causing perhaps a little sag in the larger-battery too--perhaps also reducing the voltage-differentials)... Well, if that was the case--then upon disconnecting the load, the large-battery's BMS would have then been free to put it's full discharge-current into the smaller battery's BMS.
    *Zot*... Maybe?

    My ideas here might be utter garbage... But I'm trying to sleuth it out too...
    Last edited by tklop; 02-11-2020, 03:36 AM.

  • calfee20
    commented on 's reply
    Could you have just had a bad BMS? The Two I am paralleling are both GA cells and a similar capacity. 17 and 14 amp/hours.

  • Sebz
    replied
    I wrote that article a while back and had only done basic testing on paralleling different chemistry and capacities (14S 17.5ah GA with a 14S 30Q 6ah) IT was a bench test and I was measuring currents flowing between the 2 batteries. At some point after removing a 30amp load applied the current flow from the large pack to the small one was over 10amps and then the BMS of the small one fried. Since then I have not paralleled these batteries together and put a warning on this.

    I do not know exactly what happened but if there was a current flowing from the large to the small after I removed the load from the circuit that means there was a voltage imbalance between the 2. Could be the BMS of the big one that tripped and then the small took the load and then the big one woke up with a higher voltage and pushed a current to the smaller one once I removed the load.

    But one thing is for sure if they are connected in parallel the voltage will be equal, what you don't want is current flowing from one to the other through the unprotected discharge port.

    Next time I get my test rig out, I will log current flowing between my 13.5ah against the mini 6ah pack and post the results.

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  • calfee20
    commented on 's reply
    I should have mentioned that you should do a search for "bms wiring diagram". Look at images and you will see that the charge wires are connected to the supply wires.

  • SmokerX
    replied
    Thanks for the input on paralleling different size packs working out over the long run calfee. "They both charge at the same time" my packs have 2 charge ports. 1 for each pack. This makes sense that the charge ports would jump over to the next pack over the paralled out put bridge. Over a single bms. I like the "i have no idea what im doing... This should be ok" aspect of a modified #1 solve with the jumper wire. Considering I have a handful of XT60's. Considering I have 2 chargers, 1 for each pack, hooking up both chargers should half the time of charging over a parallel wiring solve on 1 charger. I've also wired 2 chargers to double the amperage output after consulting the max charge rate, just for funsies, like a gamblers high of risking the house fire and it halfed the charge time this way. Although Im personally not overly concerned with charge times.

    Comparing solves, the modified solve #1 over a parallel solve Im having trouble identifying any advantage other than seamless transferring of amps of the packs. IE, Vs. having your power run out and manually having to switch over to the next pack. This is the only real world advantage Im able to come up with. The other advantage of the modified #1 is the 2 outputs meaning the possibility of the option of simultaneously powering 2 motors. Either a motor powered trailer. Front and rear wheel motors, or run an extension cord to your friends batteryless electric bike(lol).

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  • calfee20
    replied
    I have done it and have not had a problem. I have a 17AH Luna GA triangle connected to a 14 AH Luna GA rectangle. I matched the voltages and plugged them into a harness that I made a year and a few thousand miles ago. They both charge at the same time to. Once they are connected together the voltage of one has to be the voltage of the other. They share the load so there is very little strain on either battery. I don't know, do what you want. All I can do is tell you what I have done. Personally I think everyone is over thinking this issue. Vector ebikes was building a pak for their bikes that used 2 different chemistries in one pack so they could get the benefits of 2 different cells. One was longer range and the other was for high amps.

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  • SmokerX
    replied
    Q: Can I use a large pack and a small pack if they are at the same voltage?


    I have read other places this is ok. But I think this is incorrect and the original posters explanation of why this is a bad idea possibly makes sense. Considering the opposing argument goes into no detail on why this is OK. Im no expert on this, have no idea what book would go into a detailed explanation and have to default to gut instinct there and I guess whoever has more words wins in my mind.

    If a pack is wired internally in parallel between cells and Resistance increases with length of the wire, then the same ohm's law could argue that the battery closest to the packs output would suffer a similar effect to the "weaker pack" in the original explanation. Or vice versa(not sure considering path of least resistance). Im really not even sure if the "stronger battery will discharge through the weaker battery" Suggests an effect on the rated lifetime of charge cycles of individual cells, considering "the weaker battery" still only goes through one charge cycle. Even though it would be simultaneously being charged and discharged through the other cells in the pack and the current load.

    However. "properly configured and properly integrated BMS" "operated outside the manufacturer’s specifications" "cell matching" suggests tapping into series strings and using 2 BMS "properly configured" to each individual pack at least voids the warranty. It would also appear that you'd be throwing the concept of cell matching into the wind. Whatever any of this means to someone with a better understanding of lithium pack configuring.

    I think there's a work around here to isolate the 2 packs and keep the warranties without mucking about in the dark without fear of the unknown and doubt.

    1. The lowest tech. solution would be to swap the two packs using XT60 connectors or similar. Like swapping the battery pack on a battery powered drill. Except possibly only swapping out the connection. Not the physical pack. When one pack runs dry. This is also the cheapest.

    2. 2 Knife switches used to isolate the currents of packs. There's no point in that unless you want to make some steam punk art project. It would work but several drawbacks like exposed metal parts leading to shock and burns. or operator error closing both switches.

    3. 2 double pole single throw battery disconnect switches. That would remove the exposed metal parts from 2's equation but still have the drawback of operator error of closing both switches.

    4. Battery selector switch. Used in marine application to solve this problem. The only drawback beyond cost is most contain 4 positions. battery 1, 2, off and 1&2 or all. I think it's possible to open some models up and tamper with that "1&2" positions contacts to destroy that switch position and remove operator error, find a switch that doesnt contain that 4th position or a switch that doesn't have to cycle through that position to cycle between the off,1 and 2 positions and just never turn the switch to that position. The problem I see with some of these is parking unattended and the possibility of tampering throwing it into that "1&2" position. The benefits of #4 and #1 is not only the off switch but allowing 1 battery to be held in reserve. for example a buck converter to radio thrown into this. Switched to battery 1 with battery 2 being held for reserve to power the engine. The main reason marine application uses this solve.

    5. Remoting into a #3 using 2 contactors (or relays), particle electron board and cell phone. Im thinking you would need to run an arduino somewhere and step the voltage down to whatever voltage these things want to control the smart circuit(5v? 12v?) then write out some software and pay a monthly cell plan. Maybe you need to remote into your battery selector from a distance. Or want everything completely sealed up in a waterproof project box. Or don't like the hassle of the manual nature of a #3 or #1 solve. The downside here for me beyond complication is the slow drain on the battery from powering relays and particle electron boards. The upside is you can GPS this battery around in case it gets stolen. It cant work without giving away it's position.

    Moving back to #1 i think you could create a jumper wire. 3 XT60 connectors sitting flush to a project box. labeled 1,2 and motor or load. These 3 being female ends. Then create a jumper wire with a piece of wire and 2 male ends to jump from positions. This way you could take that jumper wire with you to prevent tampering and act as a theft deterrent since you would need this jumper wire to jump back to the motor. Without it the system would be powerless. It would also look a little nicer than just a basic #1 solve.

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