BMS parallel setup: 2 batteries 1 controller

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GumbaLL   1 mW

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BMS parallel setup: 2 batteries 1 controller

Post by GumbaLL » Jan 16 2022 10:16pm

Hey Folks,

I have a bike with Kelly KBS 72v controller and a 20s8p battery. I want the option to add on another 20s8p battery in parrallel to add more capacity on some occasions.

Each of the batteries would have its own BMS and I was thinking of adding a 3rd one to manage the two packs as I would imagine each may have varying levels of SOC. Im also thinking I would install a SOC display on each pack so I would know which one to priortise charging whilst the 3rd BMS would combine the two packs to show a combined SOC to display on the bike.

Basically the 3rd BMS would input two set of wires for each of the packs and run either one or two packs in parallel each with its own connector. And will output into controller conventionally

Would the design work like that?
As the 3rd BMS would register the each pack as a single cell, any recommendation for the kind of BMS I may need?

thanks for reading!,
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Re: BMS parallel setup: 2 batteries 1 controller

Post by john61ct » Jan 16 2022 10:41pm

Thinking too much.

Get the two sub-packs similarly balanced and to the same SoC before joining them

within 0.1V per S-count is a good guideline.

To the extent they are similar SoH, from then one treat as a single pack for normal cycling.

If you see testing / balancing is needed, easier to split and treat them individually.

There's enough complexity confirming the two BMS allow paralleling

no need adding a third.

In fact a single BMS can take care of both if you can parallel at the cell/group level.

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Re: BMS parallel setup: 2 batteries 1 controller

Post by GumbaLL » Jan 16 2022 10:58pm

john61ct wrote:
Jan 16 2022 10:41pm

Get the two sub-packs similarly balanced and to the same SoC before joining them
the problem is that this would eventualy be a retail product and Im trying to develop a more user friendly system whereby the customer wouldnt be concern with needing both packs to be the same SoC

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Re: BMS parallel setup: 2 batteries 1 controller

Post by amberwolf » Jan 16 2022 11:12pm

GumbaLL wrote:
Jan 16 2022 10:16pm
I have a bike with Kelly KBS 72v controller and a 20s8p battery. I want the option to add on another 20s8p battery in parrallel to add more capacity on some occasions.

Each of the batteries would have its own BMS and I was thinking of adding a 3rd one to manage the two packs as I would imagine each may have varying levels of SOC. Im also thinking I would install a SOC display on each pack so I would know which one to priortise charging whilst the 3rd BMS would combine the two packs to show a combined SOC to display on the bike.

Basically the 3rd BMS would input two set of wires for each of the packs and run either one or two packs in parallel each with its own connector. And will output into controller conventionally

Would the design work like that?
As the 3rd BMS would register the each pack as a single cell, any recommendation for the kind of BMS I may need?
There isn't one that works like that, that I am aware of.

It shouldn't be necessary, anyway. Generally the newer pack, if they are made identically to the old, will have lower internal resistance and so it will end up supplying more of the power needed to drive the system, and each pack will likely still "run out" around the same time. So all you should have to do is ensure both packs are within a small voltage difference (probably a volt or less, but a tenth of a volt or less would be better). The closer in voltage at connection, the better. The voltages will equalize quickly thru the resistance of the wiring, connectors, etc.; the smaller the difference the less wasted capacity as this occurs, and the less heat generated in that wiring/etc.


You can use a wattmeter to measure the combined pack output statistics, at the controller side of the discharge Y connection.

If you are also charging thru the same port, then if you use a bidirectional meter, it will also be able to measure charging statistics for the combined pack. (it will also measure regen data if you have that, as reverse currents within the meter. If the meter doesnt' measure those separately (which the Cycle Analyst does) then it will just take away some of the value in each applicable discharge statistic.)


If you also wish to measure the statistics for each pack individually, you can add another of the same meter on each battery side of the Y connection.





If the packs have separate charge and discharge ports, don't leave the packs connected at the discharge port while charging, and vice-versa.

If the packs use the same port for charge and discharge, then you can leave them connected all the time.
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Re: BMS parallel setup: 2 batteries 1 controller

Post by john61ct » Jan 16 2022 11:22pm


GumbaLL wrote:
john61ct wrote:
Jan 16 2022 10:41pm

Get the two sub-packs similarly balanced and to the same SoC before joining them
the problem is that this would eventualy be a retail product and Im trying to develop a more user friendly system whereby the customer wouldnt be concern with needing both packs to be the same SoC
I hope you are judgment proof in the target market jurisdiction 8-)

The requirement to get to similar SoC before joining is really necessary for safety.

That can maybe be automated if you supply the charging solution.

None of this has anything to do with BMS functionality.

Otherwise a robust capacitor-based current-limiting circuit could be custom designed to go between the two unequal sub-packs so long as their properties are fixed, to be used to get them equalized for say an hour before charging starts.

Hire an experienced DC power engineer…


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Re: BMS parallel setup: 2 batteries 1 controller

Post by amberwolf » Jan 16 2022 11:26pm

GumbaLL wrote:
Jan 16 2022 10:58pm

the problem is that this would eventualy be a retail product and Im trying to develop a more user friendly system whereby the customer wouldnt be concern with needing both packs to be the same SoC
Then you'll need to develop and produce the device that does what you want to happen. It isn't likely to be that complicated, and could be done with any little MCU (arduino, etc) and whatever high-current-switching circuitry you wish to build (if it works like a BMS and shuts off current under specified conditions).

If the packs are not the same voltage, then in order for the system to operate without draining one of them into the other thru the resistance of the wiring, connectors, etc.

If the difference is large enough, that could cause damage to the system, including the packs, because there is no current limiting in them besides resistance, and the high voltage pack will discharge into the low voltage pack at the current determined by the very low resistance of each pack plus that of the wiring and connectors between, and the difference in voltage.

If it's a high enough current, it could damage or destroy the charging FET in the lower voltage BMS, assuming it's a common-port BMS. If it's separate ports, then it will be charging the lower voltage pack thru the body diodes of the discharge FETs, and they will generate a fair bit of heat since they aren't being used as switches in this condition. They may be damaged if the current exceeds the design limits for this.

If they have separate ports, only the higher voltage pack is going the right way thru it's discharge port to be able to shut off, and it's very likely that the current will be within the discharge current limit, so it won't try to shut off. The lower voltage pack cannot stop the incoming current, no matter what it detects and limits for.

If the BMSs are common-port, *and* they monitor both charge and discharge currents, *and* they shut off the ports when those limits are exceeded, *then* there shouldn't be damage, because they'll turn off. But only if all those are true. Otherwise, no limiting, no shutoff, and potential damage.

The cells themselves, if the connection current exceeds their limitations (much less likely than exceeding BMS limits), could also be damaged, or overheated, and this is what can lead to fires (perhaps not at the time, but later under other conditions).


The big problem with BMS FET damage is that the most common failure mode is shorted, so the BMS can no longer turn them off to protect the packs against overcharge or overdischarge....and it is a "silent" failure, so no one will even know this is happening until something else happens, perhaps catastrophically, because the BMS doesnt' tell you.
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Re: BMS parallel setup: 2 batteries 1 controller

Post by e-beach » Jan 16 2022 11:38pm

20 balance wires to one cell group and 20 balance wires to another cell group. From your wiring diagram I don't see the 40 wires to both cell groups. If you are looking for more range, just put the second battery in a backpack and use it when the first hits LVC.

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Re: BMS parallel setup: 2 batteries 1 controller

Post by amberwolf » Jan 16 2022 11:45pm

To prevent the above situations I noted, you'd need to do one or more of the following things:

--Detect the voltage of each battery as it is connected, via a separate monitoring connection from the main Y-connection. Shut off the Y-connection to it. Compare the two readings. Re-enable the connection to the higher voltage battery, but leave the connection turned off to the lower voltage battery completely, until the higher voltage battery has drained down to that level during usage. Re-enable the connection to the lower voltage battery.

--Detect the voltage of each battery as it is connected, via a separate monitoring connection from the main Y-connection. Shut off the Y-connection to it. Compare the two readings. Connect the batteries together via a current-control circuit (passive resistance, or active PWM, etc), until they are equalized, and disable the bike until this is done, unless the current-control circuit is capable of handling the full discharge current of each battery, and is "smart" enough to deal with passing a low current from the high voltage battery to the low voltage battery, while also passing full discharge current (if demanded) from the high voltage battery to the bike (the low voltage battery can't deliver any current at this point because it's still not connected to the bike until it's equalized).


There may be other more complex things you wish the system to also do (like track statistics on each battery, etc); those would be things you need to define specifically, step by step, so that you can implement them or have someone else design/implement the system.
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Re: BMS parallel setup: 2 batteries 1 controller

Post by amberwolf » Jan 16 2022 11:57pm

e-beach wrote:
Jan 16 2022 11:38pm
20 balance wires to one cell group and 20 balance wires to another cell group. From your wiring diagram I don't see the 40 wires to both cell groups.
I'm sure you don't mean it that way, but if you connect all 20 balance wires to just one cell group (commonly defined as one parallel row of cells), the BMS can't do it's job.

If by cell group you mean an entire battery pack, and using just one single BMS to monitor two of them in parallel, then yes it would work fine to do that, because they are now effectively one pack. But you would need to make that wiring permanent, so they are no longer separately removable packs, because if a balance contact in a removable-pack connector was making poor contact, and something went wrong with the group of cells that was connected to, but not to the other ones, the BMS wouldn't know, and wouldn't do anything to protect the system against the potential fire hazard this could create.


If you are looking for more range, just put the second battery in a backpack and use it when the first hits LVC.

Unless he means something else by his other post(s), he wants to be able to connect both at the same time to sell the system commercially, to customers that can't be bothered to read instructions to not do that if they're not both fully charged. (which would make them the same voltage and avoid the problems). ;)

So while it would be better to do what you suggest (even if it's already mounted to the bike in a second slot rather than a backpack), if the voltage difference is significant, it wouldn't accomplish the accomodation of lazy people. ;)

A switch, like I think some of the original Currie systems had, between each pack and the controller, to turn them completely both off, or just one on, or just the other, would be a very simple way to aid using two packs sequentially, rather than simultaneously.

To do it simultaneously, without worrying at all about voltage differences, would require a "smart" automated switching system.

Alternately, a custom BMS on each pack could do the current control...but the entire bike would then have to be made in such a way that the entire bike cannot work at all without talking to the BMS to be sure it's there and working, so that no one can just put a normal-BMS'd battery on there, and then cause themselves a fire or other problem because they created one of the previously-posted situations and damaged the BMS, wiring, connectors, or cells in one or both packs.
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Re: BMS parallel setup: 2 batteries 1 controller

Post by GumbaLL » Jan 17 2022 2:37am

john61ct wrote:
Jan 16 2022 11:22pm
GumbaLL wrote:
john61ct wrote:
Jan 16 2022 10:41pm

Get the two sub-packs similarly balanced and to the same SoC before joining them
the problem is that this would eventualy be a retail product and Im trying to develop a more user friendly system whereby the customer wouldnt be concern with needing both packs to be the same SoC
I hope you are judgment proof in the target market jurisdiction 8-)

The requirement to get to similar SoC before joining is really necessary for safety.

That can maybe be automated if you supply the charging solution.

None of this has anything to do with BMS functionality.

Otherwise a robust capacitor-based current-limiting circuit could be custom designed to go between the two unequal sub-packs so long as their properties are fixed, to be used to get them equalized for say an hour before charging starts.

Hire an experienced DC power engineer…
I would need to be judgment proof if the problem of having unequal SoC for 2 parallel battery packs cannot be solved :lol:

I actually thought having the 3rd BMS would help mitigate safety risks as it would manage the two packs as one. guess i was wrong.

Experienced DC power engineers are tough to find in my neck of the woods, and gotta try asking when there are alot of good smart ppl online :D

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Re: BMS parallel setup: 2 batteries 1 controller

Post by GumbaLL » Jan 17 2022 4:00am

amberwolf wrote:
Jan 16 2022 11:26pm
GumbaLL wrote:
Jan 16 2022 10:58pm

the problem is that this would eventualy be a retail product and Im trying to develop a more user friendly system whereby the customer wouldnt be concern with needing both packs to be the same SoC

Then you'll need to develop and produce the device that does what you want to happen. It isn't likely to be that complicated, and could be done with any little MCU (arduino, etc) and whatever high-current-switching circuitry you wish to build (if it works like a BMS and shuts off current under specified conditions).


If the BMSs are common-port, *and* they monitor both charge and discharge currents, *and* they shut off the ports when those limits are exceeded, *then* there shouldn't be damage, because they'll turn off. But only if all those are true. Otherwise, no limiting, no shutoff, and potential damage.

WOW! Thank you :bigthumb:
Will dive deeper into what you've written here and below :D
So maybe a good BMSs would simply do it. LVC kicks in on the battery that is low SoC and for extra measure a MCU programmed to switch if off

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Re: BMS parallel setup: 2 batteries 1 controller

Post by amberwolf » Jan 17 2022 6:47pm

GumbaLL wrote:
Jan 17 2022 4:00am

So maybe a good BMSs would simply do it. LVC kicks in on the battery that is low SoC and for extra measure a MCU programmed to switch if off
No. The BMS LVC won't do anything for you in the situation you're creating, because the low pack is being charged, so voltage is increasing, not decreasing.

It's unlikely the pack that is higher voltage is going to discharge below it's LVC, because that would imply the other pack is already discharged well below it's LVC and is broken and damaged and could fail dramatically and spectacularly.


Either way the LVC of a BMS only disables the *discharge* port, which only stops the battery from providing current, not receiving it backwards thru the port (which is the situation you're creating), so even if it did engage it would only cause the discharge FETs to change from low-resistance switches into higher-resistance diodes, and cause heating in the FETs.


What you need is a completely separate device, such as what I described before. AFAIK, it doesn't presently exist, so you'll need to design it and build it (or have someone do this).


Or, as I said before, you could custom design the BMS for the packs to do the job themselves, but they will need functions and parts a typical BMS doesn't have. And you will have to have custom designed stuff on the bike that refuses operation if using anything other than that BMS, or else the situation you're creating with unequal packs will still have the same potentially hazardous results as previously noted, when end-users do what plenty of people on this forum have done, and bypassed the BMS safety functions or replaced their OEM packs with standard ones.


Something you must do whenever you are creating a product involving batteries and potential fire hazards, is to sit down and consider and list all the possible ways your product can go wrong. Not just the ways that it could fail "normally", but those ways that an end-user can break it, and still try to use it, or the ways they could misuse it either intentionally or unintentionally, and then develop the product to prevent each one of those things. The things it doesn't prevent that end up causing a fire or other harm are the things that get you sued. :(

It's not that simple a process, because you have to understand how each part of the thing you are making works, and what causes each of those things to fail. That can be a lot of information. ;)
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Re: BMS parallel setup: 2 batteries 1 controller

Post by amberwolf » Jan 17 2022 6:49pm

GumbaLL wrote:
Jan 17 2022 2:37am

Experienced DC power engineers are tough to find in my neck of the woods, and gotta try asking when there are alot of good smart ppl online :D
There are places online you can hire one. Remember that the free advice you get on forums and whatnot is worth every penny it costs. :P
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Re: BMS parallel setup: 2 batteries 1 controller

Post by docw009 » Jan 17 2022 11:08pm

I use a y-connector and some judgement as to which of my 10S packs I can run in parallel. Of course, the two packs are at the same chemistry and S-count, and are equal voltage when I connect them. I have looked at what happens if I knowingly match unequal voltages going as far as 42volt and 40 volts with two 4AH packs. They equalize pretty fast, and if I recall, the peak current was around 4 A. I wouldn't go further. I just wanted to convince myself that a small variation was OK.

I've paired a 10AH with a 4AH, and it behaved like a 14AH. I know that some people believe that if packs are severely mismatched, the larger pack can both power the motor and be diverting additional current to charge the smaller one. I haven't been able to model that in my mind.

I won't vouch for them, but they sell y-connectors with shottky diodes in them, and some of the sellers claim they can put two different voltage packs in parallel, and the diodes will isolate the lower one until the voltages match, at which point both contribute.

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Re: BMS parallel setup: 2 batteries 1 controller

Post by MrWorks » Feb 02 2022 1:26pm

Caveat: I'm just now jumping into this, so I'm catching up. My apologies if I'm off-base.

Isn't this essentially what you're looking for? https://www.aliexpress.com/item/1005003 ... 3cd28AIumC

I've been digging into the same question... knowing I have an electric mower with two batteries. It doesn't care if i put one in, which spot, two in, how charged they are... about as dummy proof as possible. Never any sense of it switching from one battery to the other. If it's available in a relatively cheap commercial product, it's out there, I'd think.

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Re: BMS parallel setup: 2 batteries 1 controller

Post by e-beach » Feb 03 2022 10:30am

That looks to me like a number of programmable BMS's that are doing their own thing individually except they can all be programed to reduce the amp output to the lowest AH in the battery chain.

So you could, for instance run a 48v 30ah battery with a 48v 40ah battery and program the bms to run a lower AH so not to cause a problem with the low powered AH cells in the system.

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Re: BMS parallel setup: 2 batteries 1 controller

Post by GumbaLL » Apr 02 2022 9:51pm

thanks everyone for all the replies. <3 :bigthumb:

In the end, i think the most simpliest and low cost idea would be a DPST (on on) switch for the user to manually switch between the two packs when one hits LVC while riding.

Although i cant seem to find one that would be able to handle the load from the 72v 20ah (with Chinese 3c cells) batteries though.
Any recommendations?

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