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16s BMS advise
#11
I am surprised that the concensus is that it works fine to parallel balance leads. I do automotive electrical work every day and will ensure the wiring is done right and tidy.

My concern is about a weak cell. The parallel cells in other modules would try to push some current through the balance leads when the weak cell's voltage dips under load. I should say dips more compared to the healthy cells. That voltage differential could push enough amps to burn up the tiny or balance lead to the weak cell. Now that cell is unmonitored and getting weaker without any way for the BMS to alarm and shut off the current.


The battery would be from a BMW 740e 9.2 kw
That is 6 modules of 16 cells each. I haven't found any data to say if the "cells" are actually parallel cells inside each "cell" casing.

Or some other production EV (Chevy Volt)
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#12
I'm using a chinese smart bms with two packs connected and the balance leads paralleled. The packs at 15s17p. My plan is to expand because I can still get more of the same packs. Each one is about 2kwh, give or take. But when I expand I plan to move to the DiyBMS which you can program up to 4 "banks" of up to 16s per bank. So that way each cell will be monitored and reported better giving me better insight into the packs.
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#13
(05-30-2020, 04:07 AM)400bird Wrote: My concern is about a weak cell. The parallel cells in other modules would try to push some current through the balance leads when the weak cell's voltage dips under load. I should say dips more compared to the healthy cells. That voltage differential could push enough amps to burn up the tiny or balance lead to the weak cell. Now that cell is unmonitored and getting weaker without any way for the BMS to alarm and shut off the current
You want to *start with* healthy batteries/cells.   If you have an unhealthy cell/battery you shouldn't use it - regardless of BMS.

When you start with healthy batteries/cells, that are charged closely to each other before paralleling - the situation you describe would be a gradual dip/self-discharge over time as the battery ages and the BMS will catch it.  Voltage differences of <=100mv (0.1v) between cells are no big deal and will not cause significant current.   If it gets to be 1000mv (1.0v) over time its a bigger deal but a BMS can detect this and its not as much current as you might think. 

The Chargery 16 has a customize-able setting for 'Diff of Cell Voltage' - with a default of 300mv (0.3v) - that you can set to trip the relay long before this becomes a 2.0v difference and current becomes an issue. 

FYI - here's a youtube done by @Daromer, one the forum's most technical/senior members on this topic:   Installing a smart BMS on a 14s LiIon battery pack  https://youtu.be/sGj9UrfEcAw

Now, the downside of paralleling 6 batteries into 1 BMS is that it won't tell you *which cell* (of the 6 in parallel) is self-discharging.  But again, this should not occur until a battery starts to become unhealty.    The upside 1 BMSs per battery - it will focus on the 1 battery.    If your batteries are healthy then maybe parallel all 6 for now and save up for seperate BMS(s) or a Batrium which can monitor a lot of cells independently of the number of batteries.
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#14
If you're worried about paralleling the balance leads for cell short type failures, you could fuse them....
The balance leads on many BMS systems are deliberately thin so they act as the "fuse", eg Batrium
Ibiza and floydR like this post
Running off solar, DIY & electronics fan :-)
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#15
Was just watching @DavidPoz's latest youtube - and notice the Chargery 16 (blue box) BMS and nest of red wires to the right in the beginning of the video.  He's got several, paralleled balance/control leads coming off that stack of LifePO4 units powering his house. Just a bit more proof that its for real that you can parallelize a group of large batteries/cells.   

Off Grid SMA Sunny Island Inverter Wiring, 48v DC   https://youtu.be/vMuDBjERZV0

If you go back a few months in his videos you can see the wall of batteries and good detail of how he hooked them up.
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#16
How are you testing each 16s module?
You don't want to break up the 16s modules which I understand.
Have a volt meter connected to each module? if one meter is lower then the other volt meters you have a weak cell in that module. The balance wires would essentially be cell level fuses like in a 18650 based battery. Each module could be fused.

Later floyd
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#17
Cell level fuses, when they blow, remove the current path from that cell. When parallel pack balance leads fail, the cell is still in the current path and the BMS doesn't have any way to monitor that cell.

I understand that one BMS with parallel packs would work under most circumstances, but we don't use a BMS to protect from "most" failures. I fell like at that point why even run a BMS?

Plan for module testing is use the inverter for a full charge/discharge cycle with the BMS connected. I may end up using a different load and/or charger. But, I don't have the components yet. Still planning, the more I plan the less likely I am to make the wrong purchase or miss some detail.
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#18
(05-31-2020, 01:05 AM)400bird Wrote: Cell level fuses, when they blow, remove the current path from that cell. When parallel pack balance leads fail, the cell is still in the current path and the BMS doesn't have any way to monitor that cell.

I understand that one BMS with parallel packs would work under most circumstances, but we don't use a BMS to protect from "most" failures. I fell like at that point why even run a BMS?

Plan for module testing is use the inverter for a full charge/discharge cycle with the BMS connected. I may end up using a different load and/or charger. But, I don't have the components yet. Still planning, the more I plan the less likely I am to make the wrong purchase or miss some detail.
You don't fuse balance leads.   Fusing is for individual 18650 cells as they join the bus is to protect the pack (not the cell or balance leads) in case of catastrophic 18650 cell failure to a dead short.  Cell fusing is not standard for Chevy EV cell hookups that I've seen - they are fundamentally different physical design (pouch) as compared to 18650 and larger/fewer in a battery as compared to many/smaller 18650 cells.

I sense that you're skeptical of parallization of cells in multiple batteries and that's perfectly fine.   The point of this thread is just to share that it's a valid technique to consider - but that doesn't mean its right for your situation.  Smile
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#19
The cells would still be connected via the positive and negitive buses. I mispoke when i said the balance leads would be like cell level fuses. All a fuse in the balance wire would protect is the balance wire. The volt meter on each module would let you know which module has a low cell/high cell if you checked them.

later floyd
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#20
Thanks for the help everyone. I'm going to order a chargery bms16. With this I can at least get each pack tested.

You are right, I am uncomfortable using just one BMS. But, I don't yet know what I will do.
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