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RagingGrandpa

How BMS Works (Gotway pack teardown)

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(If there are additional details or data that would benefit you, please let me know.)

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37 minutes ago, RagingGrandpa said:

Overcurrent in the pack output is not borne by the BMS PCB at all- the high current will flow through the leads and cell straps. Bolted shorts will flow huge amounts of current, until the cells themselves become destroyed.

On my KS wheel I noted that there are fuses (mini ATX) on the mainboard to provide overcurrent protection on the battery outputs (one fuse per pack).  This implies that the packs themselves do not provide such protection. I can attest that the fuses saved one of my packs from self-destruction when I accidentally bridged connector pins while trying to measure voltage. There was a mighty arc, my voltmeter lead was melted/fused, and one of the fuses blew.  The pack was fine, and so was the wheel after I replaced the blown fuse.  Luckily I was measuring opposite-side-pack voltage through the connector coming from the mainboard (before connecting that to the other pack), instead of pack voltage directly from the pack's own connector.

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21 minutes ago, svenomous said:

On my KS wheel I noted that there are fuses (mini ATX) on the mainboard to provide overcurrent protection on the battery outputs (one fuse per pack).  This implies that the packs themselves do not provide such protection.

KS BMS have to my knowledge mosfets that can cut off the output. Normally there are about 1 (to 2?) Mosfets for charge input and 2-?4? Mosfets for output protection.

Output protection is only used for overcurrent protection (cell undervoltage protection was cancelled "long" ago(. Seems this is additional to the fuses. BMS overcurrent protection could be a fast, high current short circuit protection, the fuses some "slow", lower current mainboard , mosfet, wire protection

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Posted (edited)

Very interesting RagingGrandpa. Thanks for taking the time.

Is the master pack different than the other packs or is it the only pack with a connected charge input? 

Do the other packs also have a charge input so that they can act as the master pack? I don't remember seeing unplugged connectors on my two tesla packs.

So all the current of the fast charger people use go through that single bms. I wonder if it can get fried by using high amperage chargers like 5 or 10A.

A point I was trying to make in another thread is that you can't combine gotway packs with generic BMS packs and expect any protection. There's zero protection for that additional pack as it is charged through the output terminals even if it does decide it wants to cut charging. (It assumes that bms design is the same on current gotway wheels.)

Edited by alcatraz

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Theoretically, how would that interruptible output work in combination with a gotway pack? Generic bms set up for euc use won't really use the onboard output terminal out of fear of a cutout. So there's no discharge protection. Charging protection is what's left and I don't see how there could be any unless it's the only pack with the charge port connected to it. 

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1 hour ago, alcatraz said:

how would that interruptible output work in combination with a gotway pack?

The output from the auxiliary pack may turn off. If it does, you're left with the Gotway packs alone (which should carry you onward nicely). Resetting this type of "trip" could be quite challenging though, depending on the reset logic used by the auxiliary pack's BMS...

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That assumes the output terminals on the bms are used which they aren't when building the pack for euc use.

And even if they were used, what's the chance of the bms cutting the output as an overcharge protection? That's for discharge or overcurrent protection I think.

And like you pointed out. How do you open that connection without getting a split second one hundred amps rushing? Hmm... 

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On 5/20/2020 at 12:19 AM, RagingGrandpa said:

The output from the auxiliary pack may turn off. If it does, you're left with the Gotway packs alone (which should carry you onward nicely). Resetting this type of "trip" could be quite challenging though, depending on the reset logic used by the auxiliary pack's BMS...

Wouldn't disconnecting it from the main pack and charging it independently usually reset the auxiliary BMS's charge and discharge stop circuits? 

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Also, while my teardown was a "old 84V BMS"... Gotway's design hasn't changed much- 2019 production 100V Nik+ BMS pictured below.

Finally a 2-layer PCB. And charging input reverse polarity / short circuit protection at C+ (diodes not populated in this example).

ACtC-3co1A-AuShjAI-jW7X9PMTIzxJFmPFJIuT4

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1 hour ago, RagingGrandpa said:

And charging input reverse polarity / short circuit protection at C+ (diodes not populated in this example)

There seems to be still no short circuit protection - or charge and discharge wires are to be connected at B+ and C-?

The 2 pairs of mosfets should be n and p channel so they could perform as reverse polarity protection. And with the R005 as current shunt a short circuit protection?

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14 hours ago, RagingGrandpa said:

Also, while my teardown was a "old 84V BMS"... Gotway's design hasn't changed much- 2019 production 100V Nik+ BMS pictured below.

Finally a 2-layer PCB. And charging input reverse polarity / short circuit protection at C+ (diodes not populated in this example).

ACtC-3co1A-AuShjAI-jW7X9PMTIzxJFmPFJIuT4

Is this the side of the bms board that's against the shrinkwrap? I'm curious to cut my tesla packs open in a way so I can expose the measue points. I'm hoping that the group voltages are all measurable at the top of that array.

Is the bms on the pack side facing inward or outward?

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Posted (edited)
8 hours ago, alcatraz said:

Is this the side of the bms board that's against the shrinkwrap?

Yes, the image above is the 'exterior' surface of the pack.

It is normally straightforward to identify the layout of the board from the bulges in the shrink of the soldered connections and balance resistor array. Feel free to send a photo of your pack if you'd like collaboration.

I presume you intend to needle-probe the cell pads shown above? Sounds reasonable if done with care...

  

21 hours ago, Chriull said:

There seems to be still no short circuit protection

* protection from short circuit at the charging port (via diodes between C+ and B+).

No protection from overcurrent / short circuit of the pack output.

 

21 hours ago, Chriull said:

R005 as current shunt

I'm perplexed by that one too. 5-milliohm current sense resistor, perhaps...

Edited by RagingGrandpa

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about that, i found on other scooter baterry packs with samsung cell leaking (brand new pack ...) and cell are still selling on the internet

DSC08357.jpg

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Posted (edited)

What is the value (in ohms) of the balance resistors?

Edited by WI_Hedgehog

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5 minutes ago, WI_Hedgehog said:

What is the value of the balance resistors?

Here 

are two 150 Ohm resistors in parallel. 

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Posted (edited)
56 minutes ago, WI_Hedgehog said:

Is the package rated at 1/16 watt? (What is the wattage?)

The mosfet above the resistors labeled 7002 is (most probably) in a sot-23 package measuring 3x1.4 mm. (2n7002 from the datasheet at digikey)

The resistors are about the same length and a bit wider. So i'd guess it's a 1206 package according to your link. There they mention 1/8 W typical. Should be just enough for 4.2V.

http://www.resistorguide.com/resistor-sizes-and-packages/ lists a bit bigger sizes for 1206 and 1/4 W...

Edit: this numbers of course don't have to mean too much in a plastic wrap if the copper tracks were not designed to distribute the heat...

Edited by Chriull

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Posted (edited)

Does anyone know or would care to guess if the cell voltages are measureable at the top or at the bottom of these balancing circuits? I'd like to make as small a cut as possible. Thank you. 

IMG_20200609_075321.jpg

Edited by alcatraz

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6 hours ago, alcatraz said:

Does anyone know or would care to guess if the cell voltages are measureable at the top or at the bottom of these balancing circuits? I'd like to make as small a cut as possible. Thank you. 

 

The cell voltages have to be on both rows of the 6 pinners (comperators).

And also from on pin of the balancing resitors (121) to one pin of the mosfets (3 legged) directly above.

As the cells are all in series and as the pcb traces seem to be the cell voltages should be measurable between the lower pins of the balancing resistors. Just for the first or last cell you have measure against another point (B+ or B-).

Seems to be B- as there is a track from B1 to the first resistor on the left...

Follow the tracks!:thumbup:

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Posted (edited)

I like the idea, but if I am going to the bother of opening up the heatshrink, it wouldnt be too much of an effort to solder on 24 small diameter leads and run them to an onboard voltmeter. Does anyone know such a meter that would do the job? Maybe one that has an onboard button to scroll through the inputs, negating the need for a big display to display all voltages at the same time?

Edit: best I can come up with is 3x 8 cell meters. Not as neat or discreet as I would like, but they do have the benefit of being able to set alarms which would be handy:

https://www.google.com/amp/s/zenid10.wordpress.com/2011/09/05/monitoring-pack-cell-voltages/amp/

Edited by Planemo

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35 minutes ago, Planemo said:

I like the idea, but if I am going to the bother of opening up the heatshrink, it wouldnt be too much of an effort to solder on 24 small diameter leads and run them to an onboard voltmeter. Does anyone know such a meter that would do the job? Maybe one that has an onboard button to scroll through the inputs, negating the need for a big display to display all voltages at the same time?

Edit: best I can come up with is 3x 8 cell meters. Not as neat or discreet as I would like, but they do have the benefit of being able to set alarms which would be handy:

https://www.google.com/amp/s/zenid10.wordpress.com/2011/09/05/monitoring-pack-cell-voltages/amp/

Did not find anything real better :(

and i could not find any specification about accuracy, just the display resolution :(

For the price asked one could consider switching to a BMS with bluetooth connection showing all the data on the mobile phone...

Other solution would be a (weatherproof! and safe) connector brought to the outside and just measuring with the DMM or just some cheapo 8s li ion cell tester.

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Posted (edited)
16 hours ago, alcatraz said:

Does anyone know or would care to guess if the cell voltages are measureable at the top or at the bottom of these balancing circuits? I'd like to make as small a cut as possible.

Yes, you can easily probe them all per below.

The spacing is tight- take care not to short them! Especially when cutting the shrink!
If soldering, tapemask one at a time... risky business with cells connected.
Soldering will be difficult- most boards have a conformal lacquer painted over this area, for corrosion resistance.

The only part that's missing is B-, which is easy to probe manually at the pack output, but challenging to find a solder spot that's small and safe... if it's this exact board, I suggest the little unused pad near 'R005'.

ACtC-3fKCVbwg-g8AHAXKU8t_p-rROEgItmtXwNG

 

5 hours ago, Planemo said:

solder on 24 small diameter leads and run them to an onboard voltmeter

I don't see a need for inspecting per-cell voltages frequently. Maybe once a year, or whenever you have the shell open, or whenever you notice an abnormality during charging... just bringing all 25 pins to an internal connector would be awesome.

(As before, take care with insulation protection for this new pigtail. It must be well-secured inside the wheel in a way that cannot chafe or wear. Smaller wire is better- they'll melt at a lower current if they short out... but you'll likely burn the balancing traces off the BMS too, destroying it.)

Edited by RagingGrandpa

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Thank you very much. I'll try at the bottom first then.

I believe the balancing works and my packs are small @425Wh each. Considering they are 3-4y old I just want to be sure and to kind of see how the bms performs. If I am to rebuild with Tesla 21700 cells that are much larger capacity I probably would attach balance leads, and in that case I'd solder to the large connection points spread all around and not the balancing circuit.

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