Bought a cheap BMS ... but what have I been given?

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xphread
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Bought a cheap BMS ... but what have I been given?

Post by xphread » Mon, 06 May 2019, 22:20

Hi all.

I am rebuilding a cheap scooter with 4x 4S Li packs (to make a 60v/16s) and took a gamble & purchased a few "no-name" BMS off banggood.com to join them together.
https://www.banggood.com/5pcs-60V-16S-4 ... rehouse=CN
(Yes, I know... Not expecting them to be the rolls-royce of BMS, but hopefully it will do the basics of the job, for a basic scooter & I need 3 of them for 3x 16s packs in parralell)

Its meant to be a BMS for a 16s pack ... but instead of the 17 wires in one harness/plug, as shown in the ad, my delivered packs have 18 wires over 2 connectors!?,
When I opened it up.... the generic design has 17 of the 20 sub circuits populated. :/

Hopefully picture of circuit can be seen here: https://drive.google.com/open?id=17b6xJ ... 6d6CMcge3v

Can I just use the first 17 wires and ignore the last?
Do I need to ask for a refund/replacement with the right device? :roll:

To test it, can I just put the 67.2volt charger between the first and the second last wires and ensure the others are appropriate sub divisions for each cell?
:?

TIA,
Tim

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jonescg
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Re: Bought a cheap BMS ... but what have I been given?

Post by jonescg » Tue, 07 May 2019, 08:23

Tim, the black wire on the small connector will probably be in common with the last positive of the large connector. Take a mulitmeter set to conductivity and test the last pin with the first pin to see if they are joined at the PCB.

The most critical point is to start wiring from battery negative and work your way up to the top. If there is a spare wire (I don't think there will be) then you can short it to B+ on the PCB to avoid any floating voltages.
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xphread
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Re: Bought a cheap BMS ... but what have I been given?

Post by xphread » Tue, 07 May 2019, 11:15

Thanks.. There are definitely 18 wires, (17 red and 1 black which is common to b-... None of the reds are common to each other, or directly connected to either b+ or b-)

I shall try working my way up and check voltage differences as I go. :? (and do a current test when I tie the last two leads to b+, rather than let it float...

Thanks.
(I still wonder if it isn't safer to desolder some key components from the 17th set of the populated board)
... Seems very strange.

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jonescg
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Re: Bought a cheap BMS ... but what have I been given?

Post by jonescg » Tue, 07 May 2019, 13:31

These cheap BMS units also tend to draw power from the first two cells, so if left for a while they can unbalance the pack in short order.
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Re: Bought a cheap BMS ... but what have I been given?

Post by brendon_m » Tue, 07 May 2019, 19:32

jonescg wrote:
Tue, 07 May 2019, 13:31
These cheap BMS units also tend to draw power from the first two cells, so if left for a while they can unbalance the pack in short order.
Thats quite a design flaw, especially with that small 60ma balance current it'll take a while to level out any significant difference in Soc.

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Re: Bought a cheap BMS ... but what have I been given?

Post by xphread » Tue, 07 May 2019, 21:49

I'm an idiot... I wrote the above from memory, rather than having it infront of me... there are 2 black wires (the first of each connector), but the rest still holds true.... they aren't common, and the pinout labels on the PCB are sequential in numbers from 0 to 17... :(

Anyway.... Now having second thoughts about them ... if its going to start draining the first 2 cells as soon as I start assembling... and it takes me a while to assemble the 3 packs and then time to fit them into the scooter.... then time to relicence it, etc... :(

If I keep going down this path, I might need to find a dummy/alternative load to keep them cycling before they become a daily runner.

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Re: Bought a cheap BMS ... but what have I been given?

Post by brendon_m » Tue, 07 May 2019, 22:20

Realistically even if it is a 17s, as jonescg said just tie the last wires together after you work your way up the board. It would just mean that the top/last cell would balance at twice the rate of the rest of the pack, so it would get in line faster (not a bad thing)
One possible issue is if you use the bms to cut out at low voltage it may do it sooner (because your pack voltage is lower). I don't know if the board does a whole pack voltage check or just relies on individual cell voltages.
I suppose it would not cut out on charge either but same basic story, when one cell reaches its top threshold it will (should) cut out, regardless of the pack voltage.

In terms of draining the first cells, wire it up and put an ammeter in line and see if the bms draws it's power from them or highest b+ pin.

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Re: Bought a cheap BMS ... but what have I been given?

Post by brendon_m » Tue, 07 May 2019, 22:45

xphread wrote:
Mon, 06 May 2019, 22:20
To test it, can I just put the 67.2volt charger between the first and the second last wires and ensure the others are appropriate sub divisions for each cell?
No, you won't get any readings like that, the bms only draws power from individual cells when over a certain voltage. It won't have any voltages there until the cells are hooked up. It only controls the charging by cutting the connection between the big solder pads (labeled C- and B-)
Unfortunately the only way to test is to wire it up.

So, to wire up start with the cells starting with B- on the far right pin (in your picture) (black wire) and then cell 1+, cell 2+, cell 3+ etc until you run out of cells. At which point just link the rest of the pins to the last cell +.
Then run wire goes from the B- pad to B-
The C- pad connects to the charger and motor controller negative etc (assuming it can handle the current of the motor, otherwise just connect the charger here and put the motor controller straight to B-, but you will lose the undervoltage / short circuit protection)
And the charger +, motor controller + and B+ are all linked (through the Fuses, contactors etc)
I would probably connect it up C- first, B- 2nd, then plug in the plug with all the cells connected and then finally connect the B+. Unless you have paper work saying different...

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Re: Bought a cheap BMS ... but what have I been given?

Post by xphread » Fri, 10 May 2019, 11:41

Thanks for that.
Will give all that a go.

Got no documentation with it unfortunately... And email request for tech support/documentation only got a request back for the product number (even though its in the order...) and nothing further! :/

Cheers,
Tim

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