DIY battery pack bms question

nethern · Feb 3, 2018

Hi
I am building a 14s battery pack with BSM bought here https://www.aliexpress.com/item/High-quality-14S-18650-lithium-battery-BMS-for-electric-bike-lithium-battery-with-20A-continuous-discharge/2030986292.html?spm=a2g0s.9042311.0.0.YcU2Cy.

After I connected all of the balance wires, the bms start to omit some burnt odor. I found out the BMS is getting very hot, all most hot to touch. Its been for a while. I guess it is doing balancing discharge/charge??

The battery i have are brand new and i have tested with multimeter on all of them, they all show 3.4v. Why would the bms trying to do so much discharge/charge? Bad bms?

Please help.

amberwolf · Feb 3, 2018

Is it on the charger when getting hot?

FWIW, 3.4v is nearly empty for a non-LiFePO4 cell.

What voltage did the cells start at?

Are the balance wires connected in the right order?

If not, it might be possible taht the balancing circuits are now damaged and draining the cells at high current.

jonyjoe303 · Feb 4, 2018

there is probably a wire connected wrong somewhere. You should never smell any burned smell coming from the bms.
The bms will usually only balance at the high end, when a cell reaches about 4.15 it starts to bleed that cell too let the other cells catch up.
At 3.4 volts the li-ion cell is close to the low end. The bms shouldnt be doing anything at that voltage.

999zip999 · Feb 4, 2018

Tell us how you hooked up the sense wires ? Where did you start start with the BMS unplugged ? Fetcher uses a method of plugging the sense wire plug from 1- 14 at and aggressive voltage sequence. Plugging it in at an angle as 1 through 14 or in a building voltage scenario.

nethern · Feb 4, 2018

Oh! i Did Connect two of the balance wire the other way around. It has drain two of the groups to 2.2. Luck i disconnect the balance plug to the bms while waiting for the reply here.

Thanks for the reminder there

999zip999 · Feb 5, 2018

I start with a balanced pack before I hook up the BMS that little BMS would have a tough time balancing a super low cell and a super-high cell

fechter · Feb 5, 2018

It sounds like the BMS is fried for sure. They are not tolerant at all for misconnections. Reversing any pair of wires is fatal.

Even if all the wires are properly routed, it can still fry when first connected to the pack if connected in the wrong sequence.
Even if everything else was done correctly, it can just fry anyway due to cheap or incorrect components on the board.

I hope you immediately disconnected it.

Try for a refund. Don't tell them you misconnected it.

When connecting a BMS for the first time, double check the voltages on the balance plug first. Make sure you measure one cell voltage between each pair of adjacent pins.

Make the B- connection first.
Plug in the balance plug and try to angle it slightly so the negative end mates first.
Make the B+ connection last.

nethern · Feb 5, 2018

i reconnect the wire correctly now. It seems fine. i Got right volt from discharge end and the bms is not hot at all This time. I thought there would be fire for a moment.

i ll watch It for a while.

fechter · Feb 6, 2018

Let it sit for a while then measure all the cell voltages again. You could have a balance shunt stuck on.

999zip999 · Feb 6, 2018

Balance the pack manually do not rely on that poor little BMS to balance out of balance pack. It's to help keep a balance pack in balance at end of charge.

nethern · Feb 8, 2018

Sorry about not mentioning i did manual charge all to 3.4, before reconnect. I Let It sit for a while at from 48.7v to 46.2v in two days. Then Charged it. It is now at 54.4v,because i am yet to buy a 14s Charger. That s 13s i used

Should i worry about the volt drop in the two days? or It s normal rate?

amberwolf · Feb 8, 2018

test and list all the individual cell group voltages as measured at the BMS balance wires. If they're not equal, the low ones are probably connected to damaged channels on teh BMS>

fechter · Feb 9, 2018

nethern said:
Sorry about not mentioning i did manual charge all to 3.4, before reconnect. I Let It sit for a while at from 48.7v to 46.2v in two days. Then Charged it. It is now at 54.4v,because i am yet to buy a 14s Charger. That s 13s i used

Should i worry about the volt drop in the two days? or It s normal rate?

Worry. It should not drop that much.

A common failure mode on low end BMS units is one of the balancing shunts blows and gets stuck on, slowly draining the cell it's attached to. Usually one or two cells will be affected.

If you measure all the individual cell voltages again, look for any that are lower than the rest.

Also keep in mind that any cell that gets drained below about 3.0v could be damaged when charged at the normal rate. Low cells need to be brought up to the safe range using a very low charge current (less than C/20). Once the cell reaches 3.3v or so, it is safe to charge at a normal rate.

nethern · Mar 20, 2018

i have put It on the bike for some rides. initially It is ok. But the range is not as far as iexpected. Since my Charger is 13s, i am not sure that causes the short mile rang or Its due to the damaged cells there. I will try and see what will happen.

DIY battery pack bms question

nethern

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amberwolf

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jonyjoe303

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999zip999

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nethern

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999zip999

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fechter

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nethern

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fechter

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999zip999

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nethern

1 mW

amberwolf

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fechter

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nethern

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