How do I test a BMS to make sure its working?

SwampDonkey

100 W
Joined
Mar 27, 2018
Messages
227
I feel like Im cursed over here. Ive been through 3 BMS units on 2 different batteries and none seem to work. They are 48V and 72V versions of this:

https://vruzend.co.uk/products/4s-1...-li-charger-balance-polymer-lipo-60a-bms-cell


They had good reviews, I dont know whats going on. Im afraid to solder anything now lol. Is there some way to test the BMS? It wouldnt charge, and the voltage from the P-/P+ was lower than the pack voltage. Measured directly, the pack is at 50V, I get 46V from the plug. Plus, it wont charge.

I was told that I could plug it in and actually measure the voltage on the balance leads, see it increase as I moved down the red charging wires coming from the 14 pin molex plug, but that doesnt work. The 72V version is having the same problem.

Do these BMS units just suck? I wired them up just as the diagram instructed. What is going on?


Edit: And here they show QC testing just as I would do it but they have a POSITIVE charging wire clamped onto the P-??? What is going on here?

https://www.aliexpress.com/item/Wat...orw7fjQ7g2ey5EHdgAcaArsUEALw_wcB&gclsrc=aw.ds
 
the diagram is clear and does not show a positive wire going to charge negative.

check your wiring.
 
Yep, like I said on the other thread. I think some how you are not wired correctly. But those pictures don't show us anything to help us tell what you may have done wrong.

However, new and broken, is not unheard of. You can zap shit instantly if you ever connect up wrong too.
 
Try posting a picture of your actual setup.
If you are not using all the channels, you may need to connect the unused wires.
 
SwampDonkey said:
I was told that I could plug it in and actually measure the voltage on the balance leads, see it increase as I moved down the red charging wires coming from the 14 pin molex plug, but that doesnt work. The 72V version is having the same problem.

Do these BMS units just suck? I wired them up just as the diagram instructed. What is going on?

The molex connector is just a wire loom. Disconnected from the BMS, if you don't see the series voltage rising as you go up the wires, you either didn't connect the leads to the right spots ob your cells, or your cell connections are broken somewhere.

If you just measure between adjacent wire pairs instead of walking up the ladder, you should get the cell voltage for each series pair.

That is the first thing you should check before connecting the molex plug,
 
flippy said:
the diagram is clear and does not show a positive wire going to charge negative.

check your wiring.

Yes, I know. I was talking about the photo halfway down the page. Shows them testing one in a bowl of water. with a positive lead on the P- wire. I followed the diagram exactly and it didnt work. I just opened the unit to look for fried components and there are none.
 
docw009 said:
SwampDonkey said:
I was told that I could plug it in and actually measure the voltage on the balance leads, see it increase as I moved down the red charging wires coming from the 14 pin molex plug, but that doesnt work. The 72V version is having the same problem.

Do these BMS units just suck? I wired them up just as the diagram instructed. What is going on?

The molex connector is just a wire loom. Disconnected from the BMS, if you don't see the series voltage rising as you go up the wires, you either didn't connect the leads to the right spots ob your cells, or your cell connections are broken somewhere.

If you just measure between adjacent wire pairs instead of walking up the ladder, you should get the cell voltage for each series pair.

That is the first thing you should check before connecting the molex plug,

Edit: I took the BMS off the pack to troubleshoot. The "sense wires", I think they're called, are not attached to the cell groups right now.

I meant from the BMS, as in checking the charging voltage coming from the BMS and going to the cell groups. I know I could measure the pack voltage if I take the reading from the connector. Its wired directly to the cell groups. The BMS isnt charging, and I cant figure out why. Shouldnt I be able to:
1) Plug it into the charger,

2) Take a voltmeter, clip the negative lead to the b- sense wire

3) Check the voltage from red b+ wires

4) Find 4.2V on B1+, 8.4V on B2+, ect all the way up to 54V?

Also, my charger is putting out 58V. Is that a problem?

Any clue as to why im getting 46V from the BMS and 50 from the pack when measured directly?
 
dogman dan said:
Yep, like I said on the other thread. I think some how you are not wired correctly. But those pictures don't show us anything to help us tell what you may have done wrong.

However, new and broken, is not unheard of. You can zap shit instantly if you ever connect up wrong too.

Edit: Link to Imjur gallery: https://imgur.com/a/e0LUEYj

Ok, how do I attach pics? I tried Imjur and it didnt work, didnt see a "browse" button under the edit screen, saw an "attach file" button and that didnt work either.

Note: I know theres some mixed-looking cells in there. They're all similar capacity and every parallel group is charged to the same 3.5v.

Ok, here we go. Might be hard to see, but the b1- (small black wire that goes to the pack negative) is soldered directly in the large B- going from the BMS to the pack. Note that its at 46V, measured from the BMS.


Heres the BMS itself, with the waterproof covering taken off.



Here's where the pack positive is soldered on.


Ive had to rebuild this pack a couple times, trying to find the problem.
 
it helps if you connect the balance wires..... :roll:
 
The sense wires are not for charging! They are for measuring voltage and draining small currents (tens of mA) from the highest voltage cells to bring those into balance with the others.
 
Congrats on one of the most sketchy DIY builds I've seen.
 
Chalo said:
The sense wires are not for charging! They are for measuring voltage and draining small currents (tens of mA) from the highest voltage cells to bring those into balance with the others.
Depends on the charger.

Lots switch over to use the balance leads for charging soon as the first cell hits the setpoint.

Some charge only through the balance wires, bo bulk pack-level charging at all.

"Sense wires" are usually used for 4-wire chargers.
 
You did load test the batteries before welding (and soldering?) right?

Show some close up pics how you connected the balance leads to the cells?

What charge sources are you using?
 
Ok, so the balance leads dont actually charge anything. Thats some new information. I was under the impression that they preferentially charged groups that get out of balance. Apparently they can only preferentially discharge?

Any idea on why I get 46V out of a pack at 50v?
 
liveforphysics said:
Congrats on one of the most sketchy DIY builds I've seen.

Well, Im using one of those "boss level welders" and it either burns the hell out of nickle strips or they dont stick. The adjustment dial broke almost immediately so its stuck on a fairly high setting. Having to take a pack apart to replace dead cells and reweld 3 times doesnt help either.
 
SwampDonkey said:
Ok, so the balance leads dont actually charge anything. Thats some new information
And false as I just pointed out.

Depends on the hardware involved.

Please give the answers to questions asked and try to more precisely describe the details of your setup, what you are doing to get the data you're reporting, what conditions, where you're measuring, what instruments etc.

 
SwampDonkey said:
liveforphysics said:
Congrats on one of the most sketchy DIY builds I've seen.

Well, Im using one of those "boss level welders" and it either burns the hell out of nickle strips or they dont stick. The adjustment dial broke almost immediately so its stuck on a fairly high setting. Having to take a pack apart to replace dead cells and reweld 3 times doesnt help either.
Maybe your lack of QC procedures are leading to junk results.

Have to start with good cells.

Likely your welding is damaging them too.
 
john61ct said:
SwampDonkey said:
Ok, so the balance leads dont actually charge anything. Thats some new information
And false as I just pointed out.

Depends on the hardware involved.

Please give the answers to questions asked and try to more precisely describe the details of your setup, what you are doing to get the data you're reporting, what conditions, where you're measuring, what instruments etc.

Ok, the balance leads arent connected. I had to replace some dead cells and I was operating under the assumption that the balance wires were for charging, and could be tested by checking the voltage coming off of them. I asked a bunch of questions about BMSs' and that was one of them. Another member says they dont actually charge, you say it depends. Im confused.

The batteries are all load tested. Actually they're rewrapped Tesla and Panasonic NCR18650BD of the same capacity (3100mAh). The orange and pink cells are just there for testing purposes to actually get the thing working so I can replicate the configuration, but they're new and working.

The charging source is a potential issue, now that I look at it. Its putting out 59V. Im building this pack for a friend of mine as a spare and this is the "48V" charger he gave me. His old BMS and 48V pack is a 14s rather than a 13s configuration.
 
john61ct said:
SwampDonkey said:
liveforphysics said:
Congrats on one of the most sketchy DIY builds I've seen.

Well, Im using one of those "boss level welders" and it either burns the hell out of nickle strips or they dont stick. The adjustment dial broke almost immediately so its stuck on a fairly high setting. Having to take a pack apart to replace dead cells and reweld 3 times doesnt help either.
Maybe your lack of QC procedures are leading to junk results.

Have to start with good cells.

Likely your welding is damaging them too.

Im sure lol. I want to make sure I understand how it all works before dropping hundreds of dollars on new cells. One big issue I had early on was the damn fake nickle strip. Bought on ebay, it was apparently coated steel since it rapidly corroded and had to be ripped off. When that happens you have to grind a clean surface on the battery terminals and ugly it up before replacing the strip.
 
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