Battery output problem

[Carinagti]

Hi,
I am new to this forum and looking for some help. I have been running electric bikes for a number of years and in the past couple of years changed over to Li cells rather than lug about Lead acid.

I now appear to have a problem with the output from my 36v battery pack. I measured only 5.2 v at the output connector, but 37v at the charging socket. On opening up the pack I get 37v at B-, but only 5.2v at P-.



Any ideas?

Thanks,
Ian

 

[Nealh]

Could be that the BMS is faulty and not outputting the req'd voltage or though it could be crappy cells as yours look like they are paper wrapped.

On each side there I see two sets of sense wire jst connectors scrape the silicone gunk off them and disconnect them, using your meter probes you should get 10 separate cell string voltage readings from the wired connectors.

Depending how you measure the voltages you should expect to see 10 readings nearly the same or 10 incremental voltage readings e;g 3.7, 7.4, 11.1 etc,etc. If this is the case then you probably need a new BMS.
Write down each of the 10 readings and post then back here.

 

[spanos]

Is that 37v fully charged ?

That’s too low if it is and tends indicate a weak cell string /strings

But Nealh suggested readings should confirm either way

 

[Nealh]

Quite right K, that's why I didn't go further.
We need to see the 10 voltage readings first.

 

[Deleted member 4366]

You get 5.2v on the output because the BMS has switched off output. The normal reason for that is that one of the ten cell-groups is below 3.1v, so you need to measure the cell voltages to confirm that. You can find them on those two white connectors. If you're lucky, they'll be marked B0 to B10, otherwise you have to measure at all pins to find them. sometimes the sequence is unusual, like B0, B2, B3, B5 on one side and the rest on the other. Just record all voltages and put them in the sequence from 4v to 40v or whatever they are. It's possible that one cell has zero volts, so it will measure the same as the next in the numerical sequence.

Measure from B-, and take a final measurement from B- to B+ to guarantee that you get all 10 cell-groups.

 

[Carinagti]

Hi,
Thanks for the prompt replies.

The reading across the pack was 37.6v

The readings for the white wires are:
0v
4.1v
8.2v
12.4v
16.5v
20.8v
21.1v
25.2v
29.3v
33.5v
37.6v
(The readings alternate from the connectors on either side)
Does that mean that the cell group between 20.8 and 21.1 is faulty?

Ian

 

[anotherkiwi]

Yes it certainly looks like it.

 

[Deleted member 4366]

You now need to find that cell-group in the pack and measure it directly. Normally, that means you have to strip off insulation.

 

[Nealh]

String no 6 reads 0.3v from that it appears to be toast.. Find the sense wire that gave you that reading and once you have removed the insulation etc, follow trace that wire to the defective string.
A direct reading will tell you/us if the string is dead, whether their is a wire break down or a bad weld connection with the bus bar.

 

[Carinagti]

String no 6 reads 0.3v from that it appears to be toast.. Find the sense wire that gave you that reading and once you have removed the insulation etc, follow trace that wire to the defective string.
A direct reading will tell you/us if the string is dead, whether their is a wire break down or a bad weld connection with the bus bar.

 

[Carinagti]

Hi,
Thanks again for your replies.

Yes, that string of cells reads less that 0.5v. All the spot welds seem sound as does the soldering.

What options do I have to get the pack working?

Ian

 

[Nealh]

Hi,
Thanks again for your replies.

Yes, that string of cells reads less that 0.5v. All the spot welds seem sound as does the soldering.

What options do I have to get the pack working?

Ian

They are knackered, kaput , fooked, no good .
With such a low voltage not even worth trying to recover them.
There is a method to recover unbalanced or low voltage cells at or above 2.8v or so but 0.5v is dead.

Do you know what cells they are make/designation/mah ?

You could try finding new cells of similar quality and welding a new string in it's place.

 

[Deleted member 4366]

Your only options are to replace them and the BMS.

There's one exception. If you didn't use the battery for a very long time before you experienced the problem, it could be that the BMS drained down that cell if that's where it gets its power from. It might be worth charging that group separately, which you can do with any 4.2v charger, or you can use any 5v USB charger if you monitor the voltage manually and switch off at 4.2v

 

[Carinagti]

Thanks for the advice. I will try to charge that string. The bike had been unused over the winter.

If that fails and I am unable to get replacement cells, would it be possible to reduce the overall capacity by removing one cell from each string and then using some of those to replace the defective string? (there appear to be 5 cells in each string.)

Ian

 

[Deleted member 4366]

If charging that cell doesn't work, it's probably best to buy a new battery. It could be the BMS that's faulty or it could be that a single cell had a fault. Unless you can find the cause of it going down, you might not solve anything by attempting a repair. You can get one for about £150, though I would always advise to get a good one if you can afford it.

 

[Nealh]

If it charges and works and seems to hold a charge then replace the bms plenty around just get one that fits.

If charging proves fruitless then pulling that string and using a cell from other strings will reduce the ah of the battery and the amp load of the strings. You need to know which cells are in use to see if it is a viable option, other wise the battery will sag under load eve more and capacity will decline quicker.

To get mobile best to think about a new battery then repair your old one and then combine both and use them in parallel so that both are user rather then have one just sitting about.

 

[spanos]

Hang on a moment

Why does the BMS need replacing. I’m not clear it’s knackered ?

If the bad string is at 0.3v then forget it, it’s beyond repair

Forgive me D8veh but I don’t understand why one would try the usb approach on 0.3v cell ? That’s a goner surely?

To Ian, can you tell me anything about the cells ? I could prob find you a decent enough match for 5 cells in my spares which you can have for the postage cost.

That way you could replace the whole string . Maybe the bms is buggered , maybe not

But if you got the time and inclination , happy to help

You’d lose time and a few pennies if it didn’t work out ?

 

[spanos]

In particular I’ve a bunch of Samsung 2600mah which are used but still serviceable . Happy to let you have the best of them. They came out of a 36v hailong where water ingress ruined 2 cell strings. I’ve been using a few of them to power my lights. They still hold good capacity (90% guesstimate but happy to properly capacity test a couple for you)

 

[Deleted member 4366]

The cells don't go down for nothing. Until you know the reason, can you trust the BMS?

Fitting a new one will require a new connector on the sense wires and possible some extra wiring instead of the two white connectors you have because most BMSs have all the sense wire connections in an incremental line on one connector, but you can get B0 to B10 (11 pin), B1 to B10 (10 pin with B0 taken from B-), B1 to B9 (9 pin with B10 taken from B+) and B1 to B9 (8 pin with B0 taken from B- and B10 taken from B+). Whichever it is doesn't matter because it's the same work to cut off the old connectors and solder on the new one.

 

[spanos]

The cells don't go down for nothing. Until you know the reason, can you trust the BMS?.

Gotcha but still, there are numerous reasons from duff cells to water ingress etc.

I suppose on the duff batteries I've repaired (which I appreciate is far fewer than you) it's never been the BMS at fault

But granted , changing both cells and BMS covers both bases.

However , that's easy to say that when you have your skills. A bms change is harder than swapping out a cell string Imho . Not difficult but need to be careful and methodical not to cause damage.

I guess I don't see the harm in doing cells first. And, if still an.issue, BMS after ?

Especially if I can give some matching (enough) cells for nought?