New LiFePO4-battery pack is underperforming

I recently bought this battery pack on ebay. The building quality is fine, the BMS is solid.
Specs: 48V40Ah (16s13p) BMS 60A

HOWEVER, after one month there are a few concerns:

• when I ride full throttle, the voltage sags from 53,8V down to 49V. The current drawn at that moment is 40A.

• fully charged the voltage is 58.5V, but a few hours later the voltage has dropped down to 53,8V

• I did a capacity test at 10A using a resistor and after 34 Ah the BMS cuts out the battery. The overall voltage at that moment was still 48V, so probably one string was at LV

My explanation would be that the pack is still not well balanced and therefore is underperforming.
What might be the other possibilities and what is the solution?
Thanks!

Edit: I add the information about the cells that were used to build the pack, according to the Chinese vendor.
> Battery Material LiFePO4, Model CDP-26650ID
> Nominal Voltage (V) 3.3V
> Nominal Capacity
> (Ah) 3100mAh
> Internal Resistance (mOhm) <15
> Life Cycles >1500
> Maximum Charge Voltage 3.65V
> Cut-off Discharge Voltage 2.0V
> 10 sec pulse discharge 5A
> Continuous Discharge Current 3.1A

i would say after a month it should be balanced.

It more sounds like you have a couple cells bad or not connected in nthe pack. If the cells are 3100mah then you have 2 duds or loose. When I test my LiFePo packs they don't shut down at 48v but more like 44v.

then again it's a Chinese made pack and 90% of them are over rated by at least 10 to 20 percent.

Check all the cell group voltage when they are at LVC, that will tell where your problem moght be.

40AH, that's a big pack. What are you using it on, bike or a cycle?

Dan

I am using it for a scooter: 2000W HUB-motor, 60 km/h, range should be at around 60 km.

It still seems low capacity for what it's rated at.

Check the cells, maybe a bad weld to a cell or two.

Good luck finding the missing 6AH.

Dan

.

Not underperforming that much, in terms of sag, or in terms of standing voltage of 54v after surface charge dissipates. Very similar to how my pingbattery behaves now at the same c rate discharge. Maybe a bit dissapointing that it doesnt hold more like 56v when newer, but from 56v-54v is only a few watthours for a lifepo4 anyway. Then you get into the big flat part of the discharge curve, at 50-54v.

Capacity could be better, and at 10 amps, .25 c you should see all of the 40 ah. But if it's a round cell pack with the cheezy spot welds, the chances that some welds popped while the shipper kicked the package around is near 100%. One single solitary cell disconnected in that huge pile of cells, and the whole pack is now 2.5 ah smaller capacity.

That's why we don't recomend that type of pack to anybody, and haven't for more than 4 years. The only exceptions are guys like cellman that do a better job on the tabs, and use A123 cells.

If you are lucky, it's just that out of balance. Not disconnected cells after all. Next time it's been discharged, look for the string that is the culprit, that is much lower than the others. Try to bring the low one back up even with the others with a single cell charger. Then do a normal charge and hope it's now balanced a lot better. At the top end, you might have to bring up a cell that is still at 3.4v, while the rest are full at 3.65v. Give it a try.

Or, put it on the charger, and leave it charging 24-7 for oh, about a month. No shit, 7 ah out of balance, it might take even longer.

I am hoping for the out of balance option. When charging, the BMS gets a little warm. That might be a sign that BMS is transferring some energy from one series to another, isn't it?
The capacity test was done two weeks after arrival. The voltage sag is still hapening, although a little better than two weeks ago.
The next time I charge the pack, I will measure the voltage and charge the series that are not at 3,65V yet.
Thanks for the advice.

That's funny because they advertise it as a super controlled environment, with a lot of testing on the cells:


That's probably false advertising :lol:


Have you tried to deal with them? Because if you put a negative review with only 21 reviews they'll directly go down to 94% satisfaction 8)

That's nice. Lucky you if you got the pack made from a matched set of high IR ones. 8)

Trust em all about as far as I could throw a truck, till they prove themselves to us. Remember when all the battery ads had the same pic of the pretty girl in the headway factory? Then they'd sell you a pack of 18650 cells that could not survive the shipping. It would come with 3/4 of the cells still connected.

Reading that ad, I never saw anything whatsoever that said what exactly they were selling you. Pouch cell, round cell? Not likely to be both. So which is it?
No indication of c rate, super cheap price, just a whole page full of red flags to me.

The solution is on its' way: I drove the scooter yesterday until LVCut-off by BMS. Then, I measured all groups: the first one (the one between the ground and the first group) was indicating only 2.55V, the others all showed a healty 3.28 or 3.27. Is it normal that the ground is not connected to the BMS by the thin wires?

I charged the whole night and this morning I measured again: only 3.28V for the first group and 3.75V (a little high??) for 13 of the others. The second group showed 3.61V and the third group 3.51V.

I conclude that it probably is not a capacity problem of the first group. If that would have been the case, the voltage of the first group would have ended up higher, because only the connected cells would need a refill.
So, now I am charging the first group with a Turnigy Accucel at slow pace (2A). I expect this to take a while. This evening I plan to charge the second and third group, that should take less than an hour.
Is it a coincidence that the first group was out of balance? I remember reading that it is often the first group that fails.
Thanks

Added: just checked the charging: already 12,5 Ah charged, charging conditions still at 3,5V and 2A. This group was way out of balance.
I wonder how this could happen. Might there be some kind of short in these cells??

Last request for advice: I charge the one group of cells all day long. Put a large amount of energy in it and found that the voltage does not reach 3.65V. Forced the voltage to increase up to 3.75V by increasing the current. Two hours later the voltage dropped, without applying any load, back down to 3.33V.
I am now pretty sure that this group contains a short cell. Tomorrow I plan to open the pack, locate the cell and replace it.
Is this the right way to tackle the problem?
thanks

nope. you have to test capacity by doing a bench test. using a wattmeter or ammeter.

These ebay packs seem to always have issues. Its unfortunate because its turns people off to electric vehicles.. I would post a negative review on eBay if they wont let you return it.

What's the name is the seller?

Jason27 said:

These ebay packs seem to always have issues. Its unfortunate because its turns people off to electric vehicles.. I would post a negative review on eBay if they wont let you return it.

Click to expand...

Returning the pack to China and being sent a new one or repaired one would cost as much as a new pack. Besides I'd rather do this myself. Opening a pack and repairing it is the best formation yu can get.

Jason27 said:

What's the name is the seller?

Click to expand...

See my first message

whatever.

this is why everyone hates the Vpower packs. bad welds and loose connections.

dnmun said:

nope. you have to test capacity by doing a bench test. using a wattmeter or ammeter.

Click to expand...

I am not sure what you mean. Do you mean to test each cell of the first string individually? Anyway, I did a capacity test of the pack as a whole as described above: the BMS cuts off after much less than the nominal capacity of 40 Ah because the first string has low voltage. This way I was able to locate the problem and therefore, I plan to open the pack and locate the cell that causes the first string to leak energy. I just hope that I don't have to isolate each and every cell of that first string.

This morning, I found the first string still at 3.33V. The others were at 3.6V. To me it is obvious that the first string has a short cell that slowly brings down the voltage and the capacity of the whole pack. I guess that after I replace that one cell, the problem is solved.
Anyway, I will ask the chinese seller to send me a few cells as a compensation.

fivari said:

I am not sure what you mean. Do you mean to test each cell of the first string individually? Anyway, I did a capacity test of the pack as a whole as described above: the BMS cuts off after much less than the nominal capacity of 40 Ah because the first string has low voltage. This way I was able to locate the problem and therefore, I plan to open the pack and locate the cell that causes the first string to leak energy. I just hope that I don't have to isolate each and every cell of that first string.

This morning, I found the first string still at 3.33V. The others were at 3.6V. To me it is obvious that the first string has a short cell that slowly brings down the voltage and the capacity of the whole pack. I guess that after I replace that one cell, the problem is solved.
Anyway, I will ask the chinese seller to send me a few cells as a compensation.

Click to expand...

I doubt you have a shorted cell. It's much, much, more likely to just be crappy quality cells, probably with a few cells in the pack that are way under capacity, or maybe even open circuit. A voltage test isn't going to tell you much, if anything, about the state of charge of each cell group. You need to do some testing under load, plus ideally doing a capacity test on each cell group. My guess is that you'll find that the pack capacity isn't 40 Ah (it's pretty common for the vendors of these things to just openly lie about capacity) and you'll probably also find either some dead (open circuit) cells or some crappy internal connections (probably both) if you take the pack apart.

Unfortunately there are a LOT of nasty cylindrical cell packs around, more often than not of poor internal build quality. Dogman was spot on earlier with the comment earlier about not recommending those packs.

Good luck with getting recompense from the vendor, many of these Chinese ebay sellers are just buying up unknown quality surplus cells in the big electronic street markets in Guangzhou or Shenzen, assembling them into packs, selling them on ebay until they get caught out, then starting the process again with a new ebay ID. There are a few Chinese based battery sellers who have built a solid reputation, people like Li Ping and Cellman, for example, but there are also hoards of rip-offs to catch the unwary. The unscrupulous vendors rely on the high cost of postage and the absence of any consumer protection laws for international sales, so can get away with what we'd call fraud. I doubt the photos on their ebay advert have anything to do with their batteries, either, they were probably just copied from a reputable manufacturers site.

I have to agree that what seemed to be a good deal, turns out to cause some troubles.
I am just trying to work me out of this and I recognize that it will require quite some energy to overcome this.

But apart from that, is there anything wrong about the deduction I made?
Because the pack voltage drops soon after charge from 58.4V down to 54V
Because this one cell group shows low voltage after cut off,
because I was able to charge this one cell group an additional 22 Ah after fully charging the pack (a lot of which was probably lost right away because of the leaking)
because the voltage came down soon after fuly charing this first cell group,

==> i conclude that this one group has at least one cell shorted. What else could cause these three phenomena to happen simultaneously?

fivari said:

I have to agree that what seemed to be a good deal, turns out to cause some troubles.
I am just trying to work me out of this and I recognize that it will require quite some energy to overcome this.

But apart from that, is there anything wrong about the deduction I made?
Because the pack voltage drops soon after charge from 58.4V down to 54V
Because this one cell group shows low voltage after cut off,
because I was able to charge this one cell group an additional 22 Ah after fully charging the pack (a lot of which was probably lost right away because of the leaking)
because the voltage came down soon after fuly charing this first cell group,

==> i conclude that this one group has at least one cell shorted. What else could cause these three phenomena to happen simultaneously?

Click to expand...

Unfortunately. many of the cylindrical cells that find their way on to the Chinese electronic street markets are cells that have failed quality testing at the manufacturer. There are also some ebay vendors who aren't very good at assembling these cells into packs; we've seen many instances here of people with packs like this where the inter-cell connections are poor, where balance leads have fallen off and where the pack has been poorly secured inside the duct tape, allowing the cells to move and break internal connections.

It won't be a shorted cell causing the problem. If it was, then the pack would probably have gone up in flames as the other cells in that group discharged through it. It's very rare to get a shorted cell without it having been mechanically damaged, cells almost always fail by going open circuit, or by just not having the rated capacity. It's also quite possible that you have a few cells in the pack with a high self-discharge rate. You may also have a problem with the BMS, perhaps it isn't limiting cell voltage properly for each cell group.

If you have a few low capacity cells in the pack, then they will discharge more rapidly, and charge more rapidly, than the others and so will show voltage differences to the others in the pack. In all probability you'll find a few open circuit cells, together with perhaps a few that have a lower capacity than the others. You may be able to rescue the pack with new cells, but it might be a challenge to take the thing apart and find the dead cells, then fit new cells and get it balanced up again.

Thanks Jeremy.
Reading your answer, I realise that I used the wrong terms: shorted cell leaking energy. You are right, I should have called it a cell with high self discharge rate. Sorry for that. I am only beginning to learn about these topics.

I will keep this forum informed about the outcome of this problem.

fivari said:

Thanks Jeremy.
Reading your answer, I realise that I used the wrong terms: shorted cell leaking energy. You are right, I should have called it a cell with high self discharge rate. Sorry for that. I am only beginning to learn about these topics.

I will keep this forum informed about the outcome of this problem.

Click to expand...

Although cells with a higher than normal self discharge rate are a possibility, they won't be drawing much power from the pack, if they were then those cells would get hot. In practice you will get cells that discharge their "surface charge" (the bit from the hot of charge voltage to the working voltage) more quickly than others, but this doesn't have much energy and so isn't normally a major problem. It's rare to get a cell that self-discharges below the normal cell working voltage, but common to get cells with a low capacity, open circuit or high internal resistance.

If you take the duct tape off your pack we can probably advise better on the way forward. If it's a typical multicell cylindrical pack then what you'll probably find under the tape is a big stack of cells with spot welded tabs interconnecting them into cell groups. Repair will mean isolating the cell group with the low capacity (not necessarily the lowest voltage group) taking it out of the pack, splitting the cells in that group and checking for which ones may be open circuit, then rebuilding the pack by spot welding the new cells back together. You may be able to make a soldered repair, but this needs care, as it's possible to damage the cells be directly soldering to them and letting the cells get too hot. It is possible that the pack you have has been soldered together, in which case this is a possible reason for some cells to fail.

It doesn't sound like simply popped spot welds this time. If it was, would that first cell group charge up faster with less cells to charge?

Often, one cell group runs the bms. Stored lifepo4 packs often destroy the first cell group by overdischarging that group. It can also happen if the shipping is slow enough, taking a month or more. So that seems like a plausible possibility.

Did you have to plug in the bms when it arrived? If so, then that theory is also out the window. Once you dissect out the bad cell group, replacing it should help the pack. It may still tend to self discharge to about 54-55v. That's normal once a pack has some wear on it.

But it may never put out the rated capacity even when you get it fixed. Pics of cell testers do not prove your cells are the ones in the good batch.

Thanks Dogman.
The shipping did last only one week. The BMS was connected to the batterypack.
The voltage drop yesterday, happened without the BMS being connected.
Opening the pack this evening will hopefully reveal the reason for its behaviour.

Well so much for that theory. It could be the bms draws more than it should be from the cells that run it, or perhaps they had it stitting a month before they shipped. Pretty sad they still haven't learned to unplug the bms after the battery is tested. This problem has been known about for about 4 years now.

Unfortunately the bargian lifepo4 packs reputation for being a hairball of problems continues. It's been pretty much standard for a V power hong kong pack to put out 15 ah if it's a 20 ah pack in the past. Not that others we trust haven't had bms issues, or cells tested good spontaneously go bad fairly soon. For example, when odering batteries from Hobby King, we just expect and order enough to cover a 10% fail rate on the RC lipo packs. So you want 8 you buy 9. Much harder to deal with in an assembled pack. One reason we recomend pingbattery is that he will send replacement chargers, bms, and cells when needed without a paypall fight.

As promised, I keep you informed about the opening of the pack.
First comment: apperently the pack was built to never be opened again: a lot of hot glew, dubble duck taped, hard to open. But I succeeded without any damage to the wiring nor the cells.

Whereas the vendor told me that the pack consisted of 16s13p of 3100 mAh cells, the truth was 16s12p of LFP26650E which are according to internet datasheet 3000 mAh cells. I opened a paypal dispute in order to be paid back 10%. He refused using the argument that he selected cells that were tested 3300 mAh. Since the capacity test I did showed 35.7 Ah, I don't believe him.



Second mistake was the connection between the different cell groups: only two thin tabs of 0,15mm x 10mm have to carry up to 60A. This is, in my opinion, not enough. Therefore I added tabs on every cell by soldering as shown on the picture below.



Since I want to be able to measure each cell group individually I connected another set of wire to the batteries. This give me the possibility to charge a group seperately if necessary. I could have used the BMS wires, but in order to use these, I have to open the BMS, deconnect the connector. After doing this 20 times, the BMS-box would be broken.