Lithium ion packs without balancing

[BradC]

I have a couple of USB power banks here. They are set up in a 2P4S configuration using LG 3200mAH 18650 (LGGBMH11685) cells. Given the cost of the units, I suspect the cells are not exactly "A" grade and I've just had one where one pair of cells went significantly out of balance.

I stripped it to inspect and have manually topped that pair back up, so we'll see how that goes.

The pack is protected by an SH367103 which protect only. With the out of balance pack, when the lowest cell reaches cut-off it switches off, and when the highest cell is fully charged it stops charging.

This is roughly a 95WH bank, and a discharge test showed the dud was down to about 40WH. The second one is sitting about 77WH, so it's obviously slowly going out of balance also.

To quantify, when fully charged the cell pairs were - 4.16v 4.17v 3.80v 4.16v. Pick the odd one out.

I don't have room in the device to put a balancer, and I'm not really considering adding an external balance port.
I've had power tool packs that didn't have balancers in them that have lasted hundreds of cycles and are still going, so I know it's possible.

I suppose what I really want to know is if I were to re-cell it with A grade Panasonic or Samsung cells, is it likely to stay better balanced? Anyone have any experience with packs like this?

[uer166]

I suppose what I really want to know is if I were to re-cell it with A grade Panasonic or Samsung cells, is it likely to stay better balanced? Anyone have any experience with packs like this?

Sure, there is nothing wrong with BMS without balancing as long as the cells are good, especially when only 4S (I used to think otherwise). E.g. I have a scooter with me with a >5 year old 10S pack with <30mV of unbalance when charged to 4.1V. It seems that the cells you have in there may be recycled, and didn't start from same batch?

[BradC]

It seems that the cells you have in there may be recycled, and didn't start from same batch?

Possibly. Looking at the discharge profile on both unit's even when perfectly balanced I'm seeing ~77WH (It's "supposed to be 99.16 according to the label) which calculates out almost bang on 2600mAH rather than the 3200mAH they are "supposed" to be based on the markings. So I suspect they are counterfeit cells. They all have the same date and batch code. I'll get a good look at them when I strip it to re-cell.

[Siwastaja]

Or, the cells are just fake.

I'm not sure about the existence of "B grade" cells. Proper manufacturers don't want to tarnish their names by releasing crap on purpose.

[BradC]

B Grade : https://evreporter.com/lithium-ion-cells-a-grade-vs-b-grade/
I've also seen quite a few advertised as "B" grade, so in some way shape or form they probably exist.

I did a discharge test on the fully charged and balanced 2P4S string and got 5250mAH. So yeah, 2600mAH cells labelled as 3200mAH.

I'll order a new set of 3350mAH cells from a local and reputable supplier and see how it goes from there.

[Siwastaja]

I've also seen quite a few advertised as "B" grade, so in some way shape or form they probably exist.

The manufacturers do not advertise the cells to the general public. You need to buy in millions and qualify as a customer. All we mere mortals get are from grey market. It can be any mix of A, B or whatever grade, and fakes.

If you can buy known B grade from the actual manufacturer, I'm sure there's an NDA in the process, and the cells are still good for use, and the suitability is assessed with the customer.

[BradC]

The manufacturers do not advertise the cells to the general public. You need to buy in millions and qualify as a customer. All we mere mortals get are from grey market. It can be any mix of A, B or whatever grade, and fakes.

If you can buy known B grade from the actual manufacturer, I'm sure there's an NDA in the process, and the cells are still good for use, and the suitability is assessed with the customer.

Not disputing any of that. I'm sure a Chinese company making something like a USB power bank and wanting cheap cells can tap the B grade market. Anyway, given the obvious specification disparity with these they're not B grade but counterfeit.

[BradC]

Righto. Replaced cells with 8 x Panasonic NCR18650B. Bloody expensive at $15/ea + GST, but at least I know they are genuine and I'll hopefully only do it once.

If this works I'll do the other one (I have 2 of the same power bank).

[BradC]

My gear arrived to allow me to do a load test on these power banks.

I have 2 power banks. Unit 1 was purchased in mid 2020 and failed (due to a mechanical failure of a connection) in early 2021. It was "replaced" by Unit 2 in mid-2021 and then repaired. Unit 1 saw quite a good heavy and deep cycle load before it failed. Unit 2 hadn't been used anywhere as much when I tore it down to find the pack badly out of balance.

The tests were done using a USB-C PD trigger supplying 20V and loaded with a SkyRC BD200 at a configured 25W constant power load (turned out to be an average of 22.7W) until the battery shutdown the output.

Unit 1 (un-touched) tested to 88.08Wh (4.40AH at 20.02V) and Unit 2 (with the new cells) tested to 96.26WH (4.78AH @ 20.14V).
Based on the rated discharge curve and capacity for the new cells, that gives an output efficiency around 96%. Nice.

Back calculating, that puts Unit 1 batteries at about 3050mAH. I hooked up the cells I replaced from Unit 2 to an iMax B8 balance charger and gave them a full cycle with a discharge load the same as performed on the other sets and they measured out to be 2700mAH rather than the 3200mAH they are specced at. That gives roughly 80WH and at 96% efficiency an output capacity of about 77WH, which is roughly what I measured with my previous imprecise methodology.

That, coupled with the disparity in characteristics resulting in the pack going out of balance indicates they are perhaps "not genuine".

Unit 1 is considerably older and has far more cycles on it than Unit 2 and it still showing a credible capacity, so it'll be interesting to monitor them both over the next couple of years to see how well the new Panasonic cells stay in balance as a whole.

This is the unit : https://www.auspowerbanks.com.au/product/apple-macbook-power-bank-87w-pd-with-magsafe-gen-2/


Great unit, but a complete bitch to get apart.

[BradC]

Back calculating, that puts Unit 1 batteries at about 3050mAH. I hooked up the cells I replaced from Unit 2 to an iMax B8 balance charger and gave them a full cycle with a discharge load the same as performed on the other sets and they measured out to be 2700mAH rather than the 3200mAH they are specced at. That gives roughly 80WH and at 96% efficiency an output capacity of about 77WH, which is roughly what I measured with my previous imprecise methodology.

That, coupled with the disparity in characteristics resulting in the pack going out of balance indicates they are perhaps "not genuine".

12 month update. The re-packed unit appears to be still well balanced and I still get > 95Wh out of it. I'm not going to pull it apart to check each set of cells this year as it's not easy to get into without risking damage and requiring more glue to put it back together again. I might do it next time around.

After the tests last Feb, the removed cells were all charged to ~3.85V and left on the shelf as an assembled pack with only a balance lead connected to them. A check this morning sees 3 sets of cells all > 3.8V and one set at 3.5V. So a marked difference in self discharge over a 12 month period. Just for giggles I'll do a balance charge and discharge test again.

[BradC]

12 month update. The re-packed unit appears to be still well balanced and I still get > 95Wh out of it. I'm not going to pull it apart to check each set of cells this year as it's not easy to get into without risking damage and requiring more glue to put it back together again. I might do it next time around.

18 month(ish) update. The first unit (still with the original "LG" cells) was down to about 33Wh total. "Fully charged" and then dismantled I found the 4 pairs at :
- 4.17V
- 4.17V
- 3.56V
- 3.76V

+/- 0.02V

So not in balance at all. I've manually balanced it up, but I've also found (thanks Aliexpress) a 4S balance board that I might just be able to shoehorn in (that or 2x2S balance boards). It hasn't seen much use in the last 6 months and I suspect while the cells are all pretty similar in capacity I reckon 2 pairs suffer a higher self discharge/leakage.

Unit 2 with the replacement Panasonic cells has been used heavily for the last 18 months and I finally figured out how to crack it open without breaking stuff. Pairs :
- 4.17V
- 4.17V
- 4.16V
- 4.17V

+/- 0.02V

It's seen much more use than Unit 1 with the LG cells now and is still nicely in balance. Happy with that.

I'll see if I can fit a balancer to Unit 1, and if so I might just do it to Unit 2 when I next check the cell balance.

[mikerj]

I suppose what I really want to know is if I were to re-cell it with A grade Panasonic or Samsung cells, is it likely to stay better balanced? Anyone have any experience with packs like this?

Yes, the better the cell matching the longer the pack will remain balanced.

Most laptop batteries, at least on the older ones I have repaired, have no cell balancing.  A fairly common cause of laptop battery capacity dropping to unusable levels is cell imbalance, and opening the pack and balancing the cells can restore a significant amount of lost capacity (for a while at least, they invariably start to get unbalanced again after a number of charge/discharge cycles).

[mzzj]

I suppose what I really want to know is if I were to re-cell it with A grade Panasonic or Samsung cells, is it likely to stay better balanced? Anyone have any experience with packs like this?

Sure, there is nothing wrong with BMS without balancing as long as the cells are good, especially when only 4S (I used to think otherwise). E.g. I have a scooter with me with a >5 year old 10S pack with <30mV of unbalance when charged to 4.1V. It seems that the cells you have in there may be recycled, and didn't start from same batch?

Not huge fan of battery packs without any balancing option as there is no easy way to predict self-discharge in future.
1-2 year is most probably ok, 5 years probably you need quality cells and 10 years gets to lottery ticket category. 
Any temperature gradient over the battery pack is also going to drastically worsen the differences in self-discharge rate. (ie charging electronics on the other end of the 4S pack)

[BradC]

Thanks to the wonders of Aliexpress I found a 4S passive balancer that I could actually make fit into the power bank. These use a HY2213 and 43 Ohm resistor across each cell. It's crude and a bit wasteful, but it also has an unintended advantage in that once the pack is fully charged it drops each cells voltage down to ~4.17V. That's much nicer than leaving them full at 4.22V as the charging system seems to.

After the last set of voltage measurements I had "manually" balanced the pack and started from :
- 4.17V
- 4.17V
- 4.05V
- 4.10V

Of course that's pretty far out of balance. Plugging it into a standard USB port to charge sees about a 7W charge load, which overwhelms the balancing circuit. Each charge cycle saw incremental tiny improvements but it was going to take a *lot* of cycles to get it back in balance.

Having had a think about it, given each balance shunt can do about ~100mA, what I needed was a way of reducing the charging current to make it large enough to actually charge, but low enough that the balance shunt will prevent the battery hitting the 4.22V and tripping the protection circuit to terminate the charge.

I have some USB sockets on proto-boards, so I hooked one up to my linear bench supply set to 5V and 350mA. The power bank happily fired up and started to charge. The already full cells cycled between 4.2 and 4.17v as the shunt tripped in and out, and the remaining cells (slowly) charged up. These are ~2800mAh cells and are in a 2P4S configuration. We were going out for 24 hours, so I let it run and came home to a beautifully balanced pack.

I'll now run it through a couple of test cycles charging and discharging with normal use to see how well it works. That runs my Macbook at between 15-60W and charges at 45W.

I know in a worst case scenario I can put it on a current limited PSU for a day or two and it'll come back into balance. Best case I get another couple of years out of these cells. Worst case I've spent $2 and some time on a 4S balancer that gives no real benefit. Either way I've learned something.

[BradC]

So that was a bust. Cycling wasn't an issue but the difference in self discharge over a couple of weeks was enough to quickly overwhelm the balancer.

I've ordered some new cells from Aliexpress. Fingers crossed they're not counterfeit. I'll given them a thorough test and if they are I can always buy them locally for twice the price.

[BradC]

4 Year update (or thereabouts). The cells I bought from Aliexpress were good as individuals, but not particularly well matched so I ordered genuines which I've only just got around to looking at installing in bank 1. When I got them I charged them all to 3.8V and put them in a box. 3 years later there was about a 10mV spread across the 8, so I'm happy for them to go in the unit.

I pulled bank 2 apart for a checkup. It's down to 91Wh from just over 96Wh when they were new back in 2022. It has seen a fair bit of use and I had intended to retrofit it with one of the passive balance boards. All cells still in balance to within 4mV which I am mightily impressed with and as such I won't be adding the balance board.

I just need to dig out the spot welder now and sort out the pack for bank 1.

I've salvaged and cleaned up the 16 original cells from both banks. They all vary in capacity and internal resistance by up to 20% across the population. I'm doing a month long self discharge test to weed out any obviously bad and then they're going in one of those cheap Chinese 16P1S battery banks. I figure with all in parallel there's no criticality to the cells. These things can supply up to 18W PD, so perfect for the Mrs to charge her old macbook air or phone on the go, plus it has a wireless charger on the lid. Might as well give the cells a second life.

The Aliexpress cells wound up in a 8P1S shell for my son, and they've been great. He's clumsy, so they're on their 3rd shell. I don't buy the expensive ones

Touching on the original topic, I'm incredibly impressed at the longevity of a pack without balancing when using well matched cells. This power bank has really blown me away. I'd have expected to be needing to give it a bit of love after 4 years, but it's still bang on. I recently stripped a 2009 vintage Ryobi battery for a check-up. That's a 2P5S bank and there is no balancing. It's down to 1.9Ah from the original 2.4, but all cells are within 10mV. It has not been treated well, so again impressed.