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.