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Lithium ion packs without balancing

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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:

--- Quote from: BradC on February 12, 2022, 11:32:28 am ---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?

--- End quote ---

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:

--- Quote from: uer166 on February 13, 2022, 04:25:09 am ---It seems that the cells you have in there may be recycled, and didn't start from same batch?

--- End quote ---

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.

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