Oldspot said:
PS the app is great, why they didn’t tell me it had Bluetooth I have no idea
They probably don't even know it has that. (they probably dont' know much, if anything, about what they sell or make (if they even make them themselves)).
Surely they should have been balanced
If the cells were new, well-matched cells, being used well within their specifications, they would stay balanced on their own. The BMS wouldn't need to do anything with them until they began to age over the years and began to drift away from each other in capabilities.
However, given the company's other deducible practices, these are probably not matched at all, in any way, other than possibly using the same model number cell from the same manufacturer (they might not even do that; it's a common practice in 18650 packs from some places, so could certainly be done with pouch packs like this one).
But what is more likely the problem is that one or both of the cells in that group have failed internally, and drained the group down to nearly zero volts. The BMS can't do anything about this kind of problem; failed cells would do this even if they were just sitting on the shelf by themselves not connected to anything.
It is also possible that the BMS itself has a stuck-on balancer on that group that it can't turn off. It can drain 150mA continuously according to the specs, so that means every hour it would take 150mAh out of a group of cells connected to a stuck-on balancer. Since the pack is almost certainly nowhere near 150Ah, and more like half that, we'll just use 75Ah as the capacity for the calculation: 75Ah = 75000mAh. 75000 / 150 = 500 hours. That's only about 21 days. So the balancer could've failed 21+ days ago, and been draining the group the whole time. Whenever the pack is on the charger, it would slow the draining down, but every hour it is not, it would drain the cell group at that rate. As it drains, the pack would behave worse and worse, cutting out more and more frequently, and sooner after starting out after a charge. Eventually it would empty it, where it's at now.
The same thing will happen with an internally failed cell, but there isnt' a simple pre-spec'd number for how quickly it will happen. It could take much more time than the stuck balancer, or it could take much less, depending on exactly what's wrong in the cell.
The main difference between the two is that a cell that failed internally can't be recharged anymore (even if you recharge it it will drain itself again, best case). A cell that's drained by the BMS *could* potentially be recharged and keep it's charge (but when drained as far as this one is, is internally damaged and probably will never work the way it did originally, and it's difficult to predict it's future behavior as you don't really know what specific damage has happened inside it).
The BMS, if stuck on balancer is the problem, will drain *any* cell that is connected to that balancer. If you have the tools and can get the part(s) you could replace the failed part(s) (usually a simple transistor).
