....I did tear it down in case anyone wanted to see the inside:
Thanks for the teardown vid Bean, you saved me some time! I've been building my own lithium packs for a decade, but the price of these got me to give them a shot. My initial findings are very similar to yours, but mine were even more out of balance.
The BMS's on the better one of mine hits the HVC at about 13.85v and then sends the voltage sky high and bounces back and forth like you have found. Monitoring the sense wires of the BMS shows that one cell pack hits almost 3.9v (ouch) before the HVC stops charging and disconnects. The other cell packs are still under 3.4v at that moment....so there must have been no attempt at top balancing. It is also impossible to charge correctly with a store bought charger, as they are usually either 14.4 or 14.6 volts. I'd like to use one of my multi-bank chargers for these, in series, but the lowest setting on them is 14.1v (gel) and even that is too high for the HVC.
I would not trust charging these in series with a single 48v charger because as soon as one cell hits the HVC it will not only stop charging that whole battery, but all 4 batteries! And we all know charge controllers DON'T LIKE to be disconnected from the battery, especially under a charge load. The back and forth on and off can't be good. For anything. Right now I'm using a power supply set to 13.7v, or homemade chargers made from controllers that have very customizable charge settings (one is the MSB version of your Mr. Pwr.). I will not worry about charging multiples in series until I can get one battery to accept a reasonable charge setting without triggering the HVC. I do want to order more of these batteries for my 48v needs but not unless they can be charged without hassle. The BMS should be a safeguard, NOT a charge controller nanny.
The good news is the cells themselves seem good, and the build quality is good for the price. One of mine will hit about the advertised capacity, but only if I get some cells into the knees on both ends (Overcharging some cells on the charge, and going too low on some cells on the discharge). This is probably why the warranty is only 2 years, as cells don't like to be treated like that.
I am willing to bet many of these problems can be fixed by top balancing and replacing the BMS. That is easier said than done because of the way the pack is assembled. I'm going to attempt it anyway and report back with my findings. There is a chance that junk or used cells were used, in which case balancing may not be possible. I haven't cut open the cell packs yet to see what is inside. It does seem that the cells do settle into a tighter voltage range after some discharge, so my guess is the builder just bought a bunch of these cell packs and hooked them in parallel to balance them at the same voltage, at whatever voltage they were at the time.
TLTR; Probably a great buy for the money for someone who knows how to build (rebuild) a pack and doesn't mind doing some work. The cells seem ok and the case itself is great (sealed but serviceable, they should all be like this!). At this time I would not recommend them to a total lithium neophyte who just wants a plug and play battery...it's not there yet. Wait for the verdict on balancing attempts.
PS. This is maybe the most important part: The Jim seller is a Vet, and he's the best seller I've ever dealt with in my decades of ebay use. He ships faster that you could imagine and he will answer your questions or concerns very quickly. I have no doubt he will stand behind the battery 100%. I'd rather buy from this guy than play around with Ali, for any price.
PPS: I hope he has more in a few weeks. If I can solve the balance problems I will want a few more of the identical batteries for my higher voltage projects. The problem with these china mystery batteries is one might have 80 cylindrical cells, then you order another a month later and they have 4 pouch cells. That is my biggest concern with not buying enough batteries at the same time....