So based on the much lower voltage than it should be (55.6v - 47.5v = 8.1v), and it's rapid dropping down to that voltage after taking off the charger, it probably points to at least three cell groups that are not charging up like they should, since each group in this 16s pack should be 3.475v or so if the pack is at 55.6v.
It's unlikely that all groups are failing to charge fully, so probably at least three are staying pretty far down, because they self-discharged during the time it wasnt' used, so they're far out of balance now--so far that the BMS could take days or longer of charge/discharge to balance the pack.
That is assuming that the self-discharge is slower than the balancing/charging cycles are--if the self-discharge is happening faster, then it will never balance.
The best two tests/fixes to do without disassembling it are measure the group voltages at the balance connector, and then to single-cell/group charge up the low groups to 3.65V. Then stick the whole thing back on the charger and let the BMS balance out all the other groups, if necessary.
After that, do a pack capacity test by running the bike till cutout, and noting down usage if you have a wattmeter (IIRC you don't), or at least noting mileage and comparing it to previous range capabilities. If it's not different then it can continue to be used normally but with the caveat that it needs to be put on it's charger frequently to prevent imbalance.
It is possible also that somethign has failed in the BMS, but it didnt' have any problems with the BMS dragging cells down while it was here, even over multiple weeks of sitting there. So it's less likely that the BMS is causing it than that more cells are simply self-discharging.
A simple way to test which is causing it is to first get the pack charged back up correctly and fully, then disconnect the BMS balance leads, which will remove the possiblity that the BMS is discharging things when it shouldn't. If the pack still gets out of balance sitting there for up to the same amount of time it did before, it's the cells. If it doesn't, its the BMS.
But all the troubleshooting I can think of involves testing at cell-group level first.
If you don't want to do it or cant', there should be someone you can meet up with in the area that can help out. You're in south Florida... Sangesf is also in that half of the state; dunno what city. Jay64 is on the eastern middle of the state, in St. Petersburg, as is SilverSurfer. Floont is in Tampa. Bluebottle2 is in the state; dunno where. Drewjet is in Orlando. I know there are others that have posted that they're in Florida, but I can't remember who or where just now.
I can work on it again, too, but youd' ahve to ship it here, and back.
First I really think just checking the balance lead voltages and single-cell-group-charging up the low one(s) would be the best first step, and might be all it needs for now.
