Newb question: Attempted to connect inverter to battery resulted in bright spark and some melted ring connector. Why?

Background: I don’t know much about electricity. I have a 14s5p battery that Id connect to my inverter (rated to handle 40-60w DC). The few times ive used the inverted it has never been an issue (always keeping battery under 58v). My battery would connect using Anderson connectors, id turn on battery, then id turn on inverter. Worked perfectly fine.

Today I wanted to test something so I took some old Chinese cells from a 'dead' 48v pack (was 13s) and made a '12s battery pack'. Each cell measured at 4.01v (give or take 0.01v) and the pack measures a tad over 48v. I attached the positive wire of the inverter to the battery, then the moment I touched the negative wire (black wire) to the other end of the battery there was sparks, bright light, and some metal of the ring connector was melted/charred. I don’t know what I’m doing, any idea what happened?

On a side note: Currently the inverter is also dead (does not even work on my normal battery). I cracked open the top and there are six fuses (3x20 and 3x10). Hoping if I replace the fuses it will fix the inverter, but ill have to order some fuses.

Where did you buy your inverter? A mystery back alley ebay purchase from some random seller?
Sounds to me like since your inverter is toast, the inverter was whacked out since you say your chinese grade F cells came from 0V to 4.xxV and were all within 0.01V that might have been a problem to, perhaps not all the time does a batteries voltage go down when problems occur but the inrush of a dropping cell putting pressure on what you think are healthy cells and the avalanche begins. Could have killed a string, increasing voltages elsewhere. But who knows really unless you methodically used your digital multi meter at every step of the way, like measuring the dead battery, weening them back in a very slow manner like we're talking 1 to 10mA aka 0.01A for days on end because if you did a normal charge on them from 0V you would have stressed them out big time, charging pack at say 5A, putting say 1A into each 5P. But I'm just guessing.

The inverter is a 2000w pure sinwave inverter. No clue about its quality (their 24v inverted combined have 106 votes for 3.9 rating average). From amazon: https://www.amazon.ca/gp/product/B074C8Z1M3/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

The Chinese cells definitely might have been the cause of this. That being said each cell was never 0v. In fact they were 4.1v in July 2019 (the 12 'good' cells, only one cell of the 13s pack was shot), and since then they sat idle and only dropped to 4.0x. Cells are the ones I posted in this thread: https://endless-sphere.com/forums/viewtopic.php?f=14&t=101454&p=1499483

So those cells have not been charged in over 2 years. In fact expect for maybe a little 4.2v 750mah USB charger (to charge all 12 cells individually) I have no way to charge these items.

I managed to fix the inverter with fuses. Seems I had reversed connected the wires. I also purchased a 25w 30 ohm resistor to charge the capacitors. Even with the resistor there was still a little bit of sparks, but nevertheless I was able to connect the battery to the inverter.

Connected a 61w load~. On load the voltage went from 48.2v to 47.6v. Battery hit 44.9v (45.5v after load removed) and I pulled 140wh. Each cell was about 3.81, expect one cell was 3.69v.

Even tho I really only need about 200 wh for my purposes I'm considering just of discharging these a tad more then recycling them. They kind of make me nervous.