How Lithium Polymer Batteries are Made
The protection circuit prevents the LiPo cell from being over charged (bad things happen when the cell is pushed past 4.3V), under charged (the cell can be harmed if pulled below about 2.8V), and short-circuit (shorting the positive and negative terminals together). If any of these triggers occur, the protection circuit very quickly disconnects the cell from the terminals.
Above, we finally have a formal battery with taped-in protection circuit (the yellow kapton tape) and positive and negative terminal wires (sans connector).
Dozens of different models of batteries were all over the factory. Above was a larger battery for a tablet.
I found this battery particularly intriguing. I've never seen a two part battery. Perhaps the dual cell allows for the battery pack to bend? Large cell goes under the keyboard and small cell behind the screen? I couldn't get more information about this one, but it does make me think about differently about battery options.
Each different battery model had their own line of employees.
Multiply by many lines.