Great idea and great rendering! However keep in mind that big tech doesn’t like it, as they want to extort your money by selling overpriced battery packs. As you may already know, buying individual cells and DIY assembly is way cheaper. Many laptop battery manufacturers has kill-switch built-in the BMS’s program, which will brick the pack upon cell removal. The purpose of this is to deter DIY cell replacement, railroading users into buying overpriced pre-built battery packs. Some of them even have encryption on their BMS. Advanced users have little problems getting around this as they know how to use interface modules like the EV2400 although they might have a hard time cracking the encryption.
To make matters worse, I have bought many of these DIY power banks, all but one of them died out of circuit failure instead of worn batteries, so quality control of these things are a must.
There are two approaches, either connect the battery pack directly using a BMS identical to most of laptops, or using a DC-DC converter and pack it into 20V5A PD. In the former case an interface module with USB connection is required for the user to set voltage limits and capacity. In the latter case the battery pack is less efficient as it converts the voltage to 20V, send to the laptop then the laptop’s PSU converts it again. In either case, diverting power is required as the laptop can’t accept multiple inputs at the same time. Power management is hard to deal with. You might want to research the topology of the laptop’s and the controller program as the ELBP should be designed in a way. For example the computer should use the ELBP first, then the internal battery, charge the internal battery to the possible max rate nd only use excessive power from the source to charge the ELBP. It may or may not be possible to get 240W if the ELBP and the laptop is charging at the same time. A USBC PD is required either way as for charging and using the ELBP as a power bank.
Good luck with your module development!