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Ah, I was just about to link to some of the threads asking about using other batteries. Several people have made threads or asked in existing threads, but once they find out what’s involved, that’s usually the end. Couple of them here & here. I don’t recall anyone following through, or even looking into the EC’s code to just see how easily it can be changed for an alternate batteries’ BMS. This being Framework, the EC code is open source and on their github.
Do you know what BMS chip the ThinkPad T60 battery uses? You’ll need to read the datasheet on it, if you haven’t already. The T60 is from 18 years ago.