I am building a smartphone-powered laptop similar to this. The case is an old HP Pavilion DV4000 that I got. I would like to use its battery for this project, because laptop batteries already have some protection circuits built-in. But the mainboard's charging part is integrated so I can't use that. So how can I charge it securely? I think the best way is a buck (step-down) boost converter with a current limiting feature like this one to get the 19.5V input down to the 12.6V the 3S2P battery needs to charge. What I need to power with the battery are the following:
A 12V LCD controller through a buck & boost converter because the battery's voltage will go from under 12V to over that
A Teensy or Arduino for keyboard and touchpad through a buck converter
Will this work? Or will the battery overcharge because it might not be disconnected when it is fully charged? Also, is there a good way to measure the charging percentage of the battery with the Teensy so I can make an LED light up when it's fully charged and almost discharged?