(I used to design and build electronics - HiFi, automation, digital systems up to and including CPU-based controllers - so I have a good understanding of the basics. I just stopped being involved in the field about 15 years ago, so my knowledge of what's currently available in terms of components is quite foggy.)
The device is a controller for a scientific instrument. Removing batteries for charging every once in a while will be cumbersome. Plus, I want seamless transition between batteries and AC (well, not AC per se, but external DC at a higher voltage).
The battery needs to be 12 V or a bit higher. In terms of how much energy it needs to store, something like a small laptop battery should be enough.
So, basically, I need to integrate the battery charging circuit with the device. The battery will not be (easily) removable. The device will provide a port for an external DC supply for charging. Switching between internal battery and external DC must be seamless. It must handle automatically all the chores related to battery charging (regulate the current, stop charging when fully charged, stop the device when fully discharged and there's no external DC, etc). Indicating the current charge level would be nice.
I'm pretty sure I could use an actual laptop battery for this purpose, I just don't know how to connect it.
Could I use 4 x LiPo or 4 x 18650 elements in series? I'd have to use some kind of battery charger/controller, I'm pretty sure such a thing already exists, I just don't know where to start looking for it. I know these elements are finicky.
Any help would be appreciated, thanks.