I chose a Dell Latitude D5xx/D6xx battery for this design. Reason: the form factor. This battery has a rectangular form factor unlike most of the laptops' stick-type batteries.

I bought the 9-blade connectors for the battery from AliExpress along with the supporting components to build the reference design last December [Those connectors are expensive and I must find a cheaper source in the long run] Built a small breakout for the battery connector, connected the laptop battery and tested the SMBus with my BeagleBone Black and Smart Battery test code which was functional.

First I will be building a SMBus based standalone laptop battery charger design using the TI bq24725a application circuit, and a spare Dell Latitude D5xx/D6xx battery. I've started PCB design for the charge controller, hoping to post an update soon.

I would choose a low-power ARM microcontroller to power my design, not yet decided which one. There would be a small display to display the state of charge and remaining time. Also some pretty graphs of charging profiles could be plotted.

Next up would be the power train. I would have to pick good buck converters and design the supervisor scheme with current monitors and load switches to monitor the power output. Also add suitable PU/PD switching on the D+/D- pins to make the 5V loads choose the highest possible charging current to draw.

Apart from being a useful portable power pack, this circuit could remain permanently connected to the utility supply, and provide UPS-style backup to any SBC connected - I'm thinking of a Raspberry Pi server or a BeagleBone Black smart home solution. Given a bigger battery this could even power a whole computer like the Intel NUC. I'm also interested in seeing if with a boost converter I can supply enough juice to make an emergency laptop battery pack.

On the casing, I'm planning something 3D printed for the later versions. Since this is a laptop spare battery, it has a nice latching mechanism and if I can design the case right, it can fit in and lock as smugly as it would in its parent laptop. This would also be my first 3D CAD design and printing experience, so looking forward to it.