For various reasons I'm happy to describe in the comments section, I would like to charge large (>30S @ 1-30Ahr) packs directly from USB. Speed is not critical and multi-day (or week) charging is not an issue. However, what is an issue is size, safety, and complexity, which are hard to come by at these cell counts (unless you have an extensive engineering department to test, test, test).
I reason that since a USB supply is 5V (@ 500mA-5A, depending on PD or not), it's conceptually easy to inject power into a 4.2V cell. If we could split the pack up into individual cells we could connect each cell to the 5V supply and let it soak up some charge. No need to develop any kind of switching mode power supply or balancer.
However, in a real-world case, these cells are inseparable and so the charger has to deal with the fact that there might be 100V or greater across the pack. Furthermore, the devil is in the details, since pragmatically it would be very dangerous (i.e. prone to user failure) to completely charge one cell before moving on to another.
I feel like the below approach should work for an arbitrary number of cells. The idea is that a microcontroller switches on both ground and positive terminals for a single cell, while switching off all other cells. A zener diode protects against the microcontroller running amok and trying to overcharge the cells.
- The system switches at a high rate ("high" as determined by the LC filter)
- An LC smooths the current so that the battery isn't seeing a sharp PWM signal
- A zener diode provides passive protection against the microcontroller running amok and trying to overcharge the cells
- Requires a USB power supply with good current regulation (to keep itself from blowing up)
- Not drawn are all the things required to make this work in a real-world setting, e.g. appropriate FETs vs transistors, the sensing circuit to measure input current, voltage, temperature; PWM control of the input voltage; current limiting through the zener diode, etc...
Seems simple, does it work as easily as this? Are there some obvious pitfalls in this design?
My working assumption is that this doesn't exist in the market because most people don't want to charge a >1kWhr, 100V battery off of USB. If I'm wrong, please use the comment section to disabuse me of this notion.