Uninterruptible power management board for Computer, 15W output, Li-ion battery

Question

I'm searching for a power management board for powering the portable NAS from my other question, which needs 12V-24V and at most 15W, much less on average.

Required:

• output to computer: 15W, voltage either regulated to something between 12V and 24V or completely unregulated (so I can add my own voltage control without doubling the inefficiencies)
• input from wall power: at least 30W so it can charge at a decent speed, don't care about voltage, the one for which the power supplies are cheapest
• input/output: li-ion battery charging/discharging.
• communication channel which I can monitor so I can do a graceful shutdown before the batteries run empty. Not picky about the interface. A standard one is better of course.

Optional:

• connect to individual 18650 cells in different configurations. Probably 4 or 5 cells in series would need the least voltage transformation and would thus be most efficient, but if that makes the charger too expensive then I'd rather have some more cells in total. If there is a cheaper alternative to 18650 cells then that's fine too as long it's not too large (I like how I can arrange individual cells in a pattern which fits my enclosure).

Answer 1

Mini-Box OpenUPS does the job but is much too expensive at $150 (Amazon Germany - $120 directly from the manufacturer). I won't accept this answer because I think something much cheaper should exist.

It does everything asked for in the question, plus balancing up to 6S and some other battery chemistries too and output is not just in the right range, but even configurable in a larger range.

Product page here.