Good article, although I wish you had gone into more detail on the standard Li ion battery charge cycle of trickle, then CC then CV. It helps the reader to understand the limitations of charging from a USB host -- namely that the max current available for CC mode is less than what is available from a dedicated charger.
Even 1.5A doesn't cut it for some of today's mobile devices (e.g., tablets).
Readers will still ask why the dedicated charger can fully charge their tablet in 2.5 hours, but when they plug into the USB on their PC, it takes much much longer. You came oh so close to answering that question.
"Make no mistake: selling specialized accessories is definitely in the business plan for a portable product. "
"The idea was to minimize the number of cell-phone chargers ending up in landfills, by converging on one USB-charging specification. "
These two sentences are clearly at odds.
And Frank, good to know that you are a ski bum!
Is the typical device smart enough to not over draw from a poorly built charger? Is that something end users need to worry about or are the devices themselves smart enough to not get into trouble with a cheap charger?
@Duane, yes. My wife's Kindle HD will check to see what the charger can handle and only burden it with that much charging current. When buying a wall powered USB port, shell out more for the higher current model, or be patient and pick up a real book.
I think the main reason why portable products need large capacity batteries requiring high charge current is because of LCDs requiring high power backlights to make it usable outdoors. Our company is struggling to find a 7" reflective or even transflective LCD (instead of transmissive) in anything above 5" (e-ink can't be used due to page inversion requirement when updating display). The difference is changing from a 4W backlight to a 0.1W frontlight, but the LCD is not available. Sharp has some nice memory displays starting to address this, so hopefully soon we will not need such large batteries (and charge currents) any more.
Indeed, backlights are a huge power drain, but so is the RF power amp. CPU-intensive and graphics-intensive activities hurt too, even when running on a low-power processor.
I'm not so sure that we will soon be reducing the need for high capacity batteries and large charging currents in portable devices -- in fact, probably just the opposite.
New anode materials like silicon nanowires are showing great potential (yes, pun intended!) for dramatically increasing the energy density of lithium ion batteries. They will allow much longer runtimes from batteries that are the same physical size or smaller than those we use today.
The corollary to that is a need for even higher charging currents. To charge a battery in the shortest time using the standard CC-CV method, the charging current needs to be near the "C rate" of the battery, and the total charge time will be in the range of 2 to 2.5 hours.
For a modern tablet with something like a 6000 mA-hr battery (C rate of 6 amps), you will want a charger that can safely output 6 amps in CC mode. A future battery that has 4x that capacity in the same package size will of course need 4x the charging current if you still want to complete a full charging cycle in a couple hours.
Despite the war on current drain in circuit design, the demand for longer runtimes will always be there. The standard for a smartphone today is that it should last all day on a battery charge in normal usage, but all other things being equal (especially size & weight), consumers would be a lot more delighted if their smartphone lasted all week on a single charge...and if still only took a couple hours or so to charge it up again.
None of this bodes well for charging from standard USB hosts and their 1.5A current limits.