Думаю как заряжать 4 последовательных Li-полимер аккумулятора от USB :-)
Dron_Gus: При енумерации устройства на шине, в дескрипторе устройства передавать соответствующее число.
ну все... на этом тему можно закрывать. наверное... 
И нифига подобного! Нашел очень интересную статью, прямо на сайте maxim-ic:
What Your Mom Didn't Tell You About USB
With any standard, it's interesting to see how actual practice diverges from the printed spec or how undefined parts of the spec take shape. Though USB is, with little doubt, one of the best thought out, reliable, and useful standards efforts in quite some time, it has not been immune to the impact of the real world. Some observed USB characteristics that may not be obvious, yet can influence power designs, are:
USB ports do NOT limit current. Though the USB spec provides details about how much current a USB port must supply, there are mile-wide limits on how much it might supply. Though the upper limit specifies that the current never exceed 5A, but a wise designer should not rely on that. In any case, a USB port can never be counted on to limit its output current to 500mA, or any amount near that. In fact, output current from a port often exceeds several Amps since multi-port systems (like PCs) frequently have only one protection device for all ports in the system. The protection device is set above the TOTAL power rating of all the ports. So a four-port system may supply over 2A from one port if the other ports are not loaded. Furthermore, while some PCs use 10-20% accurate IC-based protection, other will use much less accurate poly-fuses (fuses that reset themselves) that will not trip until the load is 100% or more above the rating.
USB Ports rarely (never) turn off power: The USB spec is not specific about this, but it is sometimes believed that USB power may be disconnected as a result of failed enumeration, or other software or firmware problems. In actual practice, no USB host shuts off USB power for anything other that an electrical fault (like a short). There may an exception to this statement, but I have yet to see it. Notebook and motherboard makers are barely willing to pay for fault protection, let alone smart power switching. So no matter what dialog takes place (or does not take place) between a USB peripheral and host, 5V (at either 500mA or 100mA, or even maybe 2A or more) will be available. This is born out by the appearance in the market of USB powered reading lights, coffee mug warmers, and other similar items that have no communication capability. They may not be "compliant," but they do function.
Перевод такой, что энумерация до одного места. Запрос на такой-то ток, это не более, чем правило хорошего тона. На самом деле ни один USB не отключает устройство, если оно не энумеровано или потребляет ток, выше чем положено. Защита по току ставится как правило на всю шину целиком, и в принципе можно и пару ампер взять, если ничего другого не подключать.
Ну так я изголяться не буду, естественно, но 450мА возьму. Само собой никакого интерфейса не будет 