The USB standard since has standardised what "dumb" chargers should implement for charging without IC circuitry.
Traditional USB requires the device to tell the host that it wants to draw more power, a negotiation of sorts. However, many chargers simply pump out 5V without any digital circuitry. So how does a device know if it can draw more than the 100ma mandatory minimum if it gets no response to its challenge to draw more current?
According to the USB standard, the +D and -D lines of the USB should be shorted out with no more than 200ohms (?) of resistance. This would tell the device that it can draw as much as it wants from the dumb charger. One thing to note is that if a charger is built properly, it will give out current in its rating in milliamps and no more without failing. The device charging, on the other hand, will just draw whatever amount it needs, or whatever the charger gives it and all should be fine.
According to anecdotal evidence, this works as I have personally shorted out +D and -D lines on a dumb charger and the Sony eReader does not complain about no power supply being detected. If you happen to have a charger with a USB-female port (i.e. having no built-in cable ending with a usb-male plug), and it doesn't work charging your Sony eReader (because the +D and -D lines floating and 0V), then you have two options:
- Either void the charger's warranty, risk electrocution or whatever: You can open up the charger and just solder either an appropriately valued resistor across the middle +D -D solder joints of the USB plug, or just apply a blob of solder to short it out and be done with it. However, besides the risks involved with opening up mains-connected devices, some simply don't allow you to open them, so you're left with:
- You can buy a cheap USB male-to-female adapter cable (easily found on eBay for a dollar or less). Carefully stripping the insulation, identify the wires connecting +D and -D lines, short them out and use your preferred insulator to cover your hack job... and now you've made yourself a charger cable than can be used for any standards compliant device that complains about detection or charges very slowly on your charger.
Unfortunately, you have pesky products which do expect voltage on the +D and -D lines, such as Apple products. In this case, commonly accepted values for +D and -D are 2.0V and 2.8V respectively. Like above with 1) or 2), you can either modify the charger, or create your own cable (I recommend doing this). My experience is that an un-shorted potential of +3V on both +D and -D (independently) still works to charge the Sony eReader. In that case, you can use 47K and 33K (in that order) resistors in a voltage divider fashion to obtain approximately the correct voltage, and using the (independent!) voltage dividers, apply the divided voltage onto the +D and -D lines and hopefully the device doesn't complain (it doesn't for me). This should be relatively safe as the high resistance values prevent too much current from flowing, and that very little power is drawn from the +D -D lines since only voltage needs to be detected...
Of course, if you don't want to solder, wait for cables to arrive, or aren't willing to apply voltage onto the USB data lines, then perhaps buying a cheap microusb charger that is known to work (like blackberry chargers etc.) is for you.
That should clear up the charger confusion about different chargers working, why some don't, cable issues (some chargers with included cables have +D -D lines floating!) etc.
PS: Current is just the capacity of the charger to deliver electrons at 5V
. The only problem you need to consider is what voltage the charger is outputting, and if it can deliver it at a high enough current that satisfies your requirements (500ma for many applications, 1A for "boosted" charging circuits, 2A for the iPad etc.). If the device can draw more, but the charger can't output more, then only the charger's maximum current rating will be delivered and your device won't be damaged (it just won't charge as fast) - unless the charger's circuitry design is poor and it overheats/fails thereby risking whatever was connected to it.