Hi there,

Just share of my finding in USB charging problem, not very technical.

I got few different made of Android phone and tablet, I find it specially difficult to charge Motorola and ASUS devices via USB, Samsung Tab also hard to charge, but since it uses a special non USB plug so doesn't count. What I mean hard to charge is the phone won't charge via an external battery with a standard USB cable, because the high current charging is not enabled on the cable, it is the wiring of the plug itself, in that case the phone only do a very slow charge or won't charge at all, if I turn off the phone it will charge better, but still not standard charge like with the power adapter. Most power adapter comes with the phone are specially wired to allow high current charging, so that it will charge with any USB cable (charge only and with data), but some non original power adapter no matter the output current, won't charge with standard cable. And for all external battery I used none can charge the ASUS, I need to use a special charge cable, and it does not support data ! but for all HTC phones I used none have problem with USB charging, I only need to bring one cable with the battery for both charging and data transfer.

The other problem is the over charge voltage protection of the device. I was a long time user of Nokia before Android, and I find it very picky on the charge voltage, it won't charge when slightly over 5V, for example, I got a solar charging battery pack, it produces slightly over 5V, when I plug it into the Nokia, it will charge for just few second and then cutoff, I've to insert a diode to drop the voltage a bit to charge. More interestingly, I got a dual output battery, when I use a standard USB cable I can't charge the ASUS, but when I plug the HTC into the other output of the battery the ASUS will charge, so I think the battery output is slightly over standard, and the HTC dragged it down a little so both unit can charge !

Btw, there is something to praise about the HTC, it has execellent over charge voltage protection. Once I plug the HTC into an external battery, the battery has multi voltage output from USB up to 19V for notebook, I plugged the cable into the wrong port, a full 19V is fed to the HTC, the charging stopped after a short while, but caused no damage to the phone, I don't know what would happen if I do this to other phone !