ac800s does not charge when USB tethered

Is there a trick to getting the ac800s to charge when operating as a USB connected internet connection?

My "optus" branded ac800s works great for most of my usage, however I am stuck in one circumstance that I need for work.

When I plug the ac800s into my portable device via USB, it works perfectly and seamlessly for data, however in this state it appears it will not draw power to charge and the battery eventually runs down, despite being plugged in. I've bought a powered USB 3.0 hub, but the same result, ie the ac800s can provide internet data perfectly but slowly drains the battery and no indication that is is drawing charge. This obviously means I have to preiodically disconnect my portable device to charge the ac800s back up again, meaning I cannot work during that time.

Thanks if you have any ideas or insight.

ksh93