Thanks for the quick response.
I have the new Olfi 1.5. Can you recommend a power bank? How long is the mini USB lead? I wear the camera on my helmet so it could be awkward. Another idea I am toying with is recharging the batteries with the charger plugged to my Garmin 1030 external powerpack. This would run as hub dynamo -> external power pack -> Olfi battery charger. Would this be viable? bearing in mind it can take up to 3hrs to fully charge the batteries in a conventional way.