Dynamo hubs

My strategy was going to be to have two 5000mAh USB batteries:-

At night:-

• Dynamo powers front light
  
• GPS kept topped up by one cache battery, swapping to second one if necessary
  
  

When the dynamo isn't powering the front light:

• Dynamo charges one (via Sinewave or eWerk or similar)
  
• GPS kept topped up by the other cache battery, also topping up 'phone
  
  

(This assumes a cache battery cannot be charged and used at the same time. If it could then I'd go for one USB cache battery with double the capacity.)

That way I'd never worry about going slowly affecting 'phone or GPS: sometimes my Edge 705 GPS will enter computer mode (it pretends to be removable media and doesn't function as a GPS/bike-computer) when USB power is provided, which terminates the current track and means I have to faff to get it on again, so flicking on and off as the speed drops above/below some threshold would be even more annoying.

The two USB batteries also provide a bit of redundancy. I'd also plan on never relying on the battery inside the GPS, which would mean it is available for emergencies or when I forgot to plug everything in.

For PBP 2011 I had two 5000mAh USB batteries but no dynohub to charge them and tiredness meant that I didn't do the right thing to maximise their use. My 'phone used up too much of the cache batteries as I kept on forgetting to put the 'phone in flight mode between controls, and the rural nature of some bits of France meant that it kept on chewing through its battery using full power hunting for a non-existant signal. The signs on PBP were good enough that I didn't need the GPS for navigation anyway. (If I'd had a PowerTap on the bike I would have probably wanted to keep it going to record that but I wasn't fussed about getting a full GPX tracklog.)