I have a Shimano 3n-72 dynamo hub and I have a LightCharge USB charger so I can charge an external battery pack. When I took off on a recent trip, my battery pack showed three out of five bars of charge. I hooked it up to the charger and rode for approximately 5 hours. The battery indicated that it was charging as I rode (bars lighting sequentially as I rode) but at the end of the 5 hours the battery still only showed three bars. When I got home I connected the battery to a USB port on my computer and the next morning the battery was fully charged.

My question is, from what I have described, does it appear that my battery charging while on my bike ride? Is the output of a dynamo low so that it takes a long time to charge a battery? Anyone have any experience here? I am going on a long bike tour in the next couple weeks and I want to be able to keep the battery charged up. Any advice or thoughts are appreciated.

My initial impression is that the charging worked, because after five hours in the hinterlands you'd have a dead cell phone if it didn't. This is assuming you left the cell phone on.

You can actually charge the batter up by riding faster or by turning off the phone. Some phones (iphone is a prime example) will ignore sporadic high currents until their little electronic brains decide it's worth turning the charging circuit on, so if you're on the border (say about 10 mph), you might not get all the charge you pedaled for.

Robert said he was charging an external battery, which, ostensibly, had no load on it during the charge. It does sound as if the system is providing very little current.

Is it also a correct assumption, Robert, that your dynamo was not powering a light set at the same time you were attempting to charge your external battery?

With USB charging it gets complicated as there's circuitry controlling what the charger outputs (which can depend on what it sees as a load), and circuitry in the device being charged that decides what's acceptable and what it will accommodate. There's likely a threshold that's needed before the device can effectively start charging something, and that threshold will depend in detail on the characteristics of the charger and the device it's trying to charge. Perhaps you weren't hitting that threshold for these devices.

Some solar chargers get around this by having their own battery. They design the charge circuitry to efficiently charge that battery even at low light levels and when that battery has sufficient charge, it is then used to output a useful voltage and current to charge a connected device.

Note that LightCharge offers an aux battery pack that they recommend to use between the dynamo charger and an iPhone. It's likely this pack functions in a similar way to the solar charger battery I described.