I'd never thought of connecting my gps to my dynamo fed E-werk to have always-on backlighting at night, only for charging or emergency power. What a good idea. Ample juice to run head and taillights concurrently? (I guess now that we use LED lights with dynamos rated to power incandescent bulbs there might well be)
Depends on the device as to how much of a good idea this is.
a) If it has a built in LiPo battery, running it off the ewerk is going to keep that battery at 100%, which it's not going to like, ideally for most LiPo batteries they last longest if kept between 20% and 80% (I wish devices offered this as an option).
b) if you slow down and there isn't enough power coming into the ewerk, you can end up with it getting a few seconds/minutes of charge, then nothing, this doesn't do internal batteries any good
c) If you run with the usb connector plugged in, over time the vibration can cause the connector to fatigue and come separated from the PCB inside the device, then you can't charge at all.
You are better off, using the ewerk to charge a battery pack, and then use that to charge your GPS when you stop for controls. Most devices with built in LiPo batteries can get a good few hours worth of charge in the time it takes to eat lunch (about 30 mins).
Remember, your ewerk is going to produce about 2.5Wh of power for every hour of cycling. If you are going the power bank route, then you are going to have a full cycle efficiency of at best 80%, giving you realistically 2Wh. If you cycle for 15 hours per day, that's 30-45Wh of available power. But you only get that if you are not also running the Dynamo light.
A Garmin Edge 520 has a 600mAh 3.7V battery (P=IV P=0.6*3.7=2.2Wh). Now a 10000mAh ikea battery pack has a capacity of 37Wh. Which means if you started with it fully charged, you could charge your Garmin fully 16 times over (give or take).
I run a USB-Werk on my bike, for charging a battery pack, but since I upgraded to a 26800mAh (98Wh) pack[1], I've found that actually it's not really worth the faff if I am stay in a hotel every 3 nights or so. I did have to get a specific power bank, with dual inputs, as I found that the larger packs just didn't charge fully in the short ~6 hour sleep I get on a race. But this is then enough to charge my devices (phone 1.5x per day, wahoo 2x per day, inreach+ 0.5x per day), and still have power in reserve if I decide to have an extra night in the bivvi before a hotel visit. On the second day of having recharged the bank, I may plug it into the usb-werk, but I'm not sure it really does me much benefit. But this is all calculated based on 2 week ultra race. For a 90 hour Audax, the power bank would be enough to charge everything. Battery power density has gone up, battery price has come down, it actually makes dynamo power even for lighting a questionable proposition. 3W of power to run the light. 10h of darkness, that's 30Wh of power. My Anker is 590g. A Son 28 is 440g. The anker would power lights for 30 hours (give or take). A standard Shimano hub is about 140g. So for a 400g penalty, you can have battery power, which will save you about 5W in resistance. Tho I'm not sure how much that extra 400g would slow you on the climbing... Ultimately you pays your money and makes your choice. Just be aware the variables are changing as the technology evolves[2].
J
[1] Anker Powercore+ 26800 - (
https://amzn.to/2G64YZu)
[2] The next generation of battery technology will really skew this in favour of battery over dynamo, if even half the 10x projected density improvements come about, it's just not going to be worth running a dynamo if you can get access to charge your battery every few days...