Before spending a bunch of money, I looked at whether this could possibly be helpful. Reviews I could find were spotty. I have an electrical engineering background, so I was not surprised when I saw a mishmash of electrical terms. Electrical units are not hard. Power is measured in Watts – electrical watts are exactly the same as the watts we generate when pedaling. To get power, one needs force (V, or Volts) and current flow (current, measured in A or Amps.) The relationship is very simple: W = V x A (Watts = Volts x Amps). To keep my review simple, I will always use Watts. This is a valid thing to do because electronic circuits can trade off volts for amps as needed, keeping the power constant. The circuits act exactly like your bike’s gears, trading force for speed. Of course, these circuits aren’t 100% efficient, but we can talk about watts and get a good idea of how well this should work.
I started by looking at the hub. The performance of the SON 28 hub is available here:
(Red graph on the first page). As the bike goes faster, the hub generates more power, up to a point. Hub power is limited to 3.5 W. The hub generates 2 W at 6 MPH, 2.5 W at 7.5 MPH; 3 W at 9 MPH; and 3.5 W at 12.5 MPH. If I’m bombing downhill at 30 MPH, I’m not pedaling, I’m having a great time, and my hub is generating 3.5 W. The power generated is not related to my average speed, but to how slowly I climb. If I stay above 9 MPH, I get essentially all the power. If my speeds drops to 7.5 MPH, I’m still getting 2.5 W. Below that, the power drops quickly. The key point is 7.5 MPH for two reasons. First, 2.5 W is the power drawn by a USB charge (5 V x 0.5 A = 2.5 W). Secondly, the hub starts limiting at 7.5 MPH. Going faster gives a little more power. Going slower gives a lot less.
My first conclusion is the hub generates enough power for a USB charge, but not much more than that. For example, my laptop charger is 65 W. There won’t be any laptop charging going on.
Using the hub to charge a smart phone or batteries requires several electrical conversions. This is what the Biologic ReCharge does. There are other, similar, units. I chose the ReCharge because it is available from Amazon and it has a reasonable price. The plastic blob that attaches to my fork converts the hub’s alternating power to direct power. A circuit inside the battery case charges the battery. A second circuit converts the battery voltage to 5 V and delivers up to 5 W of power to a standard USB connector. The charge status of the battery is indicated by three lights. When I press the center button, a full charge is indicated by all three lights turning on. If no lights turn on, the battery is discharged.
To test charging my Eneloop batteries, I also purchased two Sanyo USB battery chargers
(use some Velcro strips to strap the batteries in) and a 2-port hub
AA batteries will charge just about as fast as AAA if I put one battery in each charger. In this charger, one battery charges twice as fast as two.
I am a slow rider, about 12 – 13 MPH average. In hilly areas, I spend plenty of time riding less than 7.5 MPH. In spite of this, everything works reasonably well. I did an out-and-back in a moderately hilly area. I started with a fully charged Biologic battery. Here are my results:
Elapsed Time Device Charge Biologic level
1:45 phone 39% to 93% 3 lights
2:15 2 – AA (1/charger) charging 2-3 lights
(break for lunch – charger turned off)
4:00 2 – AA 1 done, 1 charging 2 lights
5:00 4 – AAA 2 done, 2 charging 2 lights
6:00 none none still 2 lights
6:30 none none 3 lights
I don’t know how close to fully discharged my batteries were. I used batteries I would normally throw into the battery charger at my desk. My GPS had issued a “low” warning for the AA batteries. The AAAs were from a front flasher light that ran about 10 hours. The last hour and a half was to let the ReCharge battery recover its charge. It had not recovered after 1 hour, so I rode another half hour to see what would happen. I conclude that if I am not climbing all day, I can easily keep my cell phone charged, and even charge some of my flasher batteries. If you climb faster than I do, you can do a little better.
If you want to try the same thing, here are some tips to keep in mind.
Like many smart phones, when I plug my phone into a computer, it does a “USB” charge (2.5 W). When I plug it into the wall wart charger, it does an “AC” charge (5 W). How does it know? USB devices are supposed to ask the computer how much power they can draw. It’s part of the USB protocol. That’s how the phone knows that 2.5 W are available. How does the phone know when it is plugged into the wall wart? It’s possible the wall wart announces it has 5 W available. Or it’s possible the phone uses some other way to detect the wall wart, either as part of the USB protocol or through some proprietary method. What happens when I plug my phone into the ReCharge? Does the ReCharge have a chip to tell my phone that 2.5 W are available? Or does the phone see no response and assume it should do a USB charge? I don’t know. I have a Samsung Android phone. Biologic includes a USB adaptor they say makes the ReCharge compatible with some Apple products. Some people have reported their Apple device won’t charge from the ReCharge. Did they not use the adaptor? Is the device looking for some Apple-proprietary signal? I don’t know. To mis-quote Abe Lincoln, you can fool some of the smart phones all of the time.
I use a dedicated GPS for two reasons. First, my phone is too expensive to put out on my handle bars. It simply is not made to withstand the punishment it can get out there. My phone goes inside a padded case, inside a zipper compartment, inside my waterproof handlebar bag, with the top firmly latched. Secondly, these little devices draw a lot of power when they are active. It is very likely that some smart phones draw more than 2.5 W when they are used with mapping software and GPS active, and the display on continuously. It’s very possible that some smart phones would lose charge if used as a GPS while receiving a USB charge. (Maybe this is why those Apple products “didn’t charge”.)
When charging batteries, it’s not necessarily important to charge them all the way. When a battery gets to 90% charge, the charger slows the rate of charge to keep the battery from overheating. When using a USB charge, my phone takes the same time for the battery to go from 40% to 90% as it takes the battery to go from 90% to 100%. That’s pretty typical. Of course, if the phone is charging more slowly, the charger is drawing less power, and the ReCharge battery has an opportunity to charge up a little bit.
There’s no way to know precisely what the difference is between 3 lights of charge and 2. It might be that the ReCharge battery loses no charge while charging my cell phone. I have reason to believe that the two battery charges plus USB hub together draw more than 2.5 W. The ReCharge can deliver up to 5 W (until the battery drains). Some hubs “shut down” to avoid damaging the computer if the devices try to draw more than 2.5 W. Just add this to the list of things I don’t know.
I don’t run lights from my hub. I just stick with the flashers. However, the ReCharge can be wired in with lights if you want. If you try to use the ReCharge and the lights at the same time, that meager 3.5 W will be shared between the two devices in some wonderfully wacky way. It might be totally unpredictable. It might depend on the sequence you turn the devices on. Power might rock back and forth between the two. Whatever it does, 3.5 W is not a lot of power to share. If you wire lights with the ReCharge, you should pay particular attention to the SON hub manual. As they state, turning the lights off while riding can create a very large voltage. For many decades, cars ran by having points turn the current off in a coil, which easily generated 1,500 V to fire the spark plug. I’m not saying the SON hub will generate that much voltage when you turn your lights off, but the principle is exactly the same. High voltages can be rough on electronic circuits. So if you wire both of these devices together, you might want to take care to come to a complete stop before turning your lights off.