Di2 Dynamo:USB-C Charger

Charging Di2 From Dynamo and/or Without the Shimano Charger

Charging Di2 from a dynamo or a regular 5V USB-C wall phone charger.

How to use Di2 for Bikepacking/Ultra-distance cycling and touring

Or how a broken cable made me learn electronics.

I am going to be honest with you. I genunely thought electronic gears were a ‘gadget’ and expensive garbage until…

I still remember the click and my legs locking up… It was my first 300km BRM (brevet) event and I downshifted to my lowest gear as I approached a steep hill. And then it happened. The shift lever got limb and a crunching noise came out of my derailleur. I had 130km to go in a headwind and I thought that I had to make the call of shame to get picked up from the middle of nowhere in France.

I stopped an upon further examination it wasn’t as bad as I had initially imagined – my rear derailleur cable had broken inside the shifter. I had two gears left by using the front derailleur. A group of cyclists said I can just tie the cable on the derailleur on some ‘usable’ cog on the cassette (the 25 which i later moved to 27 as I got tired) and it was flat so no worries… One cyclist even offered me a derailleur cable, which i politely refused. (In retrospect that was a good decision, since I later found out derailleur cable replacement was not a procedure easily undertaken in the field).

Flat it was NOT and my knees did not like it one bit. I still made it well within the cutoff time, however, as I was slogging away passing small villages, since it was a sunny day, I could see many people in their gardens milling about. And then it hit me. Probably none of those people had a spare derailleur cable and/or an of the tools to help me. Later when i fixed it it took me a good 30-45min and a specialized tiny screwdriver to remove the shifter cover to get out all the tiny cable wire pieces out), HOWEVER, almost everybody now has a 5V USB charger at home. So if i had been using electronic gears, a dead battery (which is the broken cable equivalent) I could have just politely asked/begged for a 15min charge and would have been merrily on my way.

This was the moment i *decided* that electornic gears in general and Shimano Di2 IS THE BEST thing for long distance riding. Though the limitation of having batteries still remained….

Until now….

Charging Di2 From Dynamo or USB

So let’s go back a bit

What are Electronic Gears and Should YOU Care?

Changing gears on a bicycle or a vehicle for that matter requires physically engaging/disengaging various elements (chain, cogs, etc.). The most surefire way of doing that has been manually (using you hands, feet, etc) using a cable, lever or a combination. As electronics have become a bigger and more significant part of our lives, in 2009 Shimano replaced the cables in the bicycle with electric motors. and called it Di2. The rest is…yet to be decided.

While still not a budget option , prices have gone down, removing one of the major negatives of the system. So while curmudgeons and retro grouches focus on the negatives, that is no way to live, so some of the biggest advantages that electronic gears have going for them are the following (in my humble opinion and in no particular order)


  • No adjustment and a perfect shift all the time every time: After initially set up, they will not go out of alignment. While this might sound a bit weird, here in France even in the summer, morning temperatures can go a hair above freezing (2019 Paris-Brest-Paris was one popular example) with 30+C degrees in the afternoon. I have had mechanical systems act iffy. Granted this is an extreme example it shows to prove a point. Also every shift is the same, no trimming or finicking needed.
  • No maintenance. The system is largely set and forget. In addition, the things are literally black boxes. They are sealed from the elements and currently there is nothing that can be done once they break or wear out. Though they don’t just break or wear out even in the most challenging conditions (sealing from the elements works!).They are pretty well engineered, so nothing to tinker with as well. It is sealed for good!
  • Multiple shift positions. This is something I had secretly coveted and had been VERY impressed ever since the first Di2 time trial group hit the market. Because you are not limited to an actual physical connection to move the derailleurs, you can have switches wherever and have them do whatever function you want them to! So semi automatic (aka syncro-shifitng) was only a matter of time until it become a feature as well.
One of many ways to organise your Di2 cockpit. Source: Shimano
  • Low shift effort. Fatigue and frozen fingers are something ALL cyclists have had to experience on *multiple* occasions. I’ll leave this like this. If you know you know…. With electronic gears, you still get frozen fingers though shifting is MUCH easier and in some cases changing gears is possible only because of the low effort required as compared to a mechanical system..

Granted just burying your head in the sand and ignoring things does not mean they will go away so some of the:


  • Price. Though with the introduction of mid-tier groups (such as Ultegra Di2, 105 Di2 and Sram Force AXS) price had come down significantly, though electronic gears are not a budget option.
  • Mixing and matching (aka compatibility) is almost impossible. Have a 10 speed system and want to make it 11 speed compatible. NO WAY JOSE! Firmware/software firmly (pun intended) prevents you from doing so, while with cable operated systems you can hack it with various success… almost always.
The tip of the Shimano Di2 Compatibility iceberg. Source: Shimano
  • Battery life. No matter how you put it, a cable that you haven;t touched since 1989 will still kinda sorta work while a dead battery is a dead battery. It might last a looong time (~30% over 300km/17h by my own experience so extrapolating ~1200km), however, over multiple day, bikepacking ultraendurance events which have exploded in popularity and not to ignore the classic randonne with the crown jewel Paris-Brest-Paris, a battery and charging ranges from inconvenient to impractical to downright impossible. All in all it is one extra thing to worry about besides resupplying etc. And believe me once you are half starved and baked under the afternoon sun, power outlets are NOT what you are thinking about.

So if I could only solve that, Di2 and electronic gears would be truly the greatest thing without the BUT *they require batteries part, that usually follows

Li-Ion Batteries and You

Batteries nowadays are almost exclusively of the Lithium ion/polymer variety From tiny household appliances to electric vehicles, the humble Li-ion is currently the best technology, albeit imperfect, it is vastly popular.

So Shimano or other electronic gear manufacturers did NOT reinvent the wheel (pun intended) and are using the same technology as everybody else.

What does that mean?

Charging Profiles

I am not going to get into details, though a single lion battery cell ranges from 2.9V (fully discharged) to 4.2V (fully charged) or nominally listed as 3.7V . If you connect 2 cells in series you get 7.4V, etc.

The first lesson (and several burnt components later) is that you cannot just feed the battery with its max voltage 4.2V and call it good. Li ion batteries are like women, you have to treat them right or they will explode.

Li ion batteries are like women, you have to treat them right or they will explode.

There is a specific protocol otherwise they will heat up at best, or violently catch fire at worst. Li ion batteries are actually quite dangerous if treated improperly (this came as a surprise to me…). The said charging protocol is VERY tightly respected down to millivolts (mV) with multiple safety checks during the charging process, and monitoring battery temperature.

Li ion Charging Profile. Source: Battery University

What you should get out of the above graph is the following. Most charging is done in the first ~80% of the charging cycle so the most discharged the battery is, the more charge you can pump in it. (ie fast) The more charged the battery is, it can accept less and less charge (ie slow).

So Shimano did the smart thing and are using a tried and tested technology.

So far so good.

Not Really…

Ever since the first generation of Di2 you NEEDED a dedicated Shimano charger and you HAD TO to remove the battery from the bike to charge it. So you couldn’t ride AND charge at the same time.

Shimano Di2 External Battery (Source: BikeRadar)
Shimano Di2 External Battery Charger (Source: Bike Radar)

That was changed later since a lot of people did not like an ugly external battery and once bikes got designed around hiding the battery and wires inside the frame, it became highly impractical to remove it to charge, so Shimano integrated it and put an external charging port, one version of it is shown below.

Di2 Charing Port (Source: BikeRadar)

However that used a Shimano cable that was unique in the myriad of industry and various electronic connectors, which is impressive that nothing out there worked for Shimano so they had to add their own…..

So you had to use the Shimano charger. Which while not bulky is still not what I would call convenient to use (on the go), with the obvious that you need it with you. At least it connected to a 5V port (through the rapidly disappering micro-USB port) which most people have at home to charge their smartphone, etc. For multi day events you can definitely use it with a portable power bank so the connection would go something like this.

Power bank->Di2 charger>Your bike.

A bit over the top (and not very efficient electrically due to quite significant power conversion losses mind you) but it works well while cycling. However, while still stating the obvious, you still NEED the dedicated Shimano charger. It (the charger) is also the best way to do some software updates and configuring since Bluetooth is (in my opinion at least) the buggiest most power inefficient wireless technology out there…ever.

Another hack is when the internal battery reaches 0%, you can carry a charged Di2 battery that you can connect to any E-tube port.

Fast forward to the latest (at least at the time of this article) 12 speed iteration of Di2 and while Shimano have kind of dealt away with their charger or at least hidden it way inside the rear derailleur. Since you can’t get away from Li ion’s quirks (as mentioned above) the actual charging bits are hidden inside the rear derailleur and you can use standard USB -A port. You still need an even funkier Shimano cable and being located in the rear derailleur it is VERY inconvenient to charge while riding. Is it even possible???

Shimano 12 speed Di2 Charging Port in the Real Derailleur (Source: Shimano)

So we are back to where it all started….

In all cases you need funky looking cable (and 5Volts). The 5V is not such a big problem, the funky cable though is.

Oh and another thing, which is purely on the annoying side is that ALL Shimano chargers have an indicator (LED lighting up) that charging is in progress. The LED switches off when done/charged, however, this is the same effect if someone trips over the cable when you have your bike innocently leaning on a wall while you are taking a coffee, sleeping or if the power outlet is not good/loses power. So you have no idea if charge has finished or someone just unplugged your bike.

Doing it Better?

Soooooo in the faraway 2019 when sketching out my bike to partake in the 2019 Concourse de Machines, I set forth to taking it a step further. Since dynamos (sometimes referred to as generators) are probably the best thing for keeping your lights (and devices GPS, phone) powered when out on your bike for multiple days, it was only logical that if you have this power, why don’t you use it charge the Di2 directly?

My requirements were quite simple….

Charging Di2 from Dynamo

So here I was knowing exactly jack shit about electronics, with a real fear of Ohm’s law and trying to make something that people have only timidly asked on online discussion groups with no real constructive plans.

Dynamo Powered Di2 Charger: The Beginning – August 2019

*Spoiler alert.* I neither finished my bike on time to present it and use it during the 2019 Paris-Brest-Paris and Concours de Machines, nor did I get even close to having something resembling a Di2 charger. The bike itself was a hugely complex project, not to mention the charger that took …

Almost 3 years later, me initially blowing up a Shimano junction box by feeding it 8.4V directly, a pandemic (which caused, and still is resulting in a MASSIVE semiconductor shortage) causing multiple complete redesigns. I am proud to introduce to the (cycling) world

The Savior

What is so special about it?

  • It takes either 5V USB-C connection (widely and universally available)
  • or Dynamo power from a hub/bottle, etc dynamo (compatible with virtually all dynamos on the market) selectable with a switch
Di2 from Dynamo to ANY Di2/E-tube port.
  • Overvoltage protection (when in dynamo mode)
  • LED indication for Charging (Red) and Done (Blue)
Dynamo/USB C Di2 charger LEDs
  • Connects to ANY e-tube (Di2) port (in 12 speed r9200, r8100 etc) you MUST run a cable from the battery to the charger (or connect it directly to the battery). The Savior does NOT work wirelessly.
  • Compact. Fits neatly in virtually all handlebars, replacing the end plug or tucks neatly in a (handelbar)bag, attaches to the cables with zipties, etc.
The Savior Dynamo/USB-C Charger Handlebar Integration
  • Achieving 100% output from 12-15kph (8-10mph)
  • 1.5-2h to full charge (depending on battery charge and ambient temperature) from 15kph (10mph) when in dynamo mode.


While I had to overcome a LOT of design and electronic challenges, the one thing that to this day I have not yet managed is that while you are charging using The Savior, the gear shifting does NOT work (there is no damage or anything done to the system). As soon as you switch it off, the system wakes up and works as normal.

Is this *Really* a Problem?

Honestly (and I realize I have a HUGE conflict of interest, since I am selling my stuff here) even in the quite rolling terrain (lots of shifting to keep a rhythm) in the Ile-de-France (Paris area) where I currently live and do my Randoneering long distance events, there are plenty of boringly mundane segments where I don’t shift for 10-30min of minutes at a time.

Even so, as per Shimano’s own recommendations, if you find yourself with a discharged battery before setting out, a 15min charge on a dead battery (remember a ‘dead’ battery can accept a LOT of charge very fast) should get you enough energy for your ride (or ~10-15% in my experience which is still ~150-200km). You can undoubtedly find 15min that you don’t need to shift gears.

The Savior lives up to its name, it is there to save you when your battery is not enough to go the distance you need it to, when you did not bring your Shimano charger, or you forgot to charge it, or when you simply could not because it was 2am (in the middle of nowhere…in the pouring rain). Oh and of course, it bears repeating, it takes a standard USB-C cable that is available from most gas stations, convenience stores, etc so you don’t need a funky Shimano thing and you can charge your bike while you take a coffee, take a short nap, etc. when you will not be shifting gears anyway….

Why is is like this? I am pretty sure Shimano pulled an ‘Apple’ and chipped their devices so they talk to each other (how they are recognized when working together most likely) so by using a third party charger it simply refuses to work. Granted I can’t just reach out to Shimano and ask why is that, I am left to my own assumptions.

Simply the Best!(?)

At the time of writing this post The Savior is the currently THE ONLY device that can charge directly Shimano’s Di2 system and it can do it via dynamo or using a standard USB-C cable. Even today (2023) the internet turns up the same posts (dating back to 2009ish) I found when i was doing my initial research in 2018/19 of people inquiring on online discussion groups whether you can charge Di2 from a dynamo, from a USB port and such, without any products or even definitive answers on how to do it.

The Savior has been tested to work with Ultegra R8050 (11-speed) and BT-DN100 battery. I will add more tests as I get them.


  • The Savior is NOT approved by Shimano and might void your warranty. Use at your own risk
  • Cycling is an inherently dangerous activity that can result in serous injury or death.
  • All images unless otherwise noted are property of their original owners

The Savior is available for SALE in my WEBSHOP.





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