Patent Round-Up: The Future of AXS? Axle Batteries, Solar-Charging & Self-Charging

Digging through patent documents can sometimes offer hints at what we might be riding in a few years' time. Many of the weird and wonderful designs don't see the light of day, but even then it's interesting to see what ideas the bike industry's brains are working on. Here are a few that we've spotted lately from SRAM that all relate to AXS electronic shifting components.

---

SRAM's Axle-mounted AXS battery

A patent filed by SRAM in 2021 called "power supply in a bicycle" shows a battery housed in a thru-axle that could be used to power an electronic derailleur or other electronic components. The patent art shows the axle battery connected to the derailleur in several different ways, but all the drawings show a style of direct mount derailleur like the one we saw here and which was patented earlier in 2021 - we covered that patent here.


Some embodiments (possible versions) have an external wire connecting the axle to the derailleur's "bracket" - the cage which extends down from the frame and is separate from the rest of the derailleur. Others show how the wiring could be routed inside the bracket with a charging port on the back.


The advantages? The patent mentions that the axle battery could supply power to more than just the derailleur, for example, "a derailleur, seat post, bicycle lighting, an air pump, a GPS transponder, a shock, or other component." So, perhaps, one battery could power multiple devices. Also, the patent goes on to say that the internal location protects the battery from water and dust ingress, as well as impacts. It may also have a larger capacity than external batteries, which it says "may be limited in size to reduce unbalanced weight or reduce likelihood of impacts". Finally, the document says the battery may be charged without removing it from the bicycle (something which isn't possible with current AXS batteries). "Alternatively," says the patent, "the power supply may include disposable batteries that may be replaced by the user."

---

Solar spare battery charger

Another SRAM patent filed in October 2020 describes a solar spare battery charger. The small solar array could be mounted to a fender, bottle cage, or elsewhere on the bike, and would be connected electronically to a mount for a spare AXS battery. It wouldn't be able to charge the battery while it's in use but would slowly charge up a spare and keep it charged. This is potentially useful because batteries slowly lose charge over time, so a spare battery stored long-term on the bike might otherwise be useless by the time it's needed.

In low light conditions, a capacitor could store energy from the solar cells and periodically discharge this energy into the battery, thereby allowing the battery to trickle charge even without strong sunlight. Presumably, it would take a long time to charge up a battery unless you ride in the desert (theoretically, a 10*10 cm solar panel could charge a 2.2Wh AXS battery in a couple of hours in ideal conditions, but with trees, mud, and clouds it will take a lot longer), but there is value in just stopping the battery from going flat over long periods of time.

---

Self-charging (& self-shifting) derailleur

If solar charging wasn't enough how about this: SRAM could be working on an electronic derailleur that charges itself as the rider pedals, and maybe also shifts gears automatically.

The patent, filed in May 2021, was brought to our attention once again by an article on Bikerumor. It shows a derailleur which houses a small generator in the cage assembly, which is powered by the rotation of the upper jockey wheel via a mechanism of gears and pulleys which step up the rotation speed. Potentially, this could mean never having to recharge or replace the battery. Indeed, the drawings don't depict a removable battery or a charge port.


The obvious drawback of this approach is that the energy required to do the shifting will come from the rider and not a battery. But this is true of a mechanical system too, except there the energy comes via your thumb instead of your legs. Besides, the energy requirements will be very small.

A current SRAM AXS battery contains 2.2Wh of energy and lasts for around 20 hours of riding. That means an AXS derailleur is drawing on average about 0.1 Watts of electrical energy. So if this system is 50% efficient at generating electricity (which isn't great - electric cars can recover over 80% of their kinetic energy during braking), you're still talking about 0.2 watts or so, on average. It will be more if you're shifting frequently, but potentially less if you're not. The point is, you're probably not going to notice any drag.

Also, the electronics for shifting the gears seem to have been moved from the back of the derailleur body to the derailleur cage - a bit like SRAM's electric road derailleurs.

More interestingly, the patent mentions the possibility of automatic shifting. The pulley wheel generator could act as a chain speed sensor which, along with information about the chairing size, could tell what cadence the rider is using in real time. Potentially, it wouldn't be so hard to have an automatic shifting algorithm (perhaps configurable to your tastes in an app) to keep you in a sensible cadence automatically, much like Shimano's XT Di2 drivetrain is meant to do.

Will it see production? I suppose that hinges on how many riders find it a chore to remove a battery to recharge it after every few rides. For forgetful types or those who like multi-day rides in the wilderness, it could be very appealing.

---