It is a battery, but thanks you made me look again, correcting something else I had wrong in memory. It's up to 1A output USB charging (not 100mA I had thought), only 100mA limit when using the light at the same time. I thought I'd seen 100mAh battery size mentioned, which for the light's size seemed right, but I can't find a source quickly to confirm the cache battery capacity.
From the manual here:
http://en.bumm.de/fileadmin/user_upload ... eitung.pdf
USB charging energy
Integrated into the handlebar button below a rubber cap is a USB socket which can be used to charge or power USB devices. During rain, the USB - socket cannot be used and the rubber cap has to remain closed (alternatively, the socket may be protected against moisture e.g. by sheathing it in plastic).
A red LED in the handlebar button indicates when USB charging power is available. This is the case when the integrated cache battery (see below) has sufficient energy available.
Connected devices are provided with 5 V and up to 1 A current.
The charged cache battery keeps the energy supply constant during slow rides and stops so that sensitive mobile devices may be charged. If the charge of the cache battery drops below a certain level, USB energy supply is interrupted (red LED ceases shining) until the battery is sufficiently charged once more.
Before a USB device is connected, cycle for some minutes with the light switched off so the cache battery has sufficient basic charge.
Permanent USB charging with more than 100 mA is only possible when all light functions are switched off. When the cache battery is charged, simultaneous charging and use of the light function is temporarily possible.
The duration of this depends on the charging status of the cache battery, the cycling speed, the quality of the dynamo and the energy demand of the connected USB device. While the cache battery discharges, the light output diminishes. As soon as the light output drops below 20 Lux, the USB energy is interrupted. The light will not deactivate during cycling.
Also from Peter White:
http://www.peterwhitecycles.com/b&m-hl.php
Luxos U, 70 lux, IQ2 Optics, Licht 24, USB charging (onboard AC to DC converter) Cache battery for charging, as well as powering the light to 90 lux for short periods, handlebar switch, monitors status of rear light, pictured mount included. Weight: 4 ounces without the included B&M475DPB mount. The Luxos U weighs a bit more due to the cache battery and AC to DC converter.
If the cache battery was 3.7V 100mAh, and it lasts theoretical 500 cycles to 80% original capacity, 500 x 0.1A x 3.7V = 185Wh throughput.
So if we assumed a 1W consumption riding with high 90lux on like il padrone has got 3-4 years from, we could say 185 hours at 1W consumption (because it's powering from the battery, and the battery is charging AC to DC supply). The hours will drop when it's not stored unused at around 60% charge, suffers temperature extremes in storage and use. Because of the dynamo we wouldn't really know it's degrading until it fails completely and goes open circuit, hopefully there's a bypass to operate without cache battery and just lose the switch and USB functionality.