Topeak RedLite II
Topeak's RedLite II mounts on seatposts, seat bags and seatstays. Its 4 super-bright LEDs shine through a 220-degree, wide-angle, reflective lens to alert traffic approaching from almost anywhere. The RedLite's watch-style lithium batteries (included) offer up to 100 hours of run time. Plus, you can choose from constant or blinking mode.
Run time: 60/100 hours (constant/flashing)
Weight: 20 grams