LEDs is a set of alkaline cells and rechargeable cells. In both cases the battery tends to fail quickly when it goes; it is widely considered a good practice to have two rear lights in case a battery fails en route. Low power lights are mainly for being seen, or as an emergency backup, and are the dominant choice for rear lights. Rear marker LED lamps may be operated on disposable coin-sized cells. Some high-end rear lights incorporate turn signal and brake light functions, switched via a handlebar control via radio frequencies. Power supplies Batteries The introduction of the Low self-discharge NiMH battery (LSD-NiMH) in 2005 made rechargeable AA and AAA sized batteries more viable for powering LED bicycle lights. Previously, the self-discharge effect of NiCAD and NiMH batteries caused the battery to run down over a period of weeks or a few months, even when not in use. This was particularly a problem for low powered LED lights, and for users who only used their bicycle lights occasionally. The LSD-NiMH battery greatly reduced the self-discharge effect, allowing the battery to keep its charge for a year or more. For higher-powered lights, an external battery pack of 12V VRLA battery is often required. These battery packs usually strap to the top tube of the frame, or come in the shape that fits in a water bottle cage, or can be placed in a basket. A cable connects the battery pack to the light. Another advantage with battery power (especially with 12 volt) is the ability to power other components outside of lighting, as well as all from one battery (as long as they run safe on 12 volts) though wire spaghetti from splicing can be a setback depending on how complicated the electric system is. Alternatively, Li-ion and LiPo batteries have been becoming more popular with bicyclists due to their higher capacity and lighter weight compared with conventional batteries. More specifically, the 18650 battery, which has been popular with electric car manufacturers (for the same reasons) and are commonly found in laptops and flashlights. Dynamo systems Dynamo systems require no batteries and may be permanently fitted to the bike. Bicycle dynamos are actually magnetos and produce alternating current. Types There are three main types: Hub dynamos are built into the front or rear wheel hub. These are generally the most reliable and most efficient of the three types. Bottle dynamos