Micro fuel cells are an alternative to batteries as a power source for mobile devices. They may be small enough for integration inside the electronic equipment, or a little larger for use in an external power supply. The majority of portable fuel cells use hydrogen as the base fuel, but micro fuel cells usually extract it from methanol—a hydrogen-rich liquid. Although methanol-based cells can potentially provide 10 times the energy storage capacity of a lithium battery, they are not as good at delivering bursts of high power. This makes them more useful for battery charging than for primary power roles. They can be recharged almost instantaneously by adding more methanol—by either pouring it in or clipping in a new cartridge. This is a significant advantage over batteries, which can take several hours to recharge. Micro fuel cells are also environmentally friendly; the operating waste is only hydrogen and oxygen, which appears (in the form of pure water) as slightly increased humidity around the cell. Other forms of fuel cell, such as zinc-air and ethanol, are possible fuel sources for mobile devices—but are currently less prominent.