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Unlike a battery, where the potential energy is sitting there waiting to be used, a fuel cell requires a continuous flow of reagents through it - it's like an engine. That's why miniature fuel cells have been so difficult to produce - you need physical pumps and valves and stuff like that, and it all has to be gas-tight. You therefore need to "turn on" a fuel cell - switch on all the pumps, etc - before it'll start producing power. It will have some external power source (probably a small rechargeable battery) to power all that.
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