For really small applications, you can consider super or Ultra capacitors, which can offer power densities much higher than even Lipo Bateries. They are common for robots under a cubic inch in size.
A unit about that size can have 5.5v x 1.2 farads of energy storage. They also come in disc or can configurations that could be disguised as gas or air tanks. For higher voltages wire them in series. For longer run times wire them in parallel or any combination. They aren't cheap. One that size cost about $35 USD a while back in lots of 15 ( group buy, then) I haven't checked singles prices.
http://www.ultracapacitor.co.kr/ultraca ... rison.html
Different unit between Ultracapacitor (Farad) and battery (Ampere hour) makes users to get confused when adopting Ultracapacitor in their system.
The amount of energy stored in ultracapacitor can be easily calculated by using following equation.
Energy (Joule) = ½ x Capacitance (Farad) x Voltage2 (Volts) It can be converted from Farad for Ultracapacitor to Watt hour unit which is normally used for conventional rechargeable battery.
Energy (Watt hour) = Energy (Joule) / 3600 (sec) LS Mtron recommended to operates ultracapacitor from maximum voltage to half voltage and release ¾ of total energy.
Just make sure to stay under the voltage rating or these babies go off like firecrackers, and use a protection diode to make sure you don't reverse charge them.
A charger can be as simple as a wall wart of the right regulated voltage and a power resistor to slow the charge rate a little so the wall wart doesn't get too hot. The Cap can draw a quicker charge than the wart can deliver.
The caps have very low international resistance, so they don't get hot. They can also be charged or discharged in seconds, which has advantages and disadvantages. They can be charged in a second or two, but they can also be discharged all at once by touching both leads or shorting wires. Think mini welder or taser.
It's how the flash in small cameras are energized.