I think 4kg should be fine for a hamster, a stack of benzedrine pills, and a wheel-powered dynamo.

Or for some 6kg, take a honey badger, they're incredibly tough bastards to boot.

Why, oh why, is the U.S. Govt so cheap? They have no problems blowing zillions of dollars on questionable products and initiatives but when it comes to completely changing the world of portable electronics they offer up $1mil. Seeing as the winner won't get to be the sole contractor anyway, it seems to me like anyone who could meet the specs for the prize would be better off selling to a big private sector firm.

Isn't There a Stirling Engine/Linear Alternator Entry in this contest, All the soldier has to do is keep one end of the device warm, and the other end cold...(just dont ask where he's stuck it!)

Thats the beauty of the stirling engine, the only requirement is a difference in temperature... Place next to camp fire, in a cold stream, Or even in sunlight (with parabolic reflector), or use body heat....

Oh great, now that someone (else) has finally made the connection between devices that store energy and release it fast as opposed to those that release it slowly (unless your lappy catches fire), we'll all be stopped from carrying electrical items onto planes.

On another note:

"Sir Christopher Wren's 40 _schilling_ prize motivated Newton to begin writing the Principia, a hotelier's prize launched Linbergh across the Atlantic, and the X-Prize Foundation helped put _Starship_ One into space."

Good grief!. If this is the standard of the pentagon's research, no wonder the world is such a dangerous place

if you'd thought to use the word "boffin". Perhaps your ration for the week has been used up?

Now, where's my proposal for my cold fusion pocket battery cum thermonuclear hand grenade. Must be worth the Fed printing up a few billion for meeeeeeeeee.

I think his name is Charles Lindbergh. With a d. Otherwise he would be Lucky Liny. Not Lucky Lindy. Which he was.

This tech all ready exists - although perhaps not in a practical state. Joel Schindall at MIT has been working on carbon nano-tube based capacitor batteries which will output much more power than this in a much smaller unit.

If I remember correctly he's had numerous (as have others working on capacitor-based tech) offers from mega corporations to buy out his research. No doubt they want to bury it like the greedy fuckers they are.

Still, when you work at MIT, I don't suppose you really need to enter into any military based contests to fund your work.

I saw your estimate of "several pounds" of TNT being equivalent to 1920 Watt hours of energy, and I wondered if that could be true -- after all, the entire 4kg pacakge could be described as several pounds.

But you're right - 1920 Watt hours is just over 6.9MJ. 1kg of TNT has about 4.2MJ of energy. So the device they're asking for really does contain as much energy as a bit over three and a half pounds of TNT.

Unless we've both made some terrible error in working it out.

Given that the amount of time and R&D already invested in secondary cell chemistry has only resulted in a peak of 190Wh/kg, such an improvement requires a step-change of the chemistry.

Which means that Li-Ion (regardless of the chosen other metal) won't cut it.

This leaves the esoteric power cell (not battery!) options - eg fuel cells.

I have no idea if a reversible fuel cell would be feasible, but I expect the Pentagon wouldn't mind all that much if you had to keep topping the powerpack up with fuel, if that fuel is relatively easy to come by.

Of course, that fuel will (obviously) contain the energetic capacity of ~1.5kg of TNT.

So not likely to end up in consumer devices. Shame.

1kg of TNT is 4.2MJ? Surely it must be more.

1 litre of petrol used in a car engine releases 34MJ (google for "34 MJ petrol")

You forget that petrol can only release its energy with the aid of an oxidiser - in this case oxygen (air). Lot's and lot's of oxygen.

The density of petrol is roughly 0.737 kg/L. Assuming 100% octane isomers with a molecular mass of ~96g/mol we find that there is ~7.68 mol of octanes in a litre of petrol. 1 mol of octane reacts with 12.5 mol of oxygen (O2), thus 1 L of petrol requires ~96 mol of O2 to combust completely.

Oxygen has a molecular mass of 16, so every litre of petrol requires 1.54 kg of oxygen.

At the very least we are looking at 0.737 + 1.54 = 2.28 kg worth of reactants for 1 L of petrol to release all its energy. The volume requirements, considering all that O2, is much larger still.

1kg of TNT on the other hand takes up only ~600 mL, and releases all its energy quite quickly!

Work out the energy output of a machinegun sometime. The Maxim design, slightly modified to the Vickers in 1912. could sustain 200 rounds per minute for hours, if you had the ammo, the cooling water, and spare barrels. That's multi-kilowatt energy deliver, and a huge amount of waste heat (cooling water, spare barrels, and hot brass).

Maybe they should fit MHD generators to the machineguns.

"1920 Watt hours is just over 6.9MJ. 1kg of TNT has about 4.2MJ of energy. So the device they're asking for really does contain as much energy as a bit over three and a half pounds of TNT."

And the battery is supposed to weigh no more than 4kg (optimistic for such a high output) which is nearly 9 pounds (8.8, to one decimal place) so TNT, at a mere 1.5 pounds (approx), is the clear winner in the energy density sweepstakes (and can probably release it a lot faster, though some batteries can be pretty impressive if "disposed of incorrectly")

How about a radioactive power supply, combined with a photovoltaic cell tuned to absorb gamma rays?

Nah, the lead shielding would tip the weight over the 4kg barrier.

I'm sticking with my cunning plan to make a TNT-based battery - 1.5lb of TNT leaves me with a 7.3lb margin for casing and whatever technology I need to release the stored energy s.l.o.w.l.y as electricity rather than in one impressive (devastating?) rush of heat, light and noise.

Wonder how I'd go about recharging it?

Hydrogen fuel cells are endlessly ridiculed for a reason, Namely they can't even come close to the energy storage capacity of a simple Li-ion battery when you include both the stack and the storage tank (only fair since you need both to get power like a battery does).

Let's take the provided example of Jadoo's system. Even with their largest tank (weighing 5.1 pounds, or 2.32 kg) and providing 360 watt hours. Couple that with their stack weighing another 2.32 kg for a total of 4.64 kg. That's a measly 77.6 wh/kg. Even the storage tank alone only has 155 wh/kg, which can't even compete with today's li-ion (which top out at around 220 wh/kg not 190 btw)

http://www.jadoopower.com/n_stor_fuel_canisters.html

https://jadoopower.com/fuel_cell.html

http://www.sionpower.com/applications/consumer.html

Despite what Randy says, lithium batteries continue to advance. Sion power has produced lithium-sulfur cells with over 350 wh/kg and claim a theoretical upper limit of 600 wh/kg. They also claim the next generation of their cells will reach 450 wh/kg. I'ld say they are pretty damn close and a much better bet than Jadoo.