The US military's various secret skunkworks type labs are actually producing a number of exoskeletons that are just a few slabs of ceramic plate away from being power armor.
They are deisel powered, because of the energy requirements. There's a reason why BigDog is diesel.
Batteries aren't too shoddy either.
From the wikipedia article:
Lockheed Martin announced that it is evaluating fuel cell power sources to increase the duration to support a 96-hour mission.[11]
Fuel cells leverage a high density fuel as a power source. This is because batteries simply don't even compare to what you can get out of a fuel cell. The only way to top a high density fuel is to go with controlled fusion. Since small scale controlled fusion is totally not a thing right now, Lockheed Martin is going with a fuel cell.
The issue with power armor is that you end up with a set of design factors that work against each other. This exoskeleton is not designed to be power armor. It is a human assist device for transporting heavy payloads. As such, the device does not have to calculate having a huge mass all by itself that it has to assist in transporting, (and thus, the fuel/energy reqs needed to transport, and thus the weight of the fuel and fuel tank in addition to the weight of the armor... etc.) This is similar to the technical limitations that engineers bump into when designing chemical rocket engines. (Most of the energy of the burn is taken up just lofting the remaining fuel up into the air.)
Actual power armor, that is ACTUAL ARMOR, is going to weigh a shit ton. (It has to absorb kinetic impacts that are lethal without the armor on, or else it simply isnt power armor. The best way to absorb a heavy kinetic impact is to have a large inertial rest mass. That way when you get balsted by that 50 cal machine gun, you dont get blown over by the barrage of projectiles.) That means the armor is going to consume a lot of power to move all that heavy mass. That means a very large fuel tank, which adds additional mass.
The energy needed to move that heavy mass around is nontrivial, and once it is used to move the mass of the armor, it becomes heat. This is why large hydraulic systems have radiators on the hydrualic fluid reservior.
http://www.hydac.com.au/cooling-system.aspxPower armor that is actually useful as power armor, is going to need an enormous amount of power, is going to produce non-trivial amounts of heat, and is going to need active cooling.