I can't see any power plants that might work that wouldn't otherwise be a big block on the back that could be about as vulnerable as a tank engine, but more devastating for the man in the suit. You either get batteries that are really heavy to get a long charge, and/or the material outlive the casing and rot the entire thing from the inside out. And often times these big batteries may not be the most effective their output vs discharge cycle rates. So you probably won't be much better off I'd think, if I read everything right.

Otherwise, the Lithium-Ion batteries the defense networks of the world would use have been deemed pretty unsafe. 

Traditional ion batteries can quickly melt down, especially during re-charge and cause an explosion of breakdown of the batter in or between use. They need to be handled with a considerable amount of care and I imagine the last thing anyone in a power-suit would want is for something to trigger catastrophic heating, melting the battery and shutting him down as hot acid pours down his back or it explodes.

Though Lithium-Ion batteries are safer, they carry a lower discharge rate. You can get higher energy storage though, but you would need a lot to probably get close to the same output as heavier, traditional batteries.

But when they fail...

[img]http://www.batteryuniversity.com/images/partone-5b-3.jpg[/img]

Lithium-Ion and their cousin Lithium-Ion-Polymer are only really useful in small-scale design like laptops, mobile phones, or maybe remote controlled vehicles or cars. Old-school lead acid batteries would have the discharge rate needed to maybe power a suit of power armor, but then we go back to Evan's point about the military trying to make exo-suits: it just makes them bigger and bulkier. You also can't really recharge them well, so when they're done they're done.

The closest thing to an exoskeleton suit that might even be maintained is bomb defusal armor:

[img]https://3.bp.blogspot.com/_ZPX5XDWisGI/S-FPIZJWgyI/AAAAAAAABr4/V6n8U3at5wM/s1600/hurtlocker+bomb+suit.jpg[/img]

And I don't think you could get a range of motion to throw grenades back, and heavier calibers could still knock you down.

You could probably mod them to strip them down for conventional combat. But you're likely to remove some of the bomb protection features to get a range of mobility needed to fire anything but a handgun, like removing the overlapping armor plates and removing foam blast protection. Joints might be reinforced with something like light-duty kevlar or stay to foam to absorb blast damage still, but might still render them vulnerable to shrapnel or bullet fire. You then got a lack of situation awareness from the helmet.