We made this awesome electromagnet arm bracer for the Replica Captain America shield, just in time for Captain America: Civil War. It uses a set of lithium-polymer batteries from Hobby King (http://goo.gl/pkSN00) running in series at 72V. This produces 720kg of magnetic force at each electromagnet. That's over 3000lbs of magnetic lifting force together!
The electromagnets are available on Amazon (http://amzn.to/2ae7VHr), and are rated for 12V, which produces ~120kg (1200N) of lifting force. We're running them at a much higher voltage for short periods of time to create awesome power. They will overheat if you use them for over a minute at this power level!