My understanding is that the effect of the explosion at the beginning of Iron Man 1 is that Tony Stark has metal shrapnel in his heart and needs an electromagnet to keep it from puncturing his heart.
When Obadiah Stane paralyzes him and tears out his power source, Tony tries to get to an older version, getting weaker and weaker.
How does removing the power source weaken him? It is not powering his body but a device to protect his heart. No power ultimately means a punctured heart but there is no reason to get weaker in the meantime.