That's not how you calculate the current coming out of a battery. 450mAh @ 25C means it can provide 11.25A maximum (at least in theory)...not at 1V but at it's normal voltage. To calculate the current based on the battery volt drop you would need to know the effective impedance of the battery which you don't. It's usually easier and a lot more useful to measure the current.
So basically the lipo can provide up to 11.25A. Your power supply can provide around 3A. It's not exactly surprising that it's not good enough.
Steve