Hello and welcome to the forums, luke.
I'm not sure what you mean by "phone goes dead." Do you mean when the battery drains to such a point that the device becomes unusable? Or do you mean that you have an issue wherein the device is dying unexpectedly?
At any rate, the SD card should not be affected by the device's functionality. If you are losing media, it is most certainly media that has installed into the system memory, not onto the SD card.
Perhaps you have been unknowingly not successfully installing media to your card.