Sorry, you did what? I am not 100% sure I understand. Do you mean you soldered the USB power directly to the battery terminals? That would be a sure way of frying your phone (the battery voltage is about 4.2V max; USB gives 5V). I hope this is not what you meant. The N900 needs the battery to boot. It can be replaced with a dummy circuitry but it is not as simple as just the +/- wires, even with the correct voltage. The middle terminal is also important. The phone uses it to check the battery capacity. The eMMC is non-volatile and does not need a power backup any more than any other flash storage.