So the battery part of the key is just for locking/unlocking doors and the trunk. Your key has a separate transponder that sends out a key code signal and doesn't need a battery. The transponder is charged by the car's immobilizer antenna emitting a radio frequency signal. Same principal a wirelessly charging your cell phone.

The constantly flashing key light means the car does not recognize your key. The transponder in your key could be bad. Not sure which key you have for your car. If key is a key + fob combo, changing the housing means you changed the transponder or didn't transfer the original transponder over.

If your friend that is an auto electrical tech has a scanner that can program keys, hopefully he can just reprogram the car to recognize your key. The other item that could have failed is the immobilizer antenna/control unit as part of your car. That is a replaceable part of the key cylinder/ignition switch in your car.