When that happened to me, it turned out to be a grounded compressor (magnetic) coil that was at fault.

The coil itself cost around Php 500.00-800.00 (sorry, I can't remember exactly). I paid the same amount for labor and materials.

Try disconnecting the battery with the engine running. If it dies, you can suspect the alternator.

You can check the battery by installing it in a different vehicle.