I have a 1998 Lexus with 235,000 miles. On occasion, when I first try to start it in the morning, the electrical system will be completely dead without even enough current flow to power the LED displays. This can happen any time the car is parked overnight, but only occasionally. The car can be parked for 30+ days with no problem, but go dead after one night, including no interior light, or response to the remote unlock. This will happen when all the systems, lights, air conditioning, interior lights, wipers and stereo are turned off manually. The electrical system has been thoroughly investigated, including the battery, starter, alternator, and cables, all of which are supposedly in good shape. I can start the car with a small supplimental jumper battery. Once the car has been started for the day it behaves perfectly and can be started repeatedly for days thereafter.
By stating that you can start the car with an external jump pack it leads me to ask you how your battery terminals and clamps are. They need to be VERY clean greased and tight. The clamps are very often neglected…if I were you I would remove the clamps and clean them with the 3 dollar tool they sell for this purpose…it is shaped like a bullet and has an internal and external wire brush to clean the batt and clamps…I highly reccomend that you try this first.
I agree with HB. Clean the battery connections. If that doesn’t fix the trouble then check for a wiring problem between the battery and the main electrical distribution panel under the hood. Having a test light probe to check for power would be a good tool to have for testing.