The battery on my car gave up last week. I'm used to the idea that
when the battery is dying things like lights get weak and the car gets
difficult to start.

Not this one.

The first clue (which I missed) was that the radio failed. Car was due
for a service soonish so I just put up with it. Except that one day,
driving along, the radio came on of it's own accord but didn't work.
Then the glowplug light stayed on after the car was running. Finally
it failed to start but the lights still worked.

I was still not tuned to the idea that the battery had failed.

When I had the battery replaced I asked a few questions about why I'd
missed the clues. I was told that modern cars "load shed" so when the
battery starts to play up unimportant things (like the radio) are
deemed not worthy of needing power and are "disconnected".

I've learned something!