battery charging

No you're car will NOT explode.

You will benefit from switching to "B" once your battery is fully charged though. That lets the engine do some of the slowing that was previously happening by charging the batteries.

 

When you go downhill and need to slow, Regen fills your battery.

Once you battery is 'full' the engine begins to act as a an air pump, to waste energy so the brakes do not overheat. (This has the odd effect of pressing the accelerator making the car quieter, and letting off it makes it louder, counterintuitive) If you want, you can shift to B when starting the downhill, that will use all 3 systems.

And of course, your disc brakes still work, they just get hotter and hotter. If you worry, take a rest break for your brakes.

(what direction is Vegas a 4000 foot drop?)

 

The rule is: if you need the brakes constantly, shift to "B". This helps keep the brakes from overheating. When you no longer need "B", if you forget to shift back to "D" fuel economy will go down but no other harm will be done.

 

We may have different Ideas about how big California is. It took me 17 weeks to walk up California.

I am going to guess you are coming over Cajon Pass on I-15 which does attain 4,00 foot elevation, but does not drop 4000 feet , Victorville is still 2,700 feet and Las Vegas is 2,000 feet.