When does the HV battery charge?

The HV battery charges from the gas engine and/or when braking.
A very tiny amount occurs if you brake at a stop light.
But if you are going down a long hill you'll get some regenerative braking.
When you lightly touch the brake pedal you'll also get regen.

Mike

 

Get your popcorn and keep watching the screen... It'll tell you all kinds of things about what's gonna happen next...

 

Your foot on the accelerator is just a hint to the computers what you want.

The computer may decide to make more power than that, and store some in the battery, it may decide to make less power than that and take some from the battery.

If you decide that the computer knows what it is doing, you will worry less.

It has been 7 years since a semi trailer hit my Gen 2 but I still remember being consistently surprised I would go up hill and gain battery power.

Besides the ways 3PriusMike suggested, in winter I found that it would run the engine to make the heater work, and save power to the battery while doing so. I always had more charge in the battery in Winter.