Low charge level ok for overnight(s)?

An uphill grade going to your house is better for the traction battery than an uphill grade coming from your house. Your Gas engine is warm coming home, A cold engine puts most of the traction work on the main battery.
You're overdue for a 12 volt battery if it is the original.

 

I don't think it's a concern at all.
In fact, I've read some posts that suggest turning off the Prius with a lower state of charge on the battery is better than leaving it turned off in a higher state of charge.

I wouldn't worry about it at all.

 

As Jim said, this is probably preferable to the opposite situation of having the uphill to traverse when the engine is cold. When you leave your house you'll have a half mile of downhill to replenish your battery and everything should be good. It does absolutely no harm with nickel chemistry batteries to leave them in a mid level state of charge.

 

I would also add, if it's hot out side (since you're decending a hill at first) blast ac when you're going down the hill to prevent battery heat in the first place. Also I'd get the hybrid battery cooling fan cleaned since it's at least a decade old.

Congrats on the new ride! You'll love it! Don't hesitate to ask questions, even if you think they're silly

 

Hi Priusonamountain. It's only really significant if the HV battery regularly tops out to the full 8 (green) bars when you're descending.

In this case you have three options.

1. Do nothing and let it take care of itself. This is what most people do and the Prius manages the battery charge pretty well. However with an older Prius and a slightly tired battery you may get a bit of overcharge and heating of the cells under these conditions. Mountainous driving conditions are considered to be the hardest case for the Prius.

You may hear the engine rev loudly as you descend after the battery charge tops out. This is not actually the engine "running" in the traditional sense, it's actually the electric motors dumping excess battery energy by just spinning the engine like a huge air pump (without spark or fuel injection).

2. If it's really hot then some people use the aircon at full blast on large descents. This actually has two benefits. Firstly it consumes power from the HV battery which can reduce it's peak charge, and also it helps a little to keep it cool as you mentioned.

3. You can also use "B" mode on the transmission and it applies more engine braking. This can also reduce the peak charge on the battery, though you will probably hear the engine revving even more pronounced than isncase one mentioned above.