What is a normal minimum charge level when idling?

While sitting long times idle in heavy traffic, or long multi-cycle waits at stoplights (especially with A/C turned up), or tooling around city streets on electric alone, the battery bars will drop. With engine off, it will continue dropping down to 2 bars, then should fire up the engine to prevent a drop to a single bar. Cruising around at highway speed, normal is 6 (out of 8) bars, but some variation is still seen depending on conditions.

If the engine is continuously running, then we would not expect the bars to be dropping much.

 

The Prius is designed to never let it drop to a dangerous level.
The lower the level gets, the more power the engine uses for producing electric to charge battery.

But a few bars is ok, and it is typical and normal if idle and using AC and such.

Note: not sure on the technicals (someone can add) but when the bars are at full, battery is not technically full,
and the same as in no bars, battery is not at 0%
I think it may be a 20% +/- leeway on each side full bars/no bars

 

Wanted to add:

That is why the Prius gets higher MPG than standard non-hybrid ICE car.
It will use the battery energy that is stored up.
The battery will be used for motion (traction) and also accessories such as AC, pumps, etc.

The regen brakes will charge it back up and also supplemented charging by the engine too.

 

The bars indicate from 40% state of charge (SOC) at the bottom to 80% at the top. I read here somewhere that the percent of charge indicated by each bar varies with the upper middle ones (5 & 6 I think) being broader than the others.

Normally, assuming the engine is warm and you're not calling for cabin heat from it, the engine will stop when you stop and the battery level will gradually drop to two bars if you're stopped long enough, at which point the engine will start and bring it up, but not all the way -- just enough to ensure it doesn't go too low. Once you start moving again, the system will work on bringing it back up to about six bars, which is where it likes to stay so there's room to store energy in the battery as well as energy to borrow.

 

I think that is about right.
Has been mentioned on here MANY times.

 

Do continue watching for signs of battery degradation from aging. But the patterns you have described are not sufficient to indicate such.