I noticed that this new 2010 Prius 3 i got with 140k miles was only indicating 4, maybe 3 bars of charge on hv battery pak while idling for some minutes in heavy traffic. Probably each bar is 10%? A little while later, after resuming 70 mph cruise, i noticed that the charge level was back up to 80%. Isn't the battery pak never supposed to get deeply discharged like that no matter the length of idling? Is ecu that monitors the charge level maybe bad? But techstream shows no dtc's for hv battery pak or for charge control system.
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.
Do continue watching for signs of battery degradation from aging. But the patterns you have described are not sufficient to indicate such.
