I've been driving my NAH for over three years now and I can never remember having the needle for the battery meter all the way to the top. This week I've been on vacation in NH and while driving on the down sides of the Kancamagus Highway - long steep runs - the meter was pegged on full for the first time. The odd thing was that while the meter was on full the engine would run at high rpm's when going down hill about 50 mph until I touched the brakes - not sure what the computer was trying to do by running at such high rpm's.

 

Like any battery, it presumably is possible to overcharge the NAH electric motor battery. And like any battery, this could significantly decrease its lifespan, make it less efficient, and possibly lead to explosion or fire. So I suspect that the NAH computer is programmed not to let the electric motor battery enter into an overcharged state and it finds a way to discharge the extra stored energy by running the car at higher RPM's. Prius owners have reported similar phenomena as you have, which is not surprising, as the NAH uses Toyota's licensed hybrid technology. As one Prius owner reported back in 2004:

 

I can tell you that from personal experience, on the 2nd gen Prius (04 to 09), if the HV battery SOC reaches near 80% (shows as completely full on the MFD aka multifunction LCD), the car will essentially kick into B mode (as if you shifted into B). The ICE will fire up (even though you're <42 mph and the ICE is hot) and run at potentially high RPMs (sometimes above 3000 RPM) while going downhill. Also, regen stops (as indicated by the BTA (aka HV battery amps) ScanGauge XGauge). Instead of regen, you end up seeing a draw from the HV battery while the ICE is fired up.

I hit this quite frequently given where I live and work now. I have to go down a long downgrade to get home.

If the stars align so that my HV battery is "full" (~80% SOC) while going downhill, the behavior will always repro, w/o fail.

When I lived in the SF Bay Area, I had no such long downgrade and thus never/rarely encountered this.

From what everyone has gathered, it's to prevent the HV battery from getting overcharged. Many others have experienced this. The Prius has no tach nor does it have an indicator to indicate HV battery state of charge, BUT one can monitor both of these w/Scangauge. I normally have engine RPM and soc up displayed as two of my gauges.

As for the 2nd phenomena reported by efusco, I've never personally hit it but others have been able to repro. efusco is one of the main moderators of Priuschat.