I understand that the battery runs in the 50-60% SOC with the end points being about 39.6% to 79.6%. I was driving out of the city on the freeway and the road involved about an 7km downhill run about a 500m drop so it had time to charge up. Anyway, about 2/3 of the way down, it was sitting on 79.6% and didn't increase further. Now where is that energy going as I am still charging up or does it disconnect and discharge in some other way to prevent it from going above 80%?
The Prius has several methods to avoid overcharging the HV battery. Descending a hill as you describe, these are the possibilities: 1) This one is obvious: If your speed is high enough, it actually requires power to go downhill. In this situation the Prius stops powering the ICE and coasts or draws power from the battery for MG2. 2) If braking is required, the Prius will continue to apply regenerative braking, but instead of charging the battery, it turns the ICE as an air pump to dissipate the energy. In other words, the ICE is turned by MG1 as a dead load. In Prius terms, this is called "Engine Braking", and is the same action as switching to B mode. 3) If engine braking alone is insufficient, the Prius also applies the friction brakes. Tom
I have had quite a few experiences with driving a genIII Prius in mountainous regions...and I have found it's pretty good about charging the hybrid battery fully when coasting downhill with any major embankment. Breaking does increase the charging some more...but overall coasting without adding acceleration can be drastic. There are quite a few different opinions about why/when the hybrid battery discharges or recharges. I have found that on occasion it charges all the way up...and then it stays fully charged until I go through another major acceleration uphill. This relevance with hypermiling of uphill/downhill mountain driving is why I've found that my mpg can still stay high with mountain driving.
RE: #2 above--at around 75% SOC regen essentially stops...I guess that's what you were eluding to in #3. Also, the car will spill energy into MG1--which causes the ICE to spin--to prevent battery overcharge.
Do remember that "fully charged" means about 80% charged. The HSD won't charge the battery any further than that. The instruments indicate all green or whatever to show "full charge" but it's actually about 80% charge. I can elaborate if you want to know -why- it doesn't charge further, but the short answer is it's to ensure the battery has a long life.
I don't know about that, I have a hill I drive down on occasions and I reach 80% (full on dash meter) about midway down but according to my SGII the battery is still taking 20-30 amps for a bit of time before the engine starts to rev and brake the decent. So it is still taking a charge beyond "full battery" (dash meter).
I've seen as high as 82% on Can-View a few times--but those are VERY rare. A full battery on the Energy screen is ~72-80%, give or take. So yea, you'll show "full battery" at around 72% and the battery will still take some charge after that, but never much above 80%.
Remembering that with NiMH batteries it's -impossible- to know the actual state of charge accurately. You can estimate (what the HSD does) or you can monitor the battery temperature while charging, knowing that it will start to really heat up after 80%, so you will know that a little while ago it reached 80%, but you can never know what it is at any given time. Also because you really don't know the actual capacity either. Again you can estimate and you can measure but your measurement was then and this is now. That's what makes designing charging systems so much fun!
