Constant engine cycling while stopped??

That's normal, though rare.

It's just the system protecting the battery-pack, using the start cycle to spin the engine up & down as a way of draining off the excess charge.

By the way, no gas is actually used during that process.
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I would guess if you observed that again, you should turn on the AC full blast? Maybe that would put enough load on the battery to prevent this?

 

Actually happens to me quite a bit. Especially when cold driving on the highway my Prius seems to keep either all blue or 1 green bar and if you get off a long exit ramp it's enough to give you full green bars, hence the ICE cycles off and on if theres a red light at the end of the exit ramp. This spills off the extra charge in the battery.

 

The A/C would help drain the battery, but you don't have to do anything. By spinning the engine to pump air the car is maintaining its own battery just as it was designed to do.

 

Another point, just in case it isn't obvious, is that the engine is not running in this situation. It is spinning, but it is not running. The Prius uses the engine as a big drag on MG1 to burn off excess energy. As said above, it's completely normal.

Tom

 

What a waste though. In a few seconds that energy could be used for acceleration instead.

 

You may know that, the Prius can not, so it's better to waste EXCESS charge than waste the battery life.

This happens to me frequently. I live at the bottom of a long hill and keep my speed at the posted 25 MPH in the neighborhood. Then comes my driveway, so no acceleration there. Really freaked me out until I figured out what was going on.

 

If it had more battery then it wouldn't have excess charge in many of these situations... Or if the driver had a way of reducing the SOC target prior to such conditions the excess charge could be avoided. Better not to reach the excess charge state in the first place. Unfortunately, the ability to do intelligent overrides is not available. So it is still a waste, just as failing to conserve engine heat during winter is a waste. (The weather gave me a head fake and I pulled one section of lower grill block too early as a precaution. I have been regretting it in the form of a mileage hit...)

Plus I doubt that it truly has excess charge in the first place. One thing I've noticed is that after the car is parked and sitting in the garage the SOC is not infrequently lower on the next start up (sometimes by as much as two bars, if the ambient temp is cold.) The difference appears to be that the battery is cooler and the indicated charge is less. This has been most noticeable in conditions where the battery is getting considerable use before shut down (gentle mile long EV glides--no EV button though.) A strong regen is of course yet another example of such a battery warming charge transfer.

 

I'm not sure how this would be interpreted. A NiMH battery has -lower- voltage when warm and -higher- voltage when cold. This effect is what "negative delta" chargers use to detect when to switch to trickle charge. How could the system think there was -less- charge with higher voltage once the battery cools off? Perhaps it's the electronics that's not temp compensated or incorrectly temp compensated?

 

I don't know. I can only relate what I have observed. It has made me suspicious of the SOC indication when it is supposedly in the green.

 

It's not excess charge in that the battery is in danger of being damaged, it is excess charge in the sense that the SOC is high enough to leave little headroom for additional charging. The control system tries to keep the SOC in the middle so that there is power for the electric motors, but still room for regenerative braking. The control system has no way to look ahead and make intelligent decisions about traffic and terrain, so it uses a relatively simplistic approach and discards charge when it runs out of headroom.

In many cases the driver can reduce SOC prior to these situations, but it requires prior knowledge of terrain and traffic conditions. There are numerous postings on PC relating to people modifying their driving techniques to deplete SOC prior to reaching a long downhill. It can be done, but it's not always easy or possible. If it was easy the control system would do it.

As for a bigger battery, yes, that would store additional charge, but the gains aren't as big as you might think. The HV battery in the Prius is pretty small. Even if you double the size it is still small compared to the energy needs of an automobile. Testing done at Argonne National Laboratory demonstrated little difference in mileage between a new battery and a half depleted battery. The same is true with the current battery verse a bigger battery, unless we start talking about a plug-in.

The long and the short of it is that yes, it is wastefull to have the Prius throw away energy at the bottom of a big hill, but from a practical standpoint it doesn't happen very often, and it is a realitively small amount of energy. For a small percentage of drivers a bigger battery would help a bit. For the rest it would be an unecessary expense and additional weight. Like all design decisions, it is a trade off.

Tom