Battery fully charged (saturated) during a descent, but ICE stays on for long time after that

If the battery is hot or overcharged, it will do this as normal. Also if previously driving in EV and the engine kicks on, it must complete a full warm-up cycle.

 

The ICE is indeed spinning at the times you mention, but it is not "on". Your Prius is using engine braking (Acting as a vacuum pump) to dissipate excess energy, no gasoline is being injected into the cylinders. Regenerative braking is not available at those times and you will be using friction braking to slow your car down.

There is one more time that this happens. I have "Full range" dynamic cruise control in my 2016 Prius. When it needs to slow the car down it spins up the ICE to dissipate excess energy and slow the car faster than standard regeneration would allow.

JeffD

 

You said "silently, at low revs" - which means it's not engine braking, which is far from silently.

If you display the ENERGY MONITOR it will give you a much better idea of what is going on - it's much more useful and accurate than the Hybrid System Indicator:

 

Assuming Gen4 still works like a Gen2, you need to first be aware of the hybrid warm up Stages. Final Stage 5(?) only kicks in after a full stop. So if by chance in your commute, you may never get to the last stage, that is sometimes the complaint.

 

The EV light is despicably inaccurate, I threw that screen into the "don't bother" class after a couple of weeks with my car, never used it since.

Use the screen I suggested (Energy Monitor) and it shows what's really happening.

But, that said - mine will sometimes, for reasons unbeknown to me, tell me that "EV Mode is Unavailable", and the ICE will run whenever the car is driving, but turns off at traffic lights - maybe for 5 minutes, then goes back to "normal". I haven't been able to determine why - just some secret the microprocessors are keeping to themselves.

I just go with the flow. I'm not quite sure if there is something as definable as "normal" with a PRIUS.

 

I'll go with overcharge in the OP's scenario.

The system doesn't use the full range of the battery's capacity; there is buffer at the bottom and at the top of it. This is to maximize the battery's life. There are times when the system will allow the SOC to go beyond the high and low points. Slowing down and stopping is more important than battery life for instance.

On a long descent, the car will activate engine braking when the SOC reaches its top limit. Engine braking doesn't completely stop the regen braking while coasting though. So the battery is being charged beyond that top limit. Once you reach flat ground, and the system sees that you no longer coasting and gaining speed, it wants to get the battery SOC back into its safe range. If using EV mode alone isn't doing so fast enough, the ICE will be spun by M/G1 to use up battery charge. Gas isn't being burned.

 

Can't remember who said it recently, but it is necessary for the ICE to turn from time to time simply to keep the various pumps lubricated

NB, don't overthink it!!!

 

80% is the upper limit for the NiMH battery, but the Li-ion limit will be close to that. So the full battery SOC on the car display is that amount. It isn't a hard limit though, and the charge can go higher under certain conditions. If it does, the car will try to get it back under 80% as soon as possible.

 

I drive in Italy a 2016 Prius4, with NiMh battery and ~20,000mi done.

I normally use a ScangaugeII OBD device, in which I look, among others, at the SOC and HV Current xGauges. These values appear to be consistent with Prius instrumentation and with the evolution of driving conditions.

Few months after the car purchase, some oddities started appearing at the ends of SOC range in the hybrid system, for low values (2 bars and <~43%) and, specially, for high values (7 bars and >~70%). Previously I had no OBD equipments and it is possible that I had not recognized these behaviors before.

I recently serviced the Prius. The Hybrid Check test has been regularly passed and the HV battery fan appeared clean.

Next behaviors don't depend, as far as I can evaluate, on the temperature of ICE or of Battery. Engine finished the warming-up and the Idle Check was already performed. External temperature and battery fan status did not suggest any overheating issue.

- Early Saturation -
It can start immediately after the the switching on of seventh bar (SOC> 67%), or slightly over. In any case, much earlier than usual values of other HSDs (8 bars and SOC at 77-78%). Seeing all 8 bars activated is not common.

- Engine spinning persistence -
Once the Saturation is reached, the time to restore normal conditions is often unexpectedly long. As normal conditions it’s intended, below 110kph, the switching off of the ICE when gas is released and the possibility of moving in EV under 1/2 ECO. Sometimes I drove for tens of kilometers, with the SOC even dropping up to 50%, before the ICE switched off again.

- Low SOC recharging hysteresys -
When SOC level is low, at 2 Bars and below 43%, the transaction discharging -> recharging seems to be retarded and losing an appreciable amount of energy. You can see for some time ~ -20A flowing into the battery ( > 4kW of recharging power) but the SOC, instead increasing, get lower of some %. The amount of energy dissipated before an actual charge starts and the SOC regain the initial value is appreciable. The black hole where it is absorbed is unknown.
At higher SOC values, the discharge -> recharge transition is, as usual, immediate.

- Forced Decharging after a Saturation -
In, luckily, rare cases a saturation can result in a not interruptible forced discharge, characterized by ICE rotation and by production of a large amount of electrical power, up to more than 100A. The SOC quickly comes under 40% and continues up to level that the system considers appropriate. At that point the SOC restarts increasing and the system returns in normal state once it reaches 47% (upper limit of forced recharging). The SOC lower level I've seen is 24%. Normally it remains over 30%. In any case, the instrumentation continues to display 2 Bar of charge level.

 

I had a Gen 2, and now have a Prime. They are totally different.

 

Sorry, but my Prime is apparently not a Gen 4.

 

@flic I might be repeating what others have already said. I live in a somewhat mountainous area, lots of long grades up and downhill. Going down a long downhill grade, say 5-8% for a couple of miles in "D" or "B" will charge the battery to full bars on the "hybrid system indicator" or HSI (I have learned a lot of the Prius acronyms) which translates to about 75% or more of actual charge. The battery, at least the NiMH battery, really likes to stay around 50-60% charge, 75% being near the upper limit, others have said 80%, I have seen 77% in mine, just depends on various conditions.

On a downhill grade, once the battery reaches its maximum charge you will feel the ICE kick in and the the RPM will rise. Coasting downhill in "B" you'll hear the ICE kick in even if the battery isn't fully charged. Once the battery is fully charged in "B" the RPM's will rise, quite high if you're going say 55 MPH or more (90 km/h or more).

The reason the ICE stays running after a long downhill grade is to control how quickly the battery is discharged. You might also notice that brake regeneration is reduced under these conditions. This is all in effort to keep the battery in good condition.

Nothing should be overheating, I have thought this was an issue as my Prius had a TSB out which coincidentally dealt with battery discharge/loss of power going downhill. I bought one of those OBDII WiFi dongles and started looking at inverter coolant temps, motor/generator temps and battery voltages, etc...etc...trying to find an explanation for the conditions. Nothing really gets hot from going up or downhill. The transaxles in these things are simple, less friction, less moving parts...less stuff to go wrong mechanically speaking.

One observation of mine on going down long downhill grades is that using "B" initially will charge the battery slower, around 38 Amps over regenerative braking, which can be as high as 130 Amps (reduces when battery nears maximum charge to around 20 Amps and stops charging when you approach the 75-80% mark). Not an engineer here but I would think that slower rates of charging would be better for the battery in the long run, but we can also assume that Toyota has a pretty good handle on what is acceptable for maintaining these hybrid batteries given how reliable the Prius has been since Gen 2.

Also keep in mind that even while the ICE is "running" the ICE really doesn't consume any appreciable amount of gas in compression braking. That is, on downhill grades when the ICE is running compression braking is used to maintain the car's speed instead of the motor/generator when the battery is at maximum charge.

Hopefully that helps, in additon to everyone else's responses. Unless your battery discharges to one bar of SOC after going downhill and driving a a while afterwards, nothing is wrong and you can be at ease.

 

Almost correct, but actually it is the electric motor spinning the engine to dissipate surplus electricity and no petrol is burnt/burned. (Can't remember which spelling USA prefers ).