Engine turns on and off at stop light

My 2006 with 172k miles on it does similar. I typically let my foot off the brake letting the car roll forward a few feet, then reapply brake. It typically stops. I saw another post about this that said it had to do with the 12 volt battery, but I disagree because I just installed a brand new Optima battery last week.

 

Yes but did you top off the charge on the new optima?
Sounds like 12v activity to me.

 

My car has been doing this since purchasing it 5 years ago. The Optima was purchased from the company directly and installed the same day it was received. I typically drive 300 miles a day with this car. It's NOT the 12-volt battery.

 

So you didn't top off the optima and are now possibly suffering again from a weak 12V ?

 

When this happens, look at the MFD traction battery state-of-charge gauge, to see how many bars appear. If you see 8 green bars, the engine is being spun by MG1 to remove excess traction battery charge.

If that is not the issue, measure voltage across the 12V battery when the car is IG-OFF, after it has rested overnight, and before you start the car in the morning.

 

Patrick, it's probably the traction battery being discharged. This always seems to be an issue when it is fully charged. So this is normal operation for the Prius?

 

yes. if you do a lot of regen, the computers use the ice to remove excess charge. you can't add anymore to the battery. you must have a unique commute.

EDIT:and btw, many optima batteries come from the distributer undercharged, as ursle suggests. the car will not charge them properly.

 

My Optima battery cam from the FACTORY not a distributor.

 

makes no difference, just trying to help you understand.

 

I checked the status of the battery first thing this morning and it's good to go. 12.3V and that's with dual dash cams (Front and Rear) running 24/7 off the 12-Volt battery.

 

okay.

 

Is that really how it works? I see 8 bars of charge on every drive I take, and never see this happen. I do see that charge current is reduced to zero when the battery is full, and regen no longer occurs, which is logical.

It does not make sense that there would be a systematic means of overcharging the battery that then requires dissipation. What are the conditions which require this "excess" to be wasted as heat?

Is it not that the engine has dropped out of S4 and needs to reheat? That I have seen, particularly when the engine is not used due to long coasting.

 

Well, your drive may not sufficiently overcharge the traction battery for this to happen. Just because there are eight green bars, does not mean this will always happen.

However, if the engine spins up because of overcharging, then when you look at the MFD you should see 8 green bars. The conditions that require this is when regen exceeds the traction battery's capacity.

For example, descending from Kingsbury near Stateline, Nevada at 7,800 ft altitude, down to Mottsville, NV at 4,800 ft altitude, at speeds of around 60 mph. This is just nine miles long, but the altitude drop and vehicle speed will force a traction battery overcharge. The engine will spin up as MG1 tries to burn up excess charge. If you stop the car, the engine will continue to run until MG1 has dissipated the overcharge.

 

Why nobody mentioned the heat? If you have the heat on and engine is getting cold it will come on to keep the heat running properly.

- Alex

 

because if it's calling for heat, the engine will come on until satisfied.

 

Driving down from over 1000m asl to sea level on Haleakala Highway quickly charges the battery as full as the battery ECU will allow, and charge/regen current goes to zero (using PriiDash). The battery temperature sensors will then show the attendant increase that comes with maximum SOC. If that temperature were the problem, I would expect the battery cooling fan to increase its speed considerably. Instead I see the fan voltage only goes from ~6-8 volts to maybe ~9-10 volts. This would say that the battery temperature of 47-49C which I see in this condition is well within design limits, and would not be of concern. This is contrasted with the battery fan's noisy maximum RPM heard during the elevated heat of a battery failure.

In the above case, I do hear the engine increase in speed if I use the brakes, but I consider that to be engine braking mode coming on automatically. If I select B mode manually, that will cause engine speeds of up to 4500rpm on the steepest sections. However, that would not be "burning excess battery charge" but just using the engine as an air compressor for mechanical braking purposes. The battery current stays at near zero in this scenario, since regen is no longer possible. No battery current is shown being drawn by MG1, which is driven by the internal loop in the inverter, deriving power from MG2 via the car's inertia.

At the bottom of the descent, the car will come to a stop with full SOC, elevated battery temp, and the engine simply shuts down as usual. So if someone's engine does stay on, something else is the cause. It could be due to heating requirements as mentioned above, or due to dropping out of S3. In a cold place a similar descent will have the engine cooling considerably.

This is not just a semantic point being made here. The batteries are not able to be overcharged, as that is the entire point of having the battery ECU in the first place. I would respectfully ask to see any references to correct my erroneous opinion, if such is available.