ICE Start/Stop Issue

Assuming the miles are accurate, it may be that the HV battery needs to be balanced or cycled.

Do you have access to a grid charger/discharger?

 

And vice versa. It is annoying. Sometimes it will be wrong for 30s or so and then the display will correct. Other times it never figures it out. I think the most likely scenario is that the MFD isn't getting (being sent?) the right information reliably. I think it is less likely that the ECU doesn't know if the motor is on or off. In fact, I just don't see how that could happen without the ECU throwing a code of some sort, since it has so much information to keep track of which would make no sense at all for the motor in the wrong state. That is, RPM, O2 sensor information, and other measures that the engine is on all have to hang together.

I don't know if the MFD goes out to get that information or if the ECU sends it periodically. If I had to guess, it would be the latter.

Ours is a 2007. The pack is original and in so-so shape, but has never thrown a code. The 12V has very little capacity left (12.5Ah) but I cannot see how it would muck this up since in this scenario the inverter is always running, so plenty of 12V power.

 

It might not actually have completed its warm-up into stage 4 with a 10-minute drive. See the article on the 5 stages of Prius warm-up in my signature below. To get the car to move over from stage 3a (or 3b) it involves coming to a complete stop for 5 or 10 seconds with the engine running. After that 5 or 10 seconds, the engine will stop and then the car should behave as you're used to from then on until the next cold start drive. If the engine will not stop after 5 or 10 seconds, turn the A/C off if it is running. If the engine does not continue running when you stop the car, hold your foot on the brake with your left foot and press the go pedal with your right foot until the engine starts, then release the go pedal. The engine should then continue to run for 5 to 10 seconds until it has done its thing and then will stop running. Ideally, you should have had a chance to stop the car before descending your hill, as trying to get the car to stage 4 after the hill descent and a full battery might complicate things with both these things going on at once.

I hope that is understandable and helps.

 

Long hill? Is the pack display solid green? I vaguely recall reading that if the car thinks the pack is overcharged it will use one of the electric motors to spin the ICE motor to bleed off the extra charge. That would probably sound like the ICE motor was running. Except I also vaguely recall that when it does this it is at pretty high RPM. I don't think that is what mine is doing though, it just sounds like the motor is near idle, and the pack isn't usually shown as fully charged.

Anyway, if that long hill is resulting in a solid green pack display, try turning on the headlights, defrosters, A/C, turn up the cabin fan. Basically anything to increase the electrical load on the pack. Couldn't hurt.

 

Given the above, it might be a good idea to measure/test the 12v health.

How many driving hours per week are you averaging over the last year?

 

This is probably not a good idea, if you don't want the battery to get fully charged you could use B which would accomplish this and be much safer for the control of the car and the health of the battery. In reality, you only really need to consider using B if you have a really long descent of 800-1000 ft or more of overall change in altitude when you would have a full battery and still descending. This is the real purpose of B.

Having said that you need not worry about the battery being over-full or the temperature of the battery, the car will look after this just fine. The car will never charge the battery more than 80% full (this is 8 bars on the MDF) in any case and the cooling fan will kick on if the car does not like how hot the battery is.

I would venture to say you should be more worried about the short daily runs than the things on which you are focused at present. Just saying.

 

There is more to "warming up" than just engine temp. If you read the article I mentioned in post #6 it will explain it, if you're interested.

 

Certainly true for a healthy pack. I'm going to guess though that when a pack is on its last legs there may be some cells with so little capacity that a long run down a hill could overcharge them long before any other cells in that pack are near 80% charge. The pack only has module level voltage sensing and it would be hard for it to catch that.

 

True.

I'm going to guess the pack is healthier than not just because if it was as bad as you indicate there would be codes, at least a P0A80 and maybe some P30xx codes. Another reason for my guess is as well as codes the battery fan would be audibly loud. Further, if there were symptoms of such a state I'm sure this would've been mentioned by the OP.

Well, the voltage sensing is per block, so in fact, there would be less granularity but even so, it will still detect a block that goes wildly high on charge and wildly low on discharge when compared to the other blocks. The battery ECU also takes into account whether any individual block is lower than 1.2 V than its peers in addition to how long the highest and lowest blocks have a difference greater than 0.3 V. There are many other variables involved, I'm sure.

Taking all that into account I don't think the OP is quite there yet.

 

I live on a hill of 104 m (~340) elevation and 900 m (0.6 miles) distance to the bottom. In my case, I go up the hill to get home and down the hill when I leave, which is the opposite of your use case and slightly higher and a bit steeper than your situation. I can't say I've ever heard my battery fan startup on the either the climb up or the glide down (so I'm not worried about out-of-range temperatures) and unless I start with an almost full battery (which it almost never would be from the climb up – five to six bars consistently would be normal), I could never get a full battery from this little descent. I don't think it makes a significant difference that your run is 400 m (~1/4 mile) longer. I'm still on the original battery although it has had five or six conditioning cycles since 2016 and have been driving up and down this hill for 15 years, I'm just letting you know my situation so you have another set of data with which to compare.

It would be interesting to get actual temperature data points using Techstream or a capable scanner to see if there is anything to your hypothesis.

From what you've said it doesn't sound right, and one of only a few possibilities I can think of is a low-capacity hybrid vehicle battery – which is strange given you say you had a new OEM battery installed in 2019. This new battery was bought from a bona fide Toyota dealer, right? Who installed it?

If you are concerned about heat at a module level, I would ensure the battery cooling fan (in the wheel well above the 12 V battery) is clean and functioning. In your case, I would also check the inlet and outlet ducting is clean and unobstructed. Most of the time, when everything is functioning correctly, you do not hear the cooling fan as it is on a low setting but when the battery ECU is not liking the temperatures it is seeing, it sure ramps up to high pretty quickly.

I'll post again if I think of anything else.