The Toyota Hybrid System Indicator (HSI)

In another thread, a participant raised an issue about ICE activation at low HSI status. This got me wondering about the relationship between the HSI dash display and the HSI PID. In addition to the information in the Owner’s Manual, there are several useful collations of observations about the HSI dash display (at left in the rav4 GX display above):

Hybrid System Indicator and Stealth | PriusChat
HSI
CHG portion of Hybrid System Indicator | Toyota Nation Forum

The indicator varies in appearance (and maybe in function) between models, and it is always an attempt to integrate multiple functions in a way that is informative for the driver. Contrast this with a tachometer, which shows just one thing (engine rpm). I assume that the data from the HSI PID is one of the inputs, but it must be heavily smoothed and somehow integrated with other data to drive the HSI display on the dashboard. It may be a clue that engine rpm is included as a PID within the Combination Meter module, even though no tachometer is displayed. Other parameters that are certainly used (like various contributors to stored energy regeneration) are not listed as PIDs for this module. As usual (and unfortunately) Toyota gives only the ‘explanation for Dummies’ and does not reveal the parameters used, or the algorithm used to integrate them in the HSI display.

I do not think the HSI alone is a reliable indicator of when the ICE will be used. Probably the ICE is always used when the HSI is in the PWR range, but it is also called upon to charge the traction battery as needed whenever the car is in READY mode. In my experience, the ICE sometimes starts before I can reverse (very slowly) out of my garage; but at other times it does not start until the car reaches a certain speed, far down the road (unless it is accelerated harder before that). It probably depends in part on where the traction battery SOC happened to be at the end of the previous drive, and on multiple other factors including what accessories were on to drain the auxiliary battery while parked (which causes a drain from the traction battery as soon as READY mode is engaged).

So I would not worry if the ICE starts at a low reading on the HSI, especially if the car has been parked for a few days (e.g. during COVID lockdowns). At least I would want to dig deeper into factors such as traction battery SOC that can trigger the ICE. Remember that the Toyota traction battery SOC display is also configured ‘for Dummies’. It displays only the range in which Toyota aims to keep the traction battery, and the car apparently uses a complex (and undisclosed) algorithm to decide when (and how much) to increase the SOC within this range. Some details vary between models.

 

Hi Alan, I don’t know what the percentages are but I see lots of posts about the HSI so I think many owners would like to understand it more fully than the description in the Owner’s Manual. It would not require the ability to read computer code or translate data streams. More like:

“The HSI gauge works when the car is running but not in Park or Neutral. When power flows to the wheels the gauge integrates battery power (A) and engine power (RPM), depending on car speed (KPH), using the formula x*RPM / (y*A + log KPH). When no power flows to the wheels, the gauge indicates regenerative power (from engine braking and regenerative braking applied to the wheels) and flowing to the traction battery (-A), using the formula -w*A. The zero point is the top line of CHG”

Of course, this is not the real (secret) algorithm. Maybe it uses drive torque. Probably it corrects for engine power being used to recharge the traction battery. Considering all the work that must have gone in to designing the HSI, it is hard to understand why Toyota would want to reduce its value by keeping interested owners from fully understanding what is being shown. Perhaps the company sees more value in keeping competitors in the dark than in enlightening car buyers. That may be a fair call if 99.99% of Toyota car owners (buyers) really don’t care.

 

The Energy Monitor is even more entertaining in an AWD (and colorful in a rav4). Don't take your grandchildren for a test drive in one, or they will want you to buy it

I have a few screens to monitor all temperatures available in OBDLink - and (touch wood, or thank Toyota designers) I have never seen anything worrying there. From memory, the traction battery gets hottest; I think all the others are liquid cooled. The example below was very soon after start, not sure why all the temperature readings were not equilibrated overnight.

 

That is interesting. My laptop does not have a maximum lifespan mode, and from the image you pasted yours regulates charge capacity (it is silent about charge rate).

I agree with you that charging too fast typically reduces secondary battery cycle life.

We know that Toyota regulates traction battery charge capacity, but I think we know little about whether or how it regulates charge rate. When I log traction battery current it is very spiky at all temperatures (Hybrid Battery Temperature Control | Page 2 | PriusChat).

It is said (prius3charger_buck/prius3charger_buck.ino at master · celeron55/prius3charger_buck · GitHub) that the Toyota DC converter module includes capability to control voltage and current outputs, but Toyota seems silent on this issue. Certainly for the auxiliary battery there seems to be no charging current control (except on/off) beyond resistance offered by the battery itself (More on Hybrid 12V (auxiliary) battery charge control | PriusChat).

Once again we can only speculate about Toyota algorithms, and the full set of parameters used in each. Sigh.

 

Yes, Prof Kelly makes useful (if too long/slow for my taste) videos. I once asked him by email if he had any knowledge to share about LFP auxiliary batteries for hybrid cars, but he did not reply. From his other videos, he (like all the other owners) does not even know which Lithium chemistry Toyota uses in their current traction batteries. He has never claimed to know anything about Toyota's (unpublished) control algorithms. Sigh.

 

What Toyota owners need to know is a deeper philosophical question, perhaps with more personal answers, than what they are reasonably entitled to know.

One shortcoming with Prof Kelly's useful charging videos is that they are entirely intra-mural; so he misses all the really interesting stuff by not measuring while driving. I made the same mistake when first I measured.

 

My memory was wrong in post #5 above. Under relaxed urban driving in my rav4 hybrid, the generator (MG1) and front motor (MG2) reach around 70ºC. Even the rear motor gets above 50ºC. The traction battery stays below 50ºC (in my experience). I guess the inverters are liquid cooled, but not the motors themselves.