Scangauge 3, first impressions, first results.

Source: Amazon.com

Scanguage is brand of OBD-II device that will, among other things, give you real-time monitoring of what's happening under the hood. Temperatures, pressures, operating conditions. That sort of thing.

In effect, it gives you a customizable set of numerical gauges for your car.

I bought the original Scangauge years ago, on advice of some PriusChat members. It worked as advertised, but was kind of crude, with a tiny low-res screen. I gave it away with my last (ever) straight-gas car.

Fast forward, and I now drive my wife's 2021 Prius Prime. I wanted something to monitor traction battery current, as explained below.

I splurged for the most recent model,the Scangauge 3, It costs around $270, or about $100 more than the standard Scangauge (II) model. But, having used both the original and now Scangauge 3, all I can say is, it seems well worth it.

It has a roughly 2.5" x 3.5" touch screen. The touch screen makes it absolutely intuitive to use. Initial setup took about two minutes. You can set it up to have as few as four or as many as nine gauges per screen, with three screens of gauges available. (By contrast, the original would display just four gauges..)

As a plus, it will automatically pull all of the Toyota-specific codes available on the Prime. This has some benefits.

E.g., the Tire Pressure Monitoring System (TPMS) light won't come on until at least one tire is 25% under-inflated. But with the Scangauge, you can check your tire pressures right off the gauge, because the Toyota TPMS reports actual pressures, and the Scangauge 3 can pick those up as Toyota-specific data codes. So you can pick up a low tire that's not yet low enough to set off the TPMS idiot light. Without having to put a physical tire gauge on the tire.

Finally, you can set audio alarms if something you are monitoring exceeds some user-defined threshold. When the value passes that threshold, the device "chirps". (Although, weirdly, this function does not allow for minus signs, so I could not set a warning for excess braking current, because that shows up as negative amperage draw from the battery.)

I'm not sure if it provides any fundamentally new functionality over the Scangauge II. But it's a lot easier on the eyes, and a lot easier to use.

Monitoring battery current.

I bought this so I could monitor the traction battery current and avoid high-current situations. As I understand it, those put wear-and-tear on that battery. And I'm trying to avoid that to the greatest extent possible.

I took it out for its first test-drive today, and I'm already learning useful information.



First, you've probably noticed that when you start off in EV mode, the guide bar (Eco-Accelerator Guidance) on the Hybrid System Indicator pops up a good third of the way up the curved scale.





Toyota only indicates that you should stay "within" that Eco Accelerator Guidance area, but I'm pretty sure I get my best "eco" score when I use the gas pedal to match the guide bar. At least on acceleration. I interpret that as the Prime telling me NOT to accelerate as slowly as possible, but instead to use a fairly heavy foot at takeoff, then back off as the car accelerates.

I now know that when you first start off, the location of that blue bar -- the Eco Accelerator Guidance area -- corresponds to about a 45 amp draw, which works out to just over 20 horsepower. Or somewhere between one-quarter and one-third of the max EV horsepower.

After initial takeoff, you have to take your foot off the gas to say within the guide bar. Near as I can tell, following that guidance will keep you at constant current = constant horsepower = constant power. (Recall that power = force x speed, so the faster you go, the less force you can apply to maintain constant power.)

My guess is that the Prius guidance is set that way because lightly-loaded electric motors are inefficient. But heavily-loaded ones are also inefficient, due to I-squared-R or copper or Ohmic heating losses. So it's telling you to get above the inefficient low-load portion of the electric motor efficiency curve, but not to stray into high currents that cause significant heating (copper, I-squared-R) losses.

Anyway, for better or worse, I long ago trained myself to do what the eco-meter bar tells me to do. That is, as traffic allows, to match the end of the Eco Accelerator Guidance area. Which means that for acceleration, you have more of a lead foot at slow speeds, and feather foot at high speeds, so as to keep power consumption constant over acceleration maneuvers. I think that all makes sense, as I now know it's trying to keep me from using the motors inefficiently at low load, and to avoid high load/high current as well.

Braking was the big surprise.

Let me take that ~50-amp current as some measure of "what the Prime wants to see". Not sure if that's right, but I think I'm on solid ground assuming that current flows at that level will put only minimal wear-and-tear on the battery. That's a "guaranteed to be safe" current flow.

The big surprise to me is that I routinely go way over 50 amps when stopping. And I'm not screeching to a halt, either. It's just that what feels like moderate braking force, at typical street speeds (e.g, 35 MPH), will put you well over 50 amps.

I guess this shouldn't be a surprise, either, as the car will stop a lot more quickly than it will start. Anyway, I had already figured out that constant-power stops should start out with a very light foot, and end with a lead foot as the car slows to a stop. But I had no idea just how light a touch it took to keep the car below that guaranteed-safe 50 amp threshold.

Finally, I have not tested the current draw at highway speeds, and I will add that at some point. But I think this has given me a feel for why in other PHEV forums, a common piece of advice is to avoid running in EV mode at highway speed, for best battery life. Because power = force x speed, I'm pretty sure that highway maneuvers could easily put you into the same type of current draw -- say 150 amps -- that you'd see from a typical Tesla fast-charger. It's well-established that fast-charging shortens battery life.

Toyota's only advice, in the manual, is that running at highway speed shortens your EV range. The only thing they tell you to avoid, for excess battery wear, is running long distances near the maximum speed for EV. So maybe avoiding EV highway driving may be a bit too fussy. Surely that's useless advice if your commute involves highway driving. But because that's optional for me, I think I'm going to get into the habit of switching to HV at highway speeds.





Conclusion

That's all I've figured out so far. The device is a definite upgrade from the older Scangauge version. For me, it worked correctly right out of the box. Operation is totally intuitive. And I'm already learning stuff that I would never have guessed without it. It's a little pricey, but so far it gives me almost everything I want, right out of the box.

 

Addendum 2/12/2022: Highway driving is not as bad as I expected, and a few other notes.

I did a little experiment this morning regarding current draw at highway speeds. The upshot is that:

1. Moderate maneuvers at highway speed will result in brief spikes above 100 amps.
   
2. What I would consider "normal" braking to the bottom of an off ramp can generate an extended period of over 100 amps.
3. EV AUTO model will not flip the car into HV mode merely for routine maneuvering at highway speed.
4. Braking in HV mode generates just as much current as braking in EV mode (duh).

Details follow.

Only a level road, at 55 MPH, in EV mode, with the cruise control on, the car drew about 35 amps. That's about 16 horsepower, which seems about right. Even a modest uphill would push that to 50-60 amps. Which again seems reasonable.

Rapidly adjusting the cruise control from 55 to 60 MPH (five quick taps on the lever) resulted in a brief current spike slightly in excess of 100 amps. By brief, I mean on-order-of one second. I tried that several times, and the results were consistent. So that's a very short call for around 50 HP out of the electric motors.

Retrying that in EV AUTO mode did not result in the car starting the gas engine/going into HV mode. So the car itself doesn't consider that 100 amp current spike to be much of a problem.

It is incredibly easy to spike the braking current over 100 amps when traveling at highway speed. On a down-sloping off ramps, exiting at 55 MPH, I never did manage to keep the braking current below 100 amps. That apparently requires a really extended-and-slow deceleration. So slow that I would consider it to be abnormal driving behavior. You would definitely be getting in other people's way if you insisted on coming to a stop at the end of an off-ramp slowly enough to keep braking current below 100 amps. .

In summary, I expected highway maneuvers to max out the current draw. But that's not true. As long as you keep your acceleration down to the rate at which the cruise control would normally accelerate you, you might get a brief period just over 100 amps. I'm guessing that puts relatively little wear-and-tear on the battery (though I do not know for sure, as I need to find the "C" rating and do the math for the cells in the Prime battery). What I am sure of is that the car itself allows that, even when driven in EV AUTO mode.

I'm coming to the conclusion that the brake pedal is more of a problem than the gas pedal, when it comes to routinely putting spikes of high current through the battery.

Reinforcing that, I generally don't think about the battery at all, when in HV mode. But now that I have this Scangauge in my face, I am reminded that braking currents in HV mode are the same as they are in EV mode. You can easily put a 100 amp current spike through the battery, in HV mode, just by coming to (what seems like) a relatively normal stop, from high speed.

 

That's a fair point. I have been reasoning by analogy. I know DC fast-charging shortens battery life, and that damage cumulates slowly. And I know that lithium cells subject to frequent high current draws fail rapidly.

So I had been assuming that short periods of high current would also cumulate.

In effect, I assumed that 60 one-second spikes would equal the damage you'd get from a minute at high amperage. That's not proven, and now that I think about it, I only have the haziest notion why I think that.

On point, the only thing Toyota warns about is extended driving near the EV speed limit, which would boil down to extended high current drain.

I need to do some more research on why, exactly, high current causes degradation in lithium-ion batteries.

 

Well, everything I can lay my hands on says that's its the average rate of charge or discharge current that puts the wear-and-tear on the battery. Possibly, that's just because that's what labs have measured.Possibly, because that's what's true.

As far as solid evidence goes, my obsessing with spikes in current may have been just plain stupid. Poor reasoning-by-analogy.

As I now read it, I should probably focus on the average as-driven current, keeping that low for best battery life.

The first thing I needed to grasp was the "C" rate of discharge for a battery, which is the rate that will fully discharge it in one hour. For the Prime, I calculate that as 8.8 KW/350 volts = 25 amps. So a 25-amp current is a "1 C" current for the Prime. Studies show that that the higher the C rate at which you discharge a lithium-ion cell, the lower the battery life. So, e.g., typically using this a 3C, or 75 amps, for extended periods, should definitely lower battery life, relative to using it at 1C discharge rates.

And, at least at the rates that people are willing to show on a graph, that's fairly linear. So the difference in life between 1C and 2C use looks about the same as between 2C and 3C use.

So, the idea that high-current spikes are particularly damaging -- or that this current-based wear goes up more-than-linearly with current -- I can find no hard evidence to support that. Other current-based losses, sure. Heating losses in the motors go with the square of current. But I can't find anything to say that battery wear-and-tear does that.

I.e., it now appears that the problem with fast-charging a Tesla battery isn't the high current per se -- it's not that 180 amps creates a lot of damage. It's that the car spends a considerable length of time charging at a high C rate. This, in a car where normal discharge is apparently somewhere around 0.25C.

Tellingly, it looks like only one PHEV allows fast charging, the Outlander. And that's some special Japanese standard for fast-charging.

I suspect that's because with the small PHEV battery, the C rate would be enormous if you managed to hook it up to a Tesla-style fast charger. A 180-amp fast charge would be a 7C rate of charge on a Prius Prime. Pretty sure that would be exceptionally bad for battery life, if the battery could handle it at all.

I guess what I'm getting at is that the research treats average charge/discharge current (the "C" rate) separate from other factors that reduce battery life, such as temperature.

That said, now that I take a hard look at it, I think you're right. I shouldn't be worried about brief periods of high current, within reason. Looks like the research says that for best battery life, I should drive so as to keep average discharge current and regen current as low as possible, given traffic and other road conditions.