My Hymotion: What I've Learned So Far

I've been trying to find ways around all of the things that cause the ICE (gas engine) to start up. Here's my list of causes:

1) going too fast
2) accelerating too hard
3) traction battery (the NiMH Prius battery) too cold
4) temperature too cold in general
5) Hymotion battery too full?
6) traction battery too full?
7) Hymotion error (13 flashes of red LED)

Some of these are really obvious.

I've found that I have to drive 32 mph (or slower) to stay in EV mode. I generally keep my speed around 30. (It's pretty hard to stay at 32, without occasionally going 32.5!)

I've found that relatively gentle acceleration is enough to stay in EV mode. On a slight uphill grade, MORE gentle is required.

Then, there are the big hills. I live on a fairly steep one. I've found that I can stay in EV all the way up the hill but only if I restrict myself to about 9 mph on the steepest stretches. The trick is adjusting my speed to match the slope as it changes. The other trick is to pull over when someone needs to pass. In some areas, passing is not allowed and there's no shoulder. So, I try to restrict my 9 mph behavior to late at night and use my flashers if I'm feeling really stubborn.

Since I live in New York state (Corning), I have often had to deal with the 0 degree (Celsius) temperature limit for the traction battery. Below this temperature, the Prius doesn't permit EV mode. So far, I havn't found a good solution for this one. Some people have suggested keeping the battery warm with an electric blanket. To avoid running the car's batteries down and to avoid needing an electrical outlet (none available at work), my first attempt was a rechargeable battery-powered blanket, the Chill-Buster. (They don't seem to make them any more, but my wife found one on Ebay.) Unfortunately, it doesn't seem to put out enough heat to do the job. I haven't given up on it yet, but it doesn't seem promising.

By the way, if anybody knows how to monitor the traction battery's temperature with a ScanGauge (on a 2008 Prius), I'd love to know. I tried creating an Xgauge with the codes that someone had posted, but it doesn't display anything. Perhaps it doesn't work on the 2008 model.

I bought an engine block heater through Prius Chat and had my local Toyota dealer install it. However, heating up the engine coolant really doesn't have much effect on whether you can start up in EV mode or stay in EV mode. The traction battery temperature is the limiting factor.

On cold days, using the EBH will definitely improve your initial non-EV mileage, but I'm not sure if I fully understand what results to expect from the interaction of the EBH and the "thermo bottle." For example, if my car has been sitting for a couple days, with the thermo bottle dump cold coolant into the engine and negate most of the effect of the EBH? If anybody understands this, I'd appreciate a better explanation of when and why the bottle dumps and whether this negates the effect of the EBH.

Like others have mentioned, I have noticed that I can use EV mode, even on fairly cold days (in the twenties), if my car is warmed up (not too likely, if you're trying to use EV mode most of the time).

The only useful trick I've found is to completely shut off the heating/air conditioning system. When air is needed to clear the windows, the "defrost/feet" button will often allow you to stay in EV mode (as opposed to the "defrost only" button). Keeping the temperature set lower also helps (not that the temperature matters much, if the engine is cold), but hitting the "OFF" button will give you the best chance of using EV mode.

Here's the confusing thing: Often, even though the temperature seems to be warm enough, my Prius won't go into EV mode when I first start it. At first, I thought it was because the engine (or the battery?) was cold. Gradually, I've come to suspect that it has more to do with the fact that the Hymotion battery is fully charged or is very close to being fully charged. This doesn't always prevent me from going into EV mode. So, it may be some combination of too much charge and too cold a temperature.

If anybody has experienced this or has some understanding of it, I would love to hear it.

The Prius (traction) battery seems to have a similar behavior. If the State of Charge (SOC) gets too high, I get kicked out of EV mode. Every morning, on my way down the hill, I would hear 3 beeps and the engine would start. The ironic thing was that I had figured out how to stay in EV mode on the way UP the hill (go 9 mph) but couldn't keep it in EV on the way DOWN!!! Obviously, it has something to do with exceeding a certain SOC level. When I got my ScanGauge, I found that it kicks me out of EV just before the SOC reaches 80%.

When the SOC gets into the 70's, it seems to increase much faster. I've found that I can stop it from increasing further by shifting into neutral. The SOC actually continues increasing a little bit after I shift into neutral. So, I usually shift when my ScanGauge shows an SOC of about 73% or so. That way, I capture as much braking energy as possible without being forced out of EV mode. After stopping at the bottom of the hill, I shift back into Drive and off I go (still in EV mode)!

The final problem is the 13 flashes of the red LED. Like most other people, I see this occasionally in cold weather. The explanation that others have posted seems to make perfect sense. The battery voltage drops below some threshold if you have to many devices turned on when starting the car. So, I have started turning the heating/air conditioning system (and the headlights and the radio) completely off before turning the car off. This seems to prevent the error.

This error is probably best avoided because it occasionally causes some other problems. Once, I got the error when I was leaving work. I was able to clear it by shutting off the car for several minutes, by I still noticed some strange behavior on the way home. I was going up the hill to my house and noticed that my car was NOT in EV mode, but the engine was also NOT RUNNING! So, I kept going up the hill at 9 mph until I realized that my traction battery was getting very low (and was not being recharged by the Hymotion battery). Very flaky behavior! So, I try to avoid the 13 flashes.

I've probably given the impression that I drive in EV mode most of the time. The reality is that I've been making a lot of highway trips lately and have only had two tanks that were above 60 mpg average (since I had the Hymotion installed in early October).

Today, I am hoping to make my first round-trip from home to work and back (hills and all), without using ANY gas. (By the time I figured out the downhill stuff, it was already cold weather. Today is quite a bit warmer.)

If anyone has any insights into any of the things I've mentioned, I'd love to hear them.

I've probably made it sound like the Hymotion system has a lot of weaknesses. If you're trying to make your Prius be a completely electric car, that's probably true. However, the Hymotion system was never intended to do that. As a plug-in hybrid system, it works great! It provides significantly increased gas mileage in a variety of situations, and I have no regrets about buying it.

 

Chris,

My alternative to EV at 9 mph is not 99.9+ mpg at 50 mph. It's more like 20 mpg at 30-40 mph. Remember that I'm going up a steep hill. Even with the electric motor assisting, the mileage is pretty low. If I spend too much time at 20 mpg, my average mpg takes a pretty big hit.

However, I was oversimplifying a bit. I often go halfway up the hill in EV and then (to avoid blocking traffic) speed up to 30 or 40 mph (non-EV) for the last mile or two. Also, staying in EV doesn't mean I have to go 9 mph the whole way. That's only on the steepest stretches.

The reason I have so few tanks with high mileage is that I rarely go more than a few weeks without some kind of out-of-town trip. So, I rarely finish a whole tank without putting on a bunch of non-Hymotion miles.

I could keep track of the miles separately, but I've been more interested in figuring out how to stay in EV than I have been on tracking my exact mileage.

My other problem is that, currently, I can't charge at work. So, if I need to make multiple trips to town, the second trip home usually ends up being non-Hymotion miles. That drops my average quite a bit.

 

Are you sure? 40 mph in EV mode? I've never seen that happen. (My commute isn't really long enough to get the engine warmed up very much.) Do you use grill-blocking? Is there anyone who sells grill-blocking material for the Prius? Or, do I have to make something myself? Any suggestions on how to do that would be appreciated.

I've been avoiding switching the Hymotion switch on and off. Have you experienced any problems while doing that? Has anyone else experienced any problems? It does seem like the best way to maximize the capture of braking energy on the way down the hill.

My commute is only about 7 miles each way, mostly on city streets (30 mph speed limit). That's why I tend to stay with EV mode.

I've noticed that, once you get kicked out of EV mode, the only way to get back into EV is to stop the car and turn the power off and back on. I occasionally do that when I accidentally go too fast.

When it's slightly too cold to start the car in EV mode, I sometimes find that driving a block or two, stopping the car, and starting it again will allow you to go into EV mode. When it's really cold, however, you can do that as many times as you want, and you'll never get into EV mode.

 

I just leave my spare tire strapped down in the back. I pile the grocery bags on top of it. On the rare occasions that I need more space, I take it out and leave it home. The real projectile risk is the tire jack. It doesn't really fit securely under the strap and could do some real damage in an accident.

I don't have a garage. So, I charge my car in the driveway (even in pouring rain). Yes, I have a GFCI outlet. I use a 100-ft, 10-gauge, yellow, Coleman, cold-weather cord. It has a big, round head on it that fits snugly in the car's charging socket, leaving very little room for water to get in.

I'm still struggling with strategies to keep the ICE from coming on. It depends on a lot of things. Last night (at about 10:30), my car went directly into EV mode when I started it up. The temperature was 19 F!! I drove all the way home from work (up a big hill) and the engine never came on. By the time I got home, my ScanGauge was showing that the coolant temperature to be 33 F.

At about 9 pm, I had made a short drive of a mile or two. I think that warmed up the battery enough that it was willing to stay in EV mode for the trip home.

In cold weather, I often notice that I get 3 beeps after driving the car for a block or two. I take that to mean that the battery has warmed itself up enough (just by discharging to run the motor) to allow EV mode. So, I usually just pull over and turn off the car. When I restart, it goes right into EV.

 

Yes, I'm definitely going to try the grill-blocking thing soon.

In the meantime, I'm still trying to fully understand the EV mode and battery temperature thing. In the evening, I start my car in the parking lot at work. At first, it won't go into EV mode (probably low battery temp). By the time I get to the other side of the parking lot, I hear the three beeps. The beeps mean that the car is TRYING to enter EV mode and being denied. However, the beeps ALSO means that the battery is now ready to allow EV mode.

That sounds contradictory, but it's not. There are essentially 2 ways to get into EV mode: EV mode with a cold ICE and EV mode with a warm ICE. If the battery temp is warm enough AND you don't have the defroster on, the Hymotion system will put you into EV mode when you first start the car (even when the ICE is cold). This only works when you first start the car. After the first few seconds, you can't get back into EV mode unless you either restart the car or get the ICE up to the right temp.

So, when I hear the three beeps, it tells me that the car is refusing to go into the "EV mode with a warm ICE" (because I don't have a warm ICE at that point). But, it also tells me that the car is ready to go into the "EV mode with a cold ICE." So, I stop my car, shut it off, turn it back on, and I'm in EV mode.

Yesterday, I was Googling the term "charge current limit" (CCL) and found some interesting info. It seems that the battery temp is not the limiting factor. It's just that low battery temp often causes the Prius to limit the current that is charging the battery. For some reason, EV mode is not allowed when the charging current is being limited. So, when my car lets me into EV mode after crossing the parking lot, it's not that the battery warmed up that quickly. It's just that the software decided to cancel the Charge Current Limit that was in effect when the car first started.

So, those three little beeps are really a "CCL cancellation" message.

I've been trying to figure out the exact relationship between battery temp and EV mode. It turns out that I was asking the wrong question. I should have been asking these two questions:

1) What are the exact criteria that cause the charging current to be limited?

2) Why the hell did Toyota decide that EV mode should not be allowed when there is a CCL in effect?

Anybody have the answers?

 

I'm actually starting to think that I want to avoid trying to change the way the system works (trying to heat the battery or trick the system into using EV mode when it doesn't want to).

At this point, I'd just like to have a better understanding of what the car is doing and why. This Prius/Hymotion system is fairly complicated. Having more quantities that I can monitor would be useful.

I bought a ScanGauge a couple months ago. So far, I'm monitoring RPM, SOC, engine coolant temp, and EV mode status. I tried to set up a gauge for battery temp, but it didn't seem to work on my 2008 Prius. Has anyone had any luck monitoring battery temp on a 2008 Prius?

How about other quantities? I'm going to try setting up gauges for battery current and voltage, but it would be nice to monitor the CCL/CDL, too. Does that require more than just a scangauge?