Hey Guys,

I'm new to these boards and have been in my '09 Camry Hybrid for about 6 months. It took me about two tanks to learn about warm-up modes and how to optimize the mileage (I now am getting 44 indicated / 42 calculated) in mixed driving.

Here is the question: On LONG down-hill sections, what does the charging system do when the battery is completely full? It seems that it stops charging the battery and the car has slightly less drag when transmission is selected to "B". Are there techniques to help the charging system keep the batteries cool and prolong the charging effort while coming down long hills? I noticed today that with the batteries completely full following a long down-hill that the car would not go into "E-mode" when I rolled into town.

Thoughts...

BB

 

The E or EV mode only works when your at or under 42 mph. I have seen this same thing where the battery if charged full on a long downhill, when braking. Could be the car was waiting to see if your going to climb another hill. I have a similar problem here when I go up and over a steep overpass. Many times the EV won't come on for a few miles even though it's slightly downhill.

The B-position charges the traction battery very quickly.

I did find rather than ride the regin-brakes all the way down a hill, is to let up on the brakes, then use them again to drop to like 45/50 mph.

I also have better luck by slowing way down at the very start of a known long downhill. Once from the top of a 6000 foot mountain I slowed to 35 mph to allow the car to build speed to 50 mph. I would then slow down to 45, let it gain back to 50 then slow to 45 again. This way the battery don't charge as fast. I found it was much harder to slow from 60 than from 50 mph. I was on a rather steep grade so that would vary.

Toyota gas powered autos are geared up for gas mileage so they don't gear down very well. Could be the reason toyota created the brake position. It charges the battery on our TCH's then uses the engine and includes the brakes as needed. I never have much luck with it other than going down a very steep short grade at 30/35 mph. The manual mentions not using the accelerator when using the brake position. Be sure to stop before selecting the brake position.

Your doing great with the 42 mpg. I use the mfd readout. I don't trust the calculated due to the many stations drive-ways being unlevel and various levels of the pump shutting off. Using the readout, it's calculating mpg from the fuel consumed vs the wheel rpm every time. I mentally subtract 1 1/2 to 2 mpg if you want a estimated calculated reading.

 

But not as quickly as the "non"-B position.

The B position simply provides more engine compression braking (to the detriment of full regen) to "preserve" the brakes if long duration braking periods are expected.

 

Thanks willard for the correction..

 

True, but I'm well below those speeds. I live in Colorado and will come down about 3,000 vertical feet at a grade of 4-6% trying to stay out of "B-position" as much as possible, but using it when needed to stay within a 'safe' speed .

When I arrive in whatever small town at the end of a hill, I think "hey the battery is dead full, slow to 40 MPH, and enjoy 3 miles of E-mode travel." It is below the 40 MPH that I discover the care will not go into E-mode at all. In fact, it continues in split charge mode as though the battery were LOW when it is actually full.

I have asked some other folks and done some reading, and it seems that the battery TEMPERATURE is what dictates this. When it gets hyper-charged (after a good down coast) it gets hot; and when it is hot, the EV controller will not allow the car to run on traction motor alone.

True, which adds to the heat problem, I think. Another issue MIGHT be that the traction motor ITSELF is hot. As I understand it, B-position charge comes FROM the traction motor (motor/generator); but there is other documentation that indicates that charge current comes from a dedicated generator. Does anyone have a complete system schematic of this drive system?

Yes, yes! That does work. Giving the thing short durations of charge followed by discharge or coast seems to keep it in its best drive mode. Also, I might be fooling myself; but I have tried aiming the car's air-conditioner vents at the rear deck traction battery vent when I hear the vent drawing air (very quiet system, but there is a distant fan sound when this happens). The thought is to get cooler air down that inlet to more effectively cool the TB.

I thought I was the only nut doing that!

This drives my wife nuts; but it REALLY helps. If your charge is normal (blue), you get about .5 miles of EV mode followed by coast-only charging in "D-position". The care is getting well over 100 MPG in this configuration. If the hill lets you roll up another hill to return to 30 MPH or so, you can discharge to crest the hill and begin again. HILLS HELP MPG! That is for sure; but it is the TOO BIG hill that we are talking about here.

The CVT on this car is the first non-gear-grinder I have owned; I must say that the B-position is the BEST MOUNTAIN transmission there is. In B-position, I can come down a 6% grade at 30-60 MPH by just touching the accelerator slightly. As we know, the "accelerator" is attached only to a computer, and is first a "non-decelerator". If rolling down hill in D or B with our foot fully off the gas, the car will pull a light charge and pull a small amount of drag in doing so. Pushing slightly forward on the gas removes this charge and the associated drag. In B-position, this includes the transmission gear ratio and the engine will gear up as you touch the accelerator which allows you to get the car moving faster without losing the ability to slow back down. All with NO shifts. This attribute makes the **** thing a lot less work than a standard while constantly providing more back-pressure throughout your mountain driving speed range.

When I said "calculated" I meant divide miles driven on the tank by the gallons delivered by the pump. I also find that this number is 1-2 MPG LOWER than the displayed MPG. If the car says I got 44, I will find that it got 42 after I fuel and calculate in this way. I think we are saying the same thing.

Thanks for the comments.

 

Guess I'll have to "play" with it going over Tioga Pass which I usually do a couple of times a summer. I used to get amazing mpg with my 99 Maxima coming down the long continuous downhill on the west side of the Sierra.

Great because gas is always much more expensive on the east side of the Sierra so I could let the tank get quite a ways down knowing that I'd use hardly any on the way down on the west side after the pull from 7,000 to 10,000 feet on the east side.

 

The 'B' setting on the shifter is to engage engine braking. Engine braking is used when going down very long hills, like mountains, to avoid brake fade.

In a normal car, when going down a mountain, if you were to ride the brakes all the way down the hill, they would get hot. If the mechanical brakes get too hot, they 'fade', and they might not be able to slow the car enough to safely make the corners. To avoid this, you shift to a low gear. The low gear uses engine compression to absorb some of the energy, so you don't have to ride the brakes.

In a TCH, if you were to ride the brakes all the way down the hill, a sequence of things happen. First, the generators fill the traction battery. After the traction battery is full, the system uses the mechanical brakes. The mechanical brakes on a TCH work just like the brakes on a regular car, except they are smaller and so are even more prone to get hot. If the mechanical brakes get too hot, they 'fade', and they might not be able to slow the car enough to safely make the corners.

The 'B' setting simulates shifting to a low gear. It uses engine compression to absorb some of the energy, so you don't have to ride the brakes, and the brakes don't get too hot. This avoids the possibility of brake failure while going down a mountain.

Aside from the safety issue, there is another good reason not to ride the mechanical brakes. Even if they don't actually fail to stop the car, the overheating can damage the rotors. When brake rotors get too hot, they can 'warp'. After rotors warp, you get to buy a brake job for several hundred bucks.

 

(Bold added by me to the above quote) Are you sure they are smaller?

 

I have read the TCH brakes are the same size at on the Corolla. The TCH uses regenerative braking till the car slow to about 8 mph, then it switches to the physical brakes.

The physical brakes are used anytime you make a panic stop or any rather quick braking.