Empty Battery Reverse Possible/Probable?

One of the posters in "Prius Technical Stuff" reported using this technique to measure the capacity of the battery. I attempted it two years ago:

The protocol:

• seconds 2400-2700 - force charge the traction battery to bring it up to 80% state of charge
• seconds 2800 to 3200 - backing up the hill but due to traffic, aborted the test as I reached the crest. I needed a larger hill without traffic.

Later, I noticed the traction battery temperature climb and realized force charging the battery is exothermic and this is confirmed in the literature. Notice the traction battery temperature continued to rise after the force charge. Driving fast in hilly terrain can 'heat pump' the traction battery mostly from regenerative charging on the descents.

I then used a local hill to test this hypothesis:

In the first test, there was a 1 C increase in battery temperature, nearly 2 degrees F. In the second test, using "B", we didn't see the temperature increase. Using "D", I had to touch the brakes to moderate the speed and you can see the current spikes of 30-55 A. In contrast, "B" had more MG2 torque holding the car back and there were fewer brake events. The traction battery state of charge did not go up as high.

Since then, I always recommend using "B" when descending. It puts a lot less stress on the traction battery and speed control is a lot easier with fewer occasions to touch the brakes.

Bob Wilson

 

Boiling it down to one line: yes, the ICE will kick on and provide power for MG2.

In this mode, the ICE works against itself, so it's not the most efficient mode of transportation, nor the fastest. Hopefully you don't need to back for miles or at high speed.

Tom

 

Another thing to keep in mind is that the car isn't going to let the battery go "empty" before the ICE comes on to recharge; the software is going to try to keep the SOC between 40 - 80 %.

 

this is a related observation/question...
i recently bought an 08 prius, love it.
was at parents house, live on steep hill, 200 yard long driveway straight but steep.
had to back out.
car would not go up the hill backwards!!!
even with a running start, i had to retry twice,
prius lost speed and crept slower and slower, eventually stopped.
3rd try, it barely made it.
i could have turned around and gone forward, but i wanted to see if it could make it.
now i know the answer... no, it cant make it.
i wonder did i hurt anything, trying to move uphill in reverse to the point or stalling, aka no motion.
anyone else notice that it wont go up a steep incline in reverse??
i was suprised that the electric motor was so wimpy. i guess its geared weird in reverse?
and the trac battery wasnt low. i didnt notice if the ICE came on or not, but i dont think battery was the issue.

 

No you definitely wouldn't have hurt anything. You just hit maximum MG2 torque and it wasn't quite enough. It seems that there is no way you would have made it without a run up, is that correct.

It's an interesting issue lextoy. I was told when I bought mine to be careful about driving into very steep driveways where the only option is to back out. I thought this might be a bit over conservative or an exaggeration but now I see it's very real.

BTW. You wouldn't happen to know the approximate grade of the driveway would you. It would be interesting to know just what the reverse limit is (either in degrees or as a 1:n type ratio).

 

The electric motor, MG2, is not wimpy when used to augment the engine (ICE), but it is small compared to the ICE. Likewise the gearing is not weird in reverse, unless you also consider it weird in Drive. The gearing never changes in a Prius. It is not shiftable in any fashion.

The issue is that in reverse all of the power for moving the car must flow from the engine (ICE) through MG1 to MG2. In other words, MG1 is used as a generator to make electricity, and this electricity is used to power MG2 to turn the drive wheels. Since the gearing is fixed, the mechanically coupled torque from the ICE fights against the torque supplied by MG2. You essentially have an isometric exercise, where MG2 and the ICE fight against each other. MG2 wins because of the gear ratios, but it doesn't have a lot left over for actually moving the car backward. It's a very inefficient way of driving, but fortunately we don't spend much time backing up.

Tom

 

Also if you try to back up through "soft media" you may find yourself powering out. I had problems on wood chips over mud (8" of chips over the mud). I made it (level ground) but just barely.

People just don't realize how much power they use (peaks) and how little the Prius can produce electrically. The MGs produce plenty for cruising, but when you need lots for a steep climb or accelerating only an ICE will do! But it isn't really the MG that limits the power, it's the battery current and temperature.

 

Our driveway is slightly misaligned with the road, and you have to bump the front right wheel up a kerb when reversing in. never had a problem in any other car, but of course you could just more and more clutch up until there was enough torque. totally different with the prius, i have to basically put the throttle to the floor to get enough torque out of the motor to go up. it -can- do it though. i think the gen3 has more torque available though, correct?

 

It's not so much a power limitation as a torque limitation (which is in turn due to a current limit). At low speed in reverse the power is still quite low even when the torque limit is reached.

Hi Toaster, I was just doing some "back of the envelope" calculations of the theoretical maximum reverse grade, but then Tom (qbee) threw me off with the extra info about the counter torque from MG1. I hadn't taken that into account. So now I'm just trying to figure that new info into my calculations.

It's the new info of Tom's that I'm the most unsure about how to quantify (so I’m still checking my calculations), but so far they are coming out at about a 1 : 3.5 grade, which is only about 16 degrees. And this is only taking a fairly modest amount of total frictional resistance at the wheels of 300N. In deep mud or something it's going to get much worse.

I'll post the details of the calculation as soon as I double check that I haven't made any dumb mistakes.

 

Ok here are my calculations.

Prius weight (lightly loaded) : 15 kN (approx 3300 lbs)
Rolling friction of about 2% : 300N (could be much worse on some surfaces)
MG2 max torque : 400 Nm
MG1 counter torque (at MG2) : 60Nm (estimate based on MG1 at 10KW at 6000 rpm)
Final drive ratio (MG2 to wheels) : 3.9 to 1
Wheel/tire radius : 0.29 m

So the force at the wheels/ground is approx,

(400-60) * 3.9 / 0.29 – 300 = 4.3 kN

And the corresponding maximum grade is 4.3/15 = 0.287 which is about 1 in 3.5 or about 16 degrees.

 

The maximum grade you -should- find on public roads is around 9% (though I have seen 16% on hwy 99 in B.C.). That's about 4.5 deg for the 9% and 8 deg for the 16%. Both Alberta and B.C. tell me they try to limit the grades to 7% or less, but they can get to 9% on major roads, and higher on secondary ones. Private driveways are another matter, of course.

Oh, and Pearl has no problem accelerating up the 16% grade on hwy 99, -forwards- of course.

 

wow guys!!!
sorry i was gone so long, playin with issues on my other 2 cars, they both need work at the moment
the physics calculations remind me of college, ugh i didnt like it then either, but fascinating!!!
i will try to get a few measurements of grade up the driveway, 4 or 5 should give a good indication of varying grade from bottom to top.
a picture would be tough to show the steepness, unless i rig up some string or something to show "rise over run" over 10 or 20 feet.
i can also measure the street that is also on a grade, but the prius goes backwards up it more easily.