Battery Wore Down in Funeral Procession

One of the very cool things about the Prius is that it does monitor and keep the battery charged. I rely on this feature this way:

On long trips, and at age 62, I can no longer stay awake for the whole drive and usually have to pull over for a nap on a 8-10 hour drive. I pull into a rest stop, push the Park button, lock the doors, let the seat back and take a nap. In hot weather the air conditioner keeps me comfortable and if I sleep long enough the ICE will kick in as needed to charge the battery.

Yes the car takes care of itself quite nicely.

 

And my Prius takes care of me.
I do the same on 3000km each way trips. A tip, put the park brake on then take your foot off the brake with the car in drive, then apply foot brake again then select park. This puts a slight load on the engine mounts and reduces vibration on engine start up.

 

The behaviour does seem a bit crazy that it will fire the engine up so you can creep a way and then shut down again as soon as you hit the brake. I would have thought it would be better to keep the engine running a bit to bring the charge level back up. Of course the car doesn't know that there's a mile of stop-start traffic ahead of you rather than being able to run at decent speeds for the next mile, allowing recharging with less waste.

It may depend on what stage you've got to though - when this last happened to me, last Monday, it was 7ºC outside, raining, and there'd been plenty of stops, so I'd not got out of Stage 2.

 

I have found that the Prius is very efficient when the SOC is at the 51% to 53% range in traffic going less than 10 MPH. That equates to 3 bars. Here is why. At that SOC level, the computer wants to charge immediately. As soon as the ICE comes on, it puts more of a priority to charge to battery then at higher SOC levels. This means that more of the energy goes into the battery then when at higher SOC levels.

A very efficient way for me to handle a traffic situation is to hit EV and run the battery down to the low 50's SOC and then use EV to start the car moving and then allow the ICE to come on to keep the SOC up while moving by deselecting EV. This usually allows the ICE pulses to be in the 25 MPG range and the ICE then stays off while stopped. By keeping the SOC above 50%, you have control of the ICE with the EV button.

On level terrain is the most efficient for me. As long as the traffic is moving and I am not stopped for an extended period, this technique usually will yield over 70 MPG. I acutally would expect to get higher then that but I think that is acievable consistantly.

[edit] One more note on this technique. What I am trying to do in this situation is drive the car on EV and use the ICE to keep the battery charged. The most efficient time to charge the battery is when the car is moving, as some of the energy will also go to propulsion. The last thing I want is for the ICE to be running and the car sitting still.

[edit] One more comment. There seems to be two schools of thought on most subjects regarding fuel economy. On one side you have the just drive it technique and on the other side you have the hypermiling technique. There are advantages to both. I have just driven my car on several tanks and gotten in the mid to high 40's in fuel economy and that is fine. I have also gotten a 1000 tank in which I got over 80 MPG on the entire tank. Sometimes there is a bit of a rivalry between the 2 factions. In normal everyday driving, I try to use the hypermiling techniques when I can, and just drive it when I can't. This gets me in the 60 MPG range routinely.

 

I will add another point: any car designed for 10 mph will be way more efficient at that speed than a normal car at highway speeds. Low speed means low aerodynamic drag. Aerodynamic drag goes up by the square of the speed. The problem with low speed is that almost all cars are designed to go much faster, and are therefor heavily overpowered for low speed operation.

The drive system on electric cars throttles more efficiently than an internal combustion engine. There are larger losses associated with larger electric motors, but nowhere near as much as the larger losses associated with using a large ICE at low speed.

Tom

 

That's putting it conservatively! An electric motor (over the designed range) has no real sweet spot. It is almost equally efficent at any throttle setting. Something impossible to achieve for an ICE. In an EV, the energy consumed is very much equal to the work performed. In an ICE, the energy/work ratio is all over the map.

I need you to check that one, Tom. Larger electric motors are, in general, more efficient all across the spectrum. The only downside of putting a larger motor into an EV is initial expense, weight, and of course you'll want enough battery to use the capabilities of the larger motor.

But all else equal, a larger electric motor is more efficient, not less. The opposite of an ICE.

 

In fact almost any internal combustion engine is most efficient at full load. Idling is least efficient.

 

Larger electric motors have larger losses. I stick by my original comment. Your comment is also true: larger electric motors are generally more efficient. The two statements are not mutually exclusive. A bigger motor has more losses, but also produces more work. If the work to loss ratio is larger, then the efficiency is also larger. The problem comes when you have a big motor at low output. The big motor suffers larger frictional losses, eddy currents, and hysteresis. I^2R losses are generally lower.

Since you know something about LEDs, lets use them as an example. A 3W high power LED suffers larger losses than a small indicator LED. The 3W LED makes a lot of heat. It also makes a lot of light. The efficiency of the 3W LED is higher than the small indicator LED because the generated light more than compensates for the losses, but its losses are higher than the small LED.

Your comment about the only downside to putting a larger motor in an EV is not generally correct. From a practical standpoint, EVs at present are limited to sensible motor sizes. It doesn't make sense to oversize the motor when you don't have the power to use it. If you limit your statement to reasonable sized high efficiency motors, then I will agree. But as a general point there are a lot of inefficient large electric motors.

Tom

 

Not just eddy currents and hysteresis, but the simple rotational inertia/mass of a very large electric motor, eg. That's why very large electric motors are not given variable speed duty, especially below a certain threshold

Eg: take a 20 hp 3 phase TEFC electric motor. It's fairly easy to turn the shaft by hand, right?

Products: AC Motors: ECP2334T-4: Baldor Electric Company, a leader in energy efficient electric motors, linear motors and adjustable speed drives industry

Now, take a 300 hp 3 phase TEFC electric motor. Both may have the exact same "Super E" designation from the EPA. Try to turn that shaft by hand


Products: AC Motors: ECP44304T-4: Baldor Electric Company, a leader in energy efficient electric motors, linear motors and adjustable speed drives industry

Wanna know what drives me insane? Depending on the size of the motor, there is either a grease tit or a cap to add grease to the bearings. You're only supposed to relube at specified intervals. Too much grease is worse than stretching the interval

Yet, I'd often catch a mill worker hooking up a grease gun to the grease tit, at least once a week. So much greasing, the seals were blown out

 

Its funny how many people this freaks out. I have to admit the first time it happened to me I was wondering what was going to happen. You really quickly just learn to trust the car. Here's a scenario I still wonder about though, happened to me last summer.

Its July, in Phoenix. Its 115F, and the AC is running on high just keep the car at ~78F as the afternoon sun blazes down on it. You pull into the local car wash, buy your ticket, and join the cue of 3-4 cars to wait to enter the wash. AC blasting, tunes cranking, your battery is down to 2 bars when you reach the front of the line. Now the tricky bit. You punch in your ticket number, roll up the window, put the car in Neutral, and get sucked into the car wash. About half way through the carwash, AC still blasting, tunes still cranking, it occurs to you that there are no bars on the battery display. From somewhere in the depths of your memory you remember the obscure seemingly irrelevant warning message that pops up when you open the drivers door while in drive, something about how the battery can't charge unless its in Park......

Anyone know what happens? I was too chicken to find out. I killed the AC and tunes, put it in drive, and gave it enough gas to start the ICE and charge, but not enough to hop the carwash roller. There was a part of my engineer brain that just wanted to wait and see what happened My guess is you hit the same HV battery low cutoff that happens when you run out of gas and deplete the battery, leaving enough juice to restart.

Rob

 

If a little is good, more must be better. Too much should be perfect.

Tom

 

Dammit, Tom, there may be Prius service technicians reading this!

 

I'm pretty sure events like that drove me to the bottle

 

Just like engine oil! :_>

 

Just so you know in advance: I'm not speaking from experience!

You are in danger in that scenario, but probably not as soon as you'd imagine. If you put it into N and the ICE was not running, then it would not start again, regardless of the SOC of the HV. At two bars, though, you have plenty of juice remaining in the battery. Certainly it would be enough to make it through a car wash, even with all of the accessories going.

To be safe(r), you could wait to put the car in N until after the ICE has started running. Theoretically, the ICE will stay in an idle position, burning gas an charging the HV, until you take it out of neutral. This is hardly fuel efficient or environmentally friendly, but you won't do any damage to the HV.

At the same time, I wouldn't do the N with the ICE running if the HV display already showed all green. You might be in danger of overcharging the HV at that point.

Oh well. I may have some of this wrong, but if so, then one of our ever-present prowling engineers with full knowledge will point it out (politely!!).

 

Hey! I resemble that remark!

Seriously, the car wash scenario wouldn't be that bad, as long as you kept an eye on the SOC

 

My question is is the opposite: Descending from high altitude on very long stretches with no uphill climbs. Dropping into Death Valley, for instance, from the west takes about 20 minutes of continual "B" shift driving plus breaking (unless you wish to test the terminal velocity of the Prius on winding road!) What happens to the excess charge? Is there a Flux capacitor involved?--Ha!

Lee

 

We're in agreement on all the points and clarifications you've made here! My comments are based on some of the silly stuff I see mentioned in the press lately about motor size. I should have been more specific. Obviously I never intended to mean that anybody is better off by stuffing a huge motor into a car - a motor that is beyond the capabilities of the car or batteries! - that's the realm of gasoline concept cars.

 

I've read many times that when in Neutral gear the Prius does NOT allow charging of the HV battery. I don't wish to test this on my own car but I would be interested to know the correct information. I've included a couple of links that show an opposing view.

http://priuschat.com/forums/prius-main-forum/38676-coasting-in-neutral-2.html#post504092

Or the bullet point - Neutral gear at the following link:

[ame=http://en.wikipedia.org/wiki/Hybrid_Synergy_Drive]Hybrid Synergy Drive - Wikipedia, the free encyclopedia[/ame]

 

The idea of neutral is that the car can be pushed by external forces (usually man power). To get power from a generator, electrical resistance is involved and it requires extra force to do this. Therefore the motors are electronically disconnected from the inverter.

However, it is not possible on the Prius to physically disconnect, so it is advised that the car should be on, so the software can direct power appropriately, or if not possible, that the car should be towed with at least the front wheels off the ground, so no power is generated by the motors (it's not possible to completely eliminate the electrical resistance, so some power is always generated).

Because starting the engine requires MG1 to turn, the engine cannot be started in neutral. I'm not sure why the engine isn't stopped in neutral but I suspect that it's due to the logic of positioning the engine for optimal start-up next time, and possibly ensuring that it does not counter-rotate. Regardless, the rotational force from the engine merely spins MG1, but under no electrical load, so minimal power is generated.