[Tom]

All Series Hybrid EVs’ need a separate 12/48V accessory battery to provide “Limp Home” capability in the event that the power train battery died. It is needed to turn-over the ICE and power its electronics during start-up with a dead power train battery. It would also provide power for lower voltage accessories. Attached is a simulation of the performance in this mode.

The Volt motor has a peak power capability of 209 hp (latest web info), but in the "limp home" mode this is limited by the generator to 71 hp (53 kW). It is rumored (http://blogs.motortrend.com/6259238...our-cylinders-to-be-built-in-flint/index.html) that it will be powered by a 1.4L 4 cylinder non-turbo engine (estimated output of 88 kW/120 hp and torque of 175 Nm).

The simulation simply substitutes the 71 hp generator output for the Volt’s max motor power. The nominal peak torque is used for the “Limp Home”, because for most of the operating range in this mode (i.e. above 10 mph), power and not peak torque determines performance. (See the Torque/Power curves below.) Relatively speaking, the overall effect of the “Limp Home” mode is that the reduced power starves the motor torque when above 10 mph.

The time to 60 mph is 27 secs, the quarter mile time is 23 secs, and the max speed (first gear) is 80 mph. I defined the max speed as the speed at the end of 1 minute at maximum acceleration. I used 1 minute because it is a convenient round number and I don't think you can realistically keep the road surface, grade, straight direction, and banking and wind conditions constant for any longer period of time than about 60 seconds at high speed.

This simulation is based on the parameters and methodology given in posts: http://gm-volt.com/forum/showthread.php?t=719 and http://www.gm-volt.com/forum/showthread.php?t=459.

 

[kubel]

1945 (post-war) Beetle: 34HP
0 to 60MPH: 27.5 seconds
Top Speed: 71MPH
Fuel Efficiency: 31MPG

 

[Tom]

How much extra would you pay for "Limp Home" capability?

GM may not neccesarily provide a "Limp Home" mode in its earliest versions. This Forum may be the opportunity to influence GM to provide it for early models as either basic or an option.

The intent of the "Limp Home" mode is to provide transportation capability in the event of battery failure or low SOC. What are some circumstances under which the battery could fail and you would want a "Limp Home" mode capability? Below 3 such conditions are listed.

1. The battery or its electronics shorts. An internal switch would then have to disconnect the battery so it would not short the drive from the generator.

2. The battery is the single most expensive part of your investment in a Volt. You would not want to damage or jeapardize the battery. Damage could occur if the battery does not charge properly, e.g. the average cell voltage falls below 10% SOC and charging, say 0.1C, does not raise the voltage/indicated SOC.

3. GM may not want the battery to be used at "end of life", when the battery has been charged over 1500 times and/or the coulombic efficiency falls below 80%. Rather than shut the car down completely, "Limp Home" may be a capability to drive it back to the dealer for service.

Note that regen may be disabled in the "Limp Home" mode under certain circumstances, such as 1 thru 3 above.

Edit: Since regen is not available, a means to dissipate the motor braking current must be provided, if mechanical brakes alone are not sufficient to dissipate braking heat.

 

[frankyB]

I have an easier approach...

GM should provide OnStar roadside assistance for free for all the Volt, it's new techno and they don't want bad press... If the car fails, better bring it to the dealer and have it look at.

 

[Tom]

An easier "easier approach" and its cost

frankyB

I have an easier approach...

GM should provide OnStar roadside assistance for free for all the Volt, it's new techno and they don't want bad press... If the car fails, better bring it to the dealer and have it look at.

I don't know if you, and your little smiley emoticon , were serious about this proposal or not, but waiting by the roadside, having to get another vehicle, and having your plans for the day ruined, does not seem to be an "easier approach." It is far easier be able to continue your plans and then drive to the dealer at your own time and convenience. The question is: "How much would this added 'Limp Home' convenience cost and would customers be willing to pay for it"?

 

[NZDavid]

Tom, the ICE is connected directly to the generator, which is used to start the ICE. If the main battery was unavailable the ICE would not start. A 12/48 volt battery would not have the power to spin the generator.

If the main battery fails the car won't go. the question should be, if the12V house battery fails can the car start? In the Prius, if the 12V battery fails, the car won't go.

 

[Tom]

Cost not power issue

I seems reasonable that a 48V 25AHr battery would have sufficient power/energy to start the ICE. The question is whether a buyer would want to pay for a 1kW 48V to 320V inverter (~$400) to provide the starter power.

 

[frankyB]

I don't know if you, and your little smiley emoticon , were serious about this proposal or not, but waiting by the roadside, having to get another vehicle, and having your plans for the day ruined, does not seem to be an "easier approach." It is far easier be able to continue your plans and then drive to the dealer at your own time and convenience. The question is: "How much would this added 'Limp Home' convenience cost and would customers be willing to pay for it"?

I was half serious But your "limp home" is not as secure as you would like it to be... while you are "limping" back home there still have a chance you won't make it, you using the capacity of the car in a way it is not meant to be... and since you have a problem with the car you may even create more problems... so yes I would prefer to wait

 

[Altazi]

$400 seems like really cheap "insurance" to me to have the limp-home mode available. What is that, 1% of the anticipated vehicle cost? Come on, people! Fork it over!

 

[Tom]

Reliability tradeoffs

frankyB,

First of all, modern day electronics both thrives on complexity and has an extremely high reliability as reflectedy by a failure rate of parts per billion. The concern we have is for the reliability of the drive train lithium battery. There is probably a big reliability improvement in the trade off between added "Limp Home" electronics vs. dependence on the drive train battery.

while you are "limping" back home there still have a chance you won't make it

As the simulated performance curves in the first post shows, you may not get there as quickly, but as long as the ICE/generator and electronics function, you have the power to get there.

you using the capacity of the car in a way it is not meant to be...

If the car is designed to have this function, then clearly it is not being used in a way that was not meant to be.

and since you have a problem with the car you may even create more problems...

The "Limp Home" mode would only be used when the diagnostics a specific set of conditions, e.g. a dead battery, one that is not charge properly, or if is at the end of useful life. Properly designed automotive power electronics have fault (open, short, thermal, correct status flags) detection and protection. A comprehensive Failure Mode and Effects Analysis is part of GM's product design cycle.

so yes I would prefer to wait while you are "limping" back home there still have a chance you won't make it

If the Volt designers have a "Limp Home" mode as an option, they could be more protective of the battery and shut it down and isolate it under a broader set of conditions. This would provide additional improvements in overall system reliability from reduced stress levels.

 

[simulation11]

Great work and sharing. Thank you so much for that.

simulation rachat credit​

 

[rowr12]

simulation rachat de credit​

this is just what i was looking for! thanks so much!

 

[mleskovar]

There is one assumption in your limp home strategy that in my limited understanding of Voltec won't work. Without the main battery the car will not be able to move. The ICE engine/generator won't be strong enough to directly power the car, only to sustain a level of battery charge (30%). To do it directly would take a much larger engine/generator configuration that would be too costly from a build and sustainability (mpg) perspective. Big difference. Once the charge goes to zero you are kaput. So having a standby small battery is useless. Is my thinking incorrect?

 

[mleskovar]

What you are asking for is not possible if I understand the limitations of the Volt ICE/generator properly. Powering the vehicle directly from the ICE/generator is not done nor is it feasible with the planned configuration. The purpose is to keep the main battery pack at a charge rate (30%) sufficient to operate the vehicle. If the main battery dies for whatever reason you are dead in the water until the battery is repaired or replaced. The cost and weight of a powered generator to fully power the vehicle is prohibitive from a commercial sense.

 

[prowler]

The cost and weight of a powered generator to fully power the vehicle is prohibitive from a commercial sense.

Not true. After the 40 miles or so battery-only range is depleted, the car is running (on the average) on the generator output only. At that point, the battery becomes a capacitor to level out the MINOR highs and lows vs. what is required.

BTW, does anyone know if the generator output has to go THROUGH the battery to get to the electric motor, or does the generator output bypass the battery (except for the excess that is used for recharge?)

It's all in the design and programming, a lot of which we don't yet know (and I'm not sure it's been finalized by GM yet, either.)

 

[mleskovar]

...BTW, does anyone know if the generator output has to go THROUGH the battery to get to the electric motor, or does the generator output bypass the battery (except for the excess that is used for recharge?)...

I'm guessing yes, that's my point.

 

[prowler]

I'm guessing yes, that's my point.

There's about a 15% loss in charging and then discharging the battery. If this is so, it is not optimized engineering (vs bypassing the battery). It also stresses the battery and reduces it's life. Again, we don't know - but I would doubt it.