[vltvlt972]

The Volt has two drive trains - When one is tried, go to the next

One intriguing thing in the Volt's design is the likelihood that most people will use its gas engine rarely.

Let's say that I have 80K or 100K miles on my G2 Volt and the battery starts losing efficiency.

But I still have a gas engine and a generator with 1 or 2K miles on them (almost new). Shifting over to Hold Mode is like putting a new engine in an old car - if the aging battery can still pass the generator's current to the electric motors without obstructing the current flow.

Since electric motors wear little compared to their mechanical counterparts, it seems like the Volt has two separate drive-trains. When one gets tired (the battery does not hold as much charge), the second drive-train can kick in and still give 42 MPG.

Its like having two engines in an ICE car where the driver can switch to the second engine when the first one reaches 100K miles (is worn out). All this depends on the battery or the electronics not obstructing the current going from the gas engine/generator to the electric motors when the battery nears the end of its life or at least loses some degree of charging efficiency.

 

[Barry]

In theory that is true and it will probably work that way as long as the battery fails gradually and gracefully, just losing capacity over time due to normal aging. However, it seems a common failure mode is that a portion of the battery, such as a group of cells, will fail and go to an out of range voltage, which throws a code and then the car will typically not drive at all.

I think it would have been nice if GM had programmed in a fail-over mode where no matter what happens to the battery, the engine could still safely take over and allow driving (even though performance would be reduced, obviously). Even if just to get to the dealership or get home in the middle of the night instead of getting stranded and needing a tow.

 

[vltvlt972]

In theory that is true and it will probably work that way as long as the battery fails gradually and gracefully, just losing capacity over time due to normal aging. However, it seems a common failure mode is that a portion of the battery, such as a group of cells, will fail and go to an out of range voltage, which throws a code and then the car will typically not drive at all.

I think it would have been nice if GM had programmed in a fail-over mode where no matter what happens to the battery, the engine could still safely take over and allow driving (even though performance would be reduced, obviously). Even if just to get to the dealership or get home in the middle of the night instead of getting stranded and needing a tow.

I agree - having two separate sources of power makes it seem that a reasonable design would have a failure mode where one side failing (the battery or engine) would allow the other to take over to keep the car running.

I am surprised that there is not more interest in this topic since it is one of the things that makes the Volt truly unique.

Two (mostly) separate sources of power mean redundancy in engineering, and all mechanical and electronic designs with "high reliability" have this feature. This highly desirable feature of the Volt is in addition to the longevity aspect which means the car could last twice as long as a normal car. This is because the seldom-used engine could take over when the battery loses a significant amount of capacity, and it will often have almost 0 miles on it when this occurs.

It is interesting to think of owning a car with $100K miles on it with a (almost) new gas engine but that is what many Volt owners may eventually have.

 

[FI Spyder]

Which brings up the question. Reduced propulsion can occur when cells are unbalanced. Is that right? If the battery degrades evenly the reduced propulsion does not come on. Is that right? I don't think the later has actually happened yet has it?

 

[mjones21]

Which brings up the question. Reduced propulsion can occur when cells are unbalanced. Is that right? If the battery degrades evenly the reduced propulsion does not come on. Is that right? I don't think the later has actually happened yet has it?

GM recently did a recall (I think it was a recall, it may have been a TSB) for a select manufactured date range of 2013 Volts that had a cell balancing issue. AFAIK, it is not a widespread issue for Gen 1 Volts broadly outside of this small group of 2013s, or for Gen 2 Volts at all.