REEV concerns

[siry]

@james the issue in a series hybrid system isn't so much maintaining a speed, which requires relatively little power, but rather acceleration after the battery is "depleted". A Volt genset, if it had to handle the task alone, would have a very hard time passing someone on the highway. This is why they only have 8kWh usable but the overall pack is 16kWh. It gives them a lot of cushion to allow for the battery to add energy for acceleration even after the 8kWh is done.

There could be cicumstances where someone is doing a lot of hard accelerations and passing or maintaining high speeds for long distances where the pack will truly be depleted and the car's performance is significantly reduced.

I consider this a point of advantage for EV over series hybrid, but GM has designed around it so it isn't going to be a common problem. I don't know if fisker is doing something similar. They have spec'd 50 miles range with 20kWh which seems to assume the same Wh/Mile if they are also only using 10kWh as the usable range. Since Wh/Mile is likely to be higher than the volt, they will likely need to use more of the battery to achieve 50 mile range.

But the again they spec'd a very powerful ICE for the Genset so I imagine they can do more under genset power. Downside is poor efficiency.

[James]

Siry, that is a good point. I had not considered that they might invade their reserve to maintain normal acceleration.

Regardless, I am sticking with Toyota (PHEV) and Tesla (EV) having better designs.

[TEG]

the issue in a series hybrid system isn't so much maintaining a speed, which requires relatively little power, but rather acceleration after the battery is "depleted".

Not only acceleration on the flats, but also maintaining speed on steep inclines, or into a strong headwind.
(Better not get stuck trying to accelerate uphill into a headwind... it does happen sometimes).

[donauker]

I think this ongoing concern of the Volt not having adequate performance in some hypothetical worst case situation is way overblown. The power output of the ICE in a Prius is about the same as the generator output of the VOLT. The capacity remaining in the battery of Volt at the time it is considered depleted and the generator begins providing power is much greater then the full capacity of the battery in the Prius.

For the Volt to not have acceptable performance you will have to find a stretch of road where you would be able to keep the accelerator of a Prius to the floor for probably considerably more than 15 minutes and it would be under the speed limit for the entire time.

[siry]

@donauker not a problem for a car like the volt but it was one of my major concerns when we considered REEV for WhiteStar since it was a sports sedan meant to be driven aggressively. Also - the "cost" of not worrying about this issue is a 50% usable range in the battery - that's expensive!

[James]

Also - the "cost" of not worrying about this issue is a 50% usable range in the battery - that's expensive!

Siry, as I understand it, all of those mini-cycles are really hard on the battery. If I recall (I think I read this comment from you in an article) doing the Tesla battery system as a pure EV is far more simple and reliable for a long term life expectancy of 100,000 miles. Is that accurate?

I think the logic is based on this set of circumstances comparing an EV to a PHEV.

With a Tesla Roadster having a 244 mile range, most people will use around 10% to 40% of the battery pack each given day that the car is driven. So the battery will spend most of it's life between 50% to 90% charged up. This is actually a very good usage profile to maintain the long term quality of the battery pack.

With a GM Volt having a 40 mile range, most people will be using the full approved range of the battery each day. So that battery pack will be taken down to 30% charge on a regular everyday basis. I am assuming that the max charge is 80% and minimum reserve is 30%. So that 8 kwh of the 16 kwh is used. This type of regular deep cycling is fairly hard on the batteries over time.

Is that an accurate description of the situation?

[siry]

@james - there is a key difference in cell chemistry. because of the cycle life issue you are describing, you have to use iron phospate (like A123) or manganese (like LG Chem) or other less common types in PHEV applications. These types of cells have lower energy density by a factor of about 2.

If you used cobalt oxide cells for a PHEV like the volt, they would not last 2 years (depends on a lot of things, but you get the gist). But in a pure EV like tesla, the pack is so big and contains so much energy that the lower number of cycles covers an awful lot of miles, so you see a longer life like you mention (along with all the temperature and charge balancing and other magic that treats the batteries nicely)

If you used these types of chemistries in an EV you would have a much shorter range (lets say 100-120 miles for the same weight). But a nice benefit would be you would have much better cycle life properties over time.

[TEG]

there is a key difference in cell chemistry. because of the cycle life issue you are describing, you have to use iron phospate (like A123)...

If you used these types of chemistries in an EV you would have a much shorter range (lets say 100-120 miles for the same weight). But a nice benefit would be you would have much better cycle life properties over time.

Yes, and some other wannabe EV makers are also trying to use LiPo because they don't have the capability to create a safety mechanism to keep the cobalt cells happy like Tesla did. (Or even if they did, Tesla's patents could get in their way).

[James]

@james - of course GM will deliver the Volt! They are way too far down this path to not deliver it. The only real question is whether they hit their timeline and how many they ultimately produce or sell.

I have my doubts on that type of series hybrid ever being successful. I saw a presentation that walked through the energy flow of that system. The gas engine is only being used to recharge the batteries, not drive the wheels. So after the 40 miles of driving from the fully charged battery, that gas engine needs to generate a lot of electricity to maintain a normal speed vehicle. Many have speculated that the gas engine that GM is using cannot manage a normal driving speed after 40 miles. It would likely be in "limp" mode because that size gas engine cannot generate enough electricity to propel the weight of the Volt.

When the GM Volt is actually in the hands of independent reviews and car magazines, the weakness of the design will become obvious.

Toyota is going the right way with their plug-in design.

Just my opinion.

[James]

It seems that the GM Volt design will have a key weakness during certain scenarios.

Since the ICE is only recharging the battery, and it does not recharge fast enough to maintain high acceleration/fast driving, this creates the potential for really horrible performance.

If you have been driving 40+ miles already and your VOLT battery is drained of your electric grid charge, then it is likely has a state of charge around 30% to 40% when the ICE kicks on to recharge.

So now you have to do a hard acceleration to deal with a hill or some other high energy consumption driving scenario. Supposedly the GM Volt will allow you to invade the battery reserve for a while. But what if you do this to the point where the state of charge gets really low.

Then you are in a serious limp mode where you really only have the energy available based on the ICE recharge rate.

How many watts is the on board ICE recharging at?1,500 watts? 2,000 watts? That is enough to allow about 5 to 8 miles of range per hour.

This is just speculation. But I imagine that the battery could get into a charge level where the performance is just pathetic.