Electric car question, Li-Ion cells

I don't know much about electric and hybrid cars, but I was thinking this morning. If Li-Ions typically get 500 charge cycles, how do people afford to drive these car that are probably being charged every day? Even if the cells lasted 2000 cycles, that's still only 5 years! Do they use some special type of Li-Ion?

Maybe it's of no concern to high rollers that are dropping $70,000 on a car in the first place? (Tesla in this example)

Suspect the cells being used by the big players are much higher quality than the 18650s you or I might obtain from the 'zon, thus have far longer shelf life and a greater cycle count.

As with most chemistries, lower depth of discharge extends cycle count considerably. If your driving pattern only uses, say 40% of the effective capacity, your effective cycle count will be less than 40% of the number of cycles you've performed over the life of the car.

Also curious how you arrived at 400 cycles per year - doubt that a daily driver using ~75% of its range 5 days a week would see even 200 0%-100%-0% equivalent cycles per year.

Most of the hybrid car manufacturers offer a 5 year 120,000 mile warranty on the batteries. The thing about they hybrids is that the gasoline engine runs when maximum power is required and can't be supplied by the batteries. Unless you can perform a battery capacity test, you'll have no clue how the batteries are doing as they age other than remembering that you drove X miles with engine assist and now you only get X minus miles.

Strictly electric only vehicles is where you'll see the effects of a battery failing. Chevrolet wouldn't sell the VOLT in northeastern states since you need an inductive heater to keep both you and the windshield cleared, battery life was less than 40 miles before recharge was required. Electric only vehicles make sense for an in-town fleet delivery vehicle, not a vehicle where the infrastructure for charging doesn't exist. Sort of like the CNG (compressed natural gas) cars and trucks.

NoNotAgain said:

Strictly electric only vehicles is where you'll see the effects of a battery failing. Chevrolet wouldn't sell the VOLT in northeastern states since you need an inductive heater to keep both you and the windshield cleared, battery life was less than 40 miles before recharge was required. Electric only vehicles make sense for an in-town fleet delivery vehicle, not a vehicle where the infrastructure for charging doesn't exist. Sort of like the CNG (compressed natural gas) cars and trucks.

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That makes sense. I was just thinking around here in the midwest; parking it in a black asphalt lot for 9 hours at work around 110°F, then -30° or colder at home. Seems like a torture test for batteries. Personally I would keep it in the garage, but not everyone would.

idleprocess said:

Also curious how you arrived at 400 cycles per year - doubt that a daily driver using ~75% of its range 5 days a week would see even 200 0%-100%-0% equivalent cycles per year.

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Don't know...I'm guessin a fat-finger typist! Either way, I'm unimpressed with even half that figure.

A neighbor had a Ford Escape Hybrid (company lease) - it needed the battery replaced in it prior to end of lease. I think he's the only person I knew that has / had a hybrid (or electric) vehicle.

The Tesla model S, uniquely packages the battery under the floor (so much better than the Fisker Karma, and first generation Chevrolet Volt) and should avoid large temperature swings (when parked that is)
http://cdn.shopify.com/s/files/1/0196/5170/files/tesla-batt_large.jpg?3174
So, let's say you only use 1 full charge equivalent per week-you can get over 400 miles of range, if you drive ~20-25mph. Tesla is offering 8 year/unlimited mileage warranty (on the 85 kilowatt-hour)
So if the Panasonic cells 'only' do 500 charges before they are spent, that would be 9 years, 7 months before replacement.

But longevity depends on how you discharge & recharge right? Tesla allows for ridiculous discharge rates from those 18650s. Something like 75 watts per cell, for about 5 seconds. P85d can jump from 0-75mph in 5 seconds, then acceleration tapers off.

MichaelW said:

you can get over 400 miles of range, if you drive ~20-25mph.

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Maybe it can if you drive at a constant 25 miles per hour for the whole cycle...but that will never, ever happen in the real world. If you are accelerating and decelerating regularly, as almost all people will have to do, your range will be much less. Even Tesla (who would love to claim the highest possible range) only rates it for 270 miles, with good conditions.

The list of important factors (straight from Tesla Motors) is a long one:

• Constant speed (such as using cruise control)
• Flat ground, no wind
• Climate control OFF or using vent only (no heat or air conditioning)
• 300 lbs of vehicle load (driver plus passenger or cargo)
• Windows up, sunroof closed
• Tires inflated to recommended pressures
• New battery pack (<1 year, <25,000 miles)

The last one is particularly germane to this conversation...they are acknowledging that even after one year, you will likely see a drop-off in battery performance/range. While I'm sure Tesla has the pick of the litter, cell-wise, there are inherent limitations to lithium-ion technology today. It doesn't have magical cells that can run for a decade without losing significant capacity.

It would be great for me. I drive 8 miles to work, then 8 miles back on the highway. About 1/2 mile of that is off the highway and I may not even stop once. I may add another 5-10mi few times every week to run errands on the way home from work. When I go backpacking about 250-300mi away, I could just take my wife's car. It would be ideal, but not $70,000 ideal. Of course I could get a Volt at half that price. I'm not in the market for one though, just curious.

I guess I'm just so OCD about Li-Ion from using them in flashlights. All we do here is talk about babying them and sometimes going to extremes. Storing them at 3.6v, putting them in the fridge, balancing, charging in metal containers, using them enough to keep them in shape, etc. So having them in a car seemed touchy.

I not certain, but most of the electric vehicles use a regenerative system that charges the battery when you decelerate.

Pretty sure that my nephews Honda Civic Hybrid does.

Honda a few years back were facing complaints about short battery life. They sent a service bulletin to registered owners to bring the car for a update to the software. As part of the Update, they extended the warranty on the batteries from 80,000 to 120,000 miles. Most owners experienced a 5-7 mpg loss as the programming changed when the gasoline engine kicked in, charging the battery more often than the as received factory program.

markr6 said:

I guess I'm just so OCD about Li-Ion from using them in flashlights. All we do here is talk about babying them and sometimes going to extremes. Storing them at 3.6v, putting them in the fridge, balancing, charging in metal containers, using them enough to keep them in shape, etc. So having them in a car seemed touchy.

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Most hybrid cars - including the Chevy Volt - are designed to keep the batteries within a given state of charge so as to maximize operating life and ensure predictable voltage/current delivery characteristics. Toyota has had the best luck and I gather that the Prius pack has been one of the most reliable parts of the vehicle.

BMS's on pure depletion mode EV's work like 18650 protection circuits but do so much more - monitoring environmental and electrical characteristics to balance pack lifespan and vehicle performance.

NoNotAgain said:

I not certain, but most of the electric vehicles use a regenerative system that charges the battery when you decelerate.

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They almost all do - it's a simple way to extend range by ~10% since you need only switch the motor to generator mode and you can recapture a slice of that kinetic energy you put into accelerating the vehicle.

I'm a long time Prius driver... Mine is 13 years old this June. I did a lot of research before buying it, and I'm happy to say that the doom-sayers were sadly mistaken.

First, regen braking. It would be great if regenerative braking were able to recapture all the energy used when accelerating, but the amount that is captured is limited by how much current the batteries can handle while charging. Like all batteries, you can usually draw higher current than you can put back when charging. Adding supercapacitors as momentary buffers may help, but they need to be pretty big to have a significant impact. An unexpected side effect is 150K miles between brake jobs.

Second; li-ion are not created equal. There are designs that use an anode that has an extremely large surface area via nano tech. These are able to handle a couple thousand FULL DISCHARGE cycles. As mentioned above, a partial discharge does less to reduce battery life than a full discharge.

Third; most hybrids are still using NiMh, not lithium. The Ford Escape hybrid was still using one in 2012. The Escapes are lasting far in excess of 100K miles. A taxi got 500,000 miles. Mine (a Prius, remmeber? ) is at 140,000 miles and still on the original battery pack.

Last; one of the things that Tesla is doing to prolong battery life is active cooling of the batteries. They are air conditioned.

Dan