Electric Car Battery Packs and Longevity

It's 2016.  We've all messed around with lithium battery powered devices.  Cell phones, laptops, tablets, GPS units, smart watches, hoverboards... whatever.  After a few years worth of use, they all have absolutely terrible battery life, and you need to either replace the battery (if that's even possible) or buy a new one (with the manufacturers, obviously, preferring you to buy a new one).

Why should electric cars be any different?  Won't they just need their battery replaced in a few years, for some absurdly large sum of money?

Probably not - at least if the auto maker is doing things properly.  There's quite a few reasons involved, and I'm about to dive into most of them.

The biggest factors in determining battery longevity are battery chemistry and additives, temperature of the cells when charging and discharging, state of charge when stored, cycle life, and pack capacity.

Keep reading for a dive into all of these!

Battery Chemistry

One of the biggest factors in battery longevity is the battery chemistry.  Everyone hears about "lithium ion" or "lithium polymer" batteries, but those terms are incredible generic - they're about as precise as "internal combustion engine."  Do you mean gasoline, diesel, or natural gas?  Otto cycle?  Atkinson Cycle? Diesel cycle?  Gas turbine?  Piston?  Rotary?  Wankel?

"Lithium ion" refers to a battery that uses lithium ions (surprise!) to transport charge around.  This includes an incredibly broad selection of actual battery chemistries.

"Lithium polymer" or "LiPo," at this point, just refers to a lithium ion chemistry in a semi-flexible pouch cell.  Nothing more.

What really matters is the actual chemistry involved in the battery reactions.  This will be something like lithium cobalt oxide (LiCoO2, ICR), lithium manganese (LiMn2O4, IMR), nickel cobalt aluminum (LiNiCoAlO2, NCA), nickel manganese cobalt (LiNiMnCoO2, NMC), lithium iron phosphate (LiFePO4, LFP), lithium titanate (LTO), or one of a variety of other chemistries suited to doing specific things.

Then, within a given battery chemistry, there are other things that can have a huge impact on power, energy, and longevity.  The details of anode and cathode construction/structure, the specific blends of materials involved, and the additives (small trace chemicals that affect battery performance and longevity in significant ways) will all vary between manufacturers, and even between different battery product lines for a given manufacturer.  Sadly, most of these details aren't available to random people on the internet.

Ideally, we'd just use batteries that had incredibly high energy density (how much energy they store per unit weight or volume), incredibly high power density (how much power they can provide at any individual moment), and infinite cycle life (you can keep using them until the end of the universe without any capacity loss).  Of course, in that universe, we'd also have unicorns that poop rainbow colored ice cream.

Sadly, we don't live in Physics Utopia Universe.  We're bound by the laws of physics, chemistry, and our universe, which all generally say, "You can have more of some, but you get less of others.  And if you want more of all of them, you're not going to be happy with the cost."  So, we pick and choose - and for electric vehicles, cost is a huge factor.  Nobody would buy a Leaf-like vehicle for a million dollars, even if the battery chemistry was pure unicornium.  So, we remain bound to realistic, affordable chemistries.

Different automakers have chosen different chemistries.  Tesla uses NCA in the Model S (and presumably the Model X).  BMW is using NMC in their i3.  Chevy and Nissan are both using some variety of lithium manganese (LiMn) chemistry, perhaps a pure LiMn, perhaps not (this information is actually quite hard to find without paying hundreds of thousands of dollars for teardown reports).  Mitsubishi is of off in the weeds, using lithium titanate in their i-MiEV series.

And, of course, each manufacturer will have their own specific variants within that chemistry.

So far, Tesla's gamble with NCA seems to be paying off.  Nissan has been having pack degradation issues on their early cars in hot areas, the i3 is too new to really tell, and I don't have enough data on other EVs to have a great feel for how they're aging (though evidence seems to indicate Chevy is doing the right things).

There's an awful lot of highly technical and incredibly detailed information in Jeff Dahn's talk, Why do Lithium Ion Batteries Die? - if this stuff is interesting to you, the talk is absolutely worth watching.  Even if you don't understand all of it, there's a lot of great data in there.

What about your experience with laptops and cell phones?

Your laptop and cell phone are most likely using an aggressive lithium cobalt oxide chemistry that's being run to it's limits.  Why?  It has the highest energy density.  And "new out of box runtime" is very important - that's what all the review sites will report.  As most OEMs view cell phones and laptops as having a short life span (2 year or so seems to be considered the useful life of a cell phone), battery longevity just doesn't seem to matter much to them.  Besides, if a cell phone lasts forever, why would anyone buy a new one?

Temperature and Thermal Management

State of Charge & Charge Control

Cycle Life

Degraded Pack Remaining Range

Finally, it's worth considering how much range a car can lose before it becomes useless for it's desired purposes.  Capacity loss vs cycle count is usually fairly close to a linear curve - so if it takes 10 years of driving to drop to 80% pack capacity, another 10 years is likely to get the car to around 60% capacity (realistically, a bit lower, because it's undergoing more cycles in that time due to the reduced capacity).  There's usually no cliff where the pack becomes worthless, just an ongoing loss of capacity over time.  This is why people often refer to packs being "recycled" into stationary storage - as long as the cost per kWh is low, it doesn't really matter what the percent of original capacity is.

But, for driving, there's some minimum range below which the car becomes useless as a car, and will either need a new battery pack or will take a trip to the scrap hard.

A car with a 280 mile initial range, with a pack degraded to 60%, still has a useful range of 168 miles - which is plenty for most people's daily commuting, even factoring in heating/cooling loads, rain, wind, etc.

On the other hand, a car originally rated at 84 miles, at 60% pack capacity, only has 50 miles of range.  That's starting to cut it really close if you use your heater or have to deal with crosswinds.

Combining this effect with the reduced degradation from the lower cycle count on larger packs, and it becomes evident that a large pack will last significantly longer than a small pack before needing replacement.

Cell Phones & Laptops

If you're using your phone regularly, just use it.  However, it's not great to leave it fully charged - so consider charging in the evening or morning instead of plugging it in all night.  I've yet to meet a phone that lets you turn down the "fully charged" percent, and some of the newer devices are pushing their batteries really hard.

If you are in extreme hot or cold conditions, let your phone come back to room temperature before charging if at all possible.  Plugging your phone in to charge after a day on the ski slopes is really, really hard on the battery.

For storing a phone when it's not being used, aim for around 50% battery - it doesn't have to be perfect, just close.  60% or 70% is fine as well - but don't store it fully charged.

Laptops should be kept cool.  A blazing hot laptop with a fully charged battery will cook the battery in a hurry.  Clean your fans!  And, again, don't store it with a full battery if you can avoid it.

Conclusions