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Electric Vehicle Battery Overview

An outline of the different battery chemistries currently in use and their implications

There has been a lot of discussions surrounding the different types of batteries used in electric vehicles and this is to give an overview of the various types used in the common electric and plug-in hybrids on the road today and outline some of the pitfalls and benefits of using the various types of batteries.

Battery testing methods

Before I go into the batteries, I would like to explain the testing methods that they use to test the cycle life. Typically to test cycle life, they charge up the battery, fully discharge it and repeat the process at various temperatures.

As per IEC : 1C constant current-constant voltage charge to 4.2V with cut-off current 20mA; let the battery sit for 1 hours and then 0.2C discharge to 3.0V. Then after 500 cycles, remaining battery capacity should be 60% of original capacity.

Testing method commentary

Well, if you think about it, that's a pretty harsh test to say the least. A real word condition would be to charge it using a Tesla supercharger network routinely (says not to do it in the manual), then draining it completely and repeating the process for the life of the car. If you look at the battery curves the rule of thumb regardless of the type of lithium battery is "The faster you charge it, the more "damage" it does to the battery". There's a highly scientific explanation, but let's just keep in it layman's terms.

Realistically with a Tesla Model S, this situation is not going to happen, to mimic that test, you'll have to plug it into the supercharger each and every time. Even if using the 220 V/80A twin chargers, you'll be below the 1C mark. The Leaf and Focus electric vehicles, it is possible to drain the batteries quickly and run into the fast draining situation, fast charging, the vehicles are incapable of utilizing the Tesla supercharger, but the Nissan and MiEV at least are capable of utilizing the CHAdeMO charger (a little of a concern in terms of battery life). Plug in hybrids, not so much.

I have to comment on depth of discharge and how quickly the battery is discharged in a normal days usage. This assumes that the person goes home and plug it in ever night because that is the typical behavior of an EV owner. With Lithium batteries, the more you baby the batteries the longer they last. In general, if you have two lithium batteries, take 1 down to the minimum discharge and leave the other at 50%, top them off, and repeat the process, the latter is going to last longer.

Lithium iron phosphate

These are the batteries found in the BYD e6, Coda, and the Fisker Karma. These have the second lowest energy density of the bunch, are very safe comparatively, have a low cost and offer the second lowest cycle life. The cycle life is still relatively high though, there are some results peg the cycle life between 1000 and 2500 cycles, while some peg it as high as 7000 cycles. These are also relatively abuse tolerant, meaning they can be discharged at a high rate. The BIG advantage is price and that they are slightly more safe than lithium manganese oxide.

Lithium Titanate

There's the Altair nano or Toshiba lithium titanate. As the name suggests, it's a lithium/titanium. This is commonly used in the Mitsubishi MiEV, FIT EV, and possibly the British lightning GT. In general, these batteries have a lower energy density that the other types of lithium batteries discussed, BUT as per Altair nano, they can last as much as 20,000 charge discharge cycles. To put in perspective, you could charge/discharge these batteries for 54 years if you were a daily driver and still have a reasonable capacity. These are the most stable of any battery chemistry, they don't have any reactivity, BUT they are very, very expensive (about 3 times as much as lithium manganese and suffer from low energy density, roughly between 60-80 Wh/kg.

Lithium Cobalt Oxide (LCO)

There is lithium cobalt oxide, this is the least stable of the current battery chemistry out there, but until recently had the highest energy density on a weight basis. These also have the lowest cycle life compared to others, and have a 70% life at around 300 cycles if following the testing methods for batteries. No one uses these in any current in production EV. Tesla did use these in the Roadster though, but had a massive cooling system and battery management system. If the system in the Roadster sensed the batteries were being taxed or getting too warm, the system would dial back the performance of the car.

Side note: Boeing also used this class of batteries WITHOUT cooling and without an adequate battery management system in planes. There's nothing inherently wrong with using these batteries and they work well, but you have to be mindful of the pitfalls and that these need a decent battery management system to prevent overcharging and overheating. Also the Boeing batteries were large format cells, so if something went wrong, it's snowball the entire pack. If a cell went in a Tesla, it's not a big deal. Think of it like a string Christmas light vs a light bulb fixture. If a little LED goes out, no big deal, it works fine. I have a lighting fixture in which once one light blew, sure enough within a few seconds the other light blew, it was a poor design. Same premise applies here, if one battery blows, no big deal, if one large cell battery malfunctions, it is a big deal. The main issue turned out to be that they were overcharging the cells (no battery management system) and compounded by the large format (one goes, it's a huge exothermic reaction)

Lithium Nickel Cobalt Aluminum Oxide (NYSE:NCA)

This battery is slightly more safe than lithium cobalt oxide, has a high energy density, almost as high as lithium cobalt oxide, it's also costly, BUT the real benefit is longevity. It has one of the highest longevities of the lot. Only lithium titanate beats it in terms of longevity. I call this LCO 2.0 because it has the benefits of LCO and improved upon some of the drastic drawbacks of LCO. As an example of the longevity, these should have about 70% charge after 2000-2500 harsh full cycles. Well, 265*2500= 662,500 MILES!

Lithium Manganese Oxide (LMO)

This is the most common type found in many types of electric vehicles and found in the Leaf, Volt, and Focus. The main supplier is LG chem. It's middle of the road for many of the factors such as longevity, energy density, and safety. The standout factor is that it is the CHEAPEST!

Small format vs large format

There's a common debate about which is better, small format (Tesla build) or large format (everyone else). There is some arguments for both. The large format cells have the advantage in the low number of parts, easy to refurbish, and in some cases even not including a active cooling system. More or less, if a you have a small number of cells, it needs less "control", easier to manufacture, and if enough orders come in this can greatly reduce the costs and the modules are already done and can be easily put into the car. That's a very reasonable philosophy.

Small format, specifically the 18650 format is what Tesla uses in a modified form. Currently the small format has advantages in terms of cost because they are consumer grade cells, but they do require a bit more care to be taken, they also require specialized equipment to put them together and a very robust and complicated computer program to ensure that if a cell goes bad it doesn't spoil the whole module or pack. It does have an advantage if there is a thermal incident in one cell the energy released may be dissipated in the cooling fluid so as to not reach the activation energy of the adjoining cells. Also if there is an accident, and the cells are punctured it's not going to cause a huge fire.

Summary table

 

 

Car

Battery type

Cooling system

Daily Depth of discharge

Potential for >>1 C

charge

Potential for >>1 C discharge

*longevity

Battery format

Other notes

Fisker

Iron phosphate

Yes

high

No

Yes

medium

large

Same advantages as LMO

e6

Iron phosphate

Yes

medium

Yes

Yes

medium

large

Same advantages as LMO

Coda

Iron phosphate

Yes

mid-high

No

Yes

medium

large

Same advantages as LMO

Leaf

LMO

No

mid-high

Yes

Yes

medium

large

Cheapest, middle ground

MiEV/Fit

titanate

Yes

Very high

Yes

Yes

Very high

large

Most robust

Roadster

LCO

Yes

low

No

No

Low

small

Highest energy density, not very robust

Model S

NCA

Yes

low

No

No

Very high

small

Robust and high energy density

Fusion

LMO

Yes

mid-high

No

Yes

medium

large

Cheapest, middle ground

Volt

LMO

Yes

high

No

Yes

medium

large

Cheapest, middle ground

If you do a little matrix and weigh everything equally, yes, LMO does come out "better" but again that's weighing everything equally. I would put a higher weighting factor on energy density and longevity because you can "cheat" by getting consumer grade cells to make cost less of a factor and you can engineer a nice safety system to make the cells safer, you can not change the basic chemical structure or composition. Think of it like home shopping. You come to 2 homes, one meets all your needs except it has no yard for your dog, one has a yard, but has an awful kitchen and bathroom. You can change the kitchen and bathroom, but you can't change the fact that the other has no yard.

Known accidents and outcomes

No real world fire concerning Evs should be given any credence. More or less there are more reasonable and more mundane reasons for the fires than the battery spontaneously combusting.

To my knowledge there has only been 1 fire in the real world attributed to the battery going up after immediately after an accident. Scouring the internet for hours could only find this one, lonely case. This occurred in China when a Nissan GT-R going 110 mph T-boned a BYD e6 taxi. In all respects the passengers were dead even before the fire occurred. Let's be honest, regardless of the car those people would be dead. If I remember correctly, there was something about Ford police interceptors from the early 2000's, that if rear ended going 40 miles less than 110 mph, could catch fire, and there were quite a few incidences. High speed crashes, specifically when they hit critical areas typically result in deaths or fires or other severe injuries regardless of type of vehicle. We really should not include an accident that involves someone going over 100 mph and call that normal.

Now lets look at some other accidents. Recently there a Tesla Model S on the news that got into a horrific accident that killed 2 people (in the other car) and the Tesla Model S driver walked away with just minor injuries. The car did not burst into flames. The Roadster did have a recall due to a fire hazard, but again, NOT due to the lithium battery. The recall was due to the 12V accessory/back-up wire rubbing and possibly causing a short. Again, many vehicles are recalled for fires all the time, Ford and Toyota come to mind recently.

There have also been numerous Roadster accidents, which can be found on the Tesla motors club forum and the damage can be seen. None of those cars caught fire following the accidents.

With Fisker there were a number of fires. The most obvious one was after hurricane Sandy in New Jersey. Well, technically that's an act of God. I don't think anyone in their right mind would let a $100,000 car become submerged in SALT water and then let it dry out. Was it the batteries that started the fire? No, it was a computer board. Soaking a computer board in a highly conductive liquid is asking for trouble. Moral of that story is don't treat your Fisker Karma like submarine.

There were a few other Fisker fires, one in Sugarland/Fort Bend, Texas, this one was ruled to be the cooling fan at the front of the car. There was another one in Woodside, CA. Again, google the pictures, you can clearly see the left fender caught fire, the battery is in the center. Doesn't take a fire expert to figure out that it was not the battery in that case.

The infamous 2 Chevy Volt incidences. The first incident was a side impact at the NTSHA lot. The battery did not catch fire, but smoldered weeks after the incident occurred. Moral of the story, don't leave a wrecked car sitting in a lot without taking proper precautions for weeks at a time. If a person is trapped in a wrecked car for 3 weeks, there are more pressing concerns than the battery catching fire. GM also has this nifty feature called On-star. Remember those commercials, he guy gets into an accident, On-star calls him to ask if he's OK and sends the police?

3 weeks trapped inside a car, with no food or water, well, the person would be long dead of dehydration before the fire ever got to them.

The second NTSHA incident was a little odd. First, you need to roll over the vehicle. Then you need to puncture the battery case. After that coolant has to get onto a specific circuit board. It's unlikely that such situations occur in the real world, and it is no more, if not less dangerous than the fire risk of puncturing a fuel tank and having gasoline leak onto a spark or open flame if a regular car was in such an accident. No volts have been reported to have caught fire under real world conditions, Google "volt accident Geneseo, New York" that one was completely demolished without any fires