The Secret Life Of An EV Battery

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You are probably familiar with the lithium batteries in your smartphone, tablet, or laptop. You are probably aware that they do not last very long. The number of charge and discharge cycles is only about 600 before the battery is seriously depleted. So, if you are charging every day, 365 days in a year, the battery won’t even last for two years. I have read comments by electric vehicle detractors gleefully declaring that anyone buying an electric vehicle (EV) will find that their EV battery is defunct within a couple of years — because, typically, a lithium battery only lasts for 600 cycles, as just noted.

Of course, you and I know that an EV battery will last at least 8 years, possibly 10 years or more, depending on how it is treated. I have had my electric vehicle for 3 years, and somebody owned it for a couple of years before that, and the battery is still going strong. So, obviously, the electric vehicle detractor is talking a lot of nonsense about their 600 cycles.

The Puzzle

Yet, a long life for EV batteries did seem to conflict with our own experience of smartphone batteries and the like, which we know do not last very long. I did not like that puzzle, because it made me realize that I did not really know how this was possible. Could it be something in the battery chemistry, or the battery architecture, that was different for EV batteries? I am sure that many university research departments are working on battery chemistry to try to achieve greater efficiency and longevity in lithium batteries, or even a different kind of battery altogether. Readers of CleanTechnica.com will have seen many articles about such developments, but they are always at the early stages — promising, but a few little problems yet to be solved, not yet in commercial production, etc. Is there anything special in the chemistry of existing EV batteries that makes them last longer than smartphone and computer batteries?

I had also read that when Tesla originally chose cells for the battery packs in its cars, the company chose a cell, the “18650” cell, that was already very commonly in use — and so, very cheap to buy, readily available, and reliable. That cell was used for portable electronics and was not designed to be used in a traction battery at all. So, if Tesla was using common, ordinary batteries, with a common, ordinary lifecycle of 600 charge/discharge events, how could it be made to last such a long time in a Tesla battery pack?

A Visit to the “Library”

“Curiouser and curiouser,” said Alice, but there is no little bottle labelled “drink me” to make her battery last 5 times as long. There is only one way to solve such problems, one less likely to be taken by Alice (hitting the bottle) and more likely to be taken by Hermione Granger (a visit to the library). Well, not the library as such, but the modern-day version, with a dive into the “restricted section” of the biggest library on Earth, the Internet.

More is Less

So, what was I able to find out? Firstly, all cycles are not the same. One way of increasing the number of charging cycles is by charging between less than full and more than empty. The lowest number of cycles and the shortest battery life will come from repeated cycles of charging to 100% and discharging to close to nil%. (A lithium battery should never be entirely discharged, as this drastically shortens their life.) There are a number of possibilities, such as charging between 100% and 50%, between 85% and 25%, or between 50% and 25%. It would appear that charging between 85% and 25% gives a good balance between battery life and workable capacity

Charging Parameters	Cycles before capacity reduced to 85%
100% — 25%	2,010
100% — 40%	2,800
100% — 50%	2,800
85% — 25%	4,500
75% — 25%	7,100
75% — 45%	10,000
75% — 65%	12,000

The Whole Truth?

When your instruments say your EV battery is charged 100%, is it really? When your instruments say your EV battery is down to zero%, it definitely is not. The battery management system will keep an emergency reserve for you, and after that is used up, will protect that last precious 5% or so in order to prevent damage to your battery. Your electric vehicle will behave as if the battery was completely flat, when it is not, and might tell you it is down to 0 miles when you really have about 10 miles in reserve.

I have experienced part of this for myself, when I miscalculated somewhat and ended up on the journey home with one flashing bar left on my battery indicator, which then disappeared to leave no bars at all. I then drove about 3 miles after that, running on fairy dust, with the car behaving completely normally. I arrived in my driveway without a problem. So, obviously, my instrumentation was telling a little white lie, just to scare me.

Black Box

For the avoidance of any confusion, I need to say here that when I talk about a “Battery Management System,” I refer to a wider concept than that. There are many systems in an EV which are separately identifiable, but that all gets too technical for most readers.

Let’s use the concept of a “black box.” We do not need to know all the intricacies of what is in the box, but we can be aware of the inputs and the outputs. Think of a PC box or your tablet: you know how to use it without knowing all the intricacies of what goes on inside the box.

When I say “Battery Management System,” I am including all the complex systems between the battery and the motor, the battery and the charging ports, and the driver information outputs on the dashboard or your touchscreen. What I am saying is that there is not just a battery, a length of wire going to the motor, and a pedal like on a sewing machine to control the speed — there is a much more complex system than that, but I am not going into all the technical details.

Your Virtual Battery

What if the battery management system charged only up to, say, 80%, and kept 30% in reserve, but displayed this as 100% from zero. Say this provided a range rating of 100 miles (160 km). Then, however, as the battery lost capacity, say the system charged it up to 90% and kept 20% in reserve? You would not be aware that your battery had lost any capacity at all, because your instruments would still be displaying the virtual battery as 100% from zero, with the same range as before. That is a secret way that electric vehicle manufacturers could make it appear that your EV battery has lost no capacity when, in actual fact, it has.

 

Reserved			Virtual Battery From 0% – 100%, Range 100m					Reserved
10%	20%	30%	40%	50%	60%	70%	80%	90%	100%

 

Reserved		Virtual Battery From 0% – 100%, Range 100m							Reserved
10%	20%	30%	40%	50%	60%	70%	80%	90%	100%

 

Reserved	Virtual Battery From 0% – 100%, Range 100m
10%	20%	30%	40%	50%	60%	70%	80%	90%	100%

 

That would lull an EV owner into a false sense of security, because they would think their battery has not deteriorated at all, in however many years they have been using it, but once it reaches the stage in table 3, the virtual battery has no room left to expand, and range will begin to drop off for real.

An aging EV battery will also be more susceptible to damage from fast charging, so deterioration could become quite rapid. I am not saying that any particular manufacturer does this, because I haven’t found any information from them claiming as such, but I have read about this idea in general as if it were a common practice.

Even if that’s not what’s done, however, the rather large buffers at the top end and bottom end of battery capacity surely help to maintain battery life longer than if there were no or much lower buffers, as appears to be the case with smartphones and laptops.

Controlled Voltage Level

For the avoidance of any confusion, the voltage level is pretty much synonymous with the percentage of charge I have just mentioned, so when we talk about “100%” charged, we are talking about a battery fully charged up that will then have a voltage in each cell of around 4.2V. A flat battery will have a voltage in each cell of 3V or less. So, in some ways the following table is just a different way of expressing the same thing, but is more precise. Percentage of charge is not to be confused with percentage of capacity. The capacity of a new 40kWh battery is to provide 40kW for a whole hour. If the capacity reduces to 75%, it will only be able to provide 40kW for 45 minutes, for example, or 30kW for an hour, as another example.

Where a lithium battery cell has a nominal voltage of 4.2V, it can be charged up to slightly more than 4.2V or slightly less than 4.2V. The difference between charging to only 3.9V and 4.2V can be as much as four times the number of cycles and longevity of the battery. That gain in longevity has to be balanced against the loss of some of the battery’s full capacity. This is one secret that manufacturers employ in their battery management systems. Because this reduces the effective capacity of the battery, the battery has to be much bigger, physically, to provide the same level of capacity. This is one reason why EV batteries are so big and heavy, with relatively low efficiency. Manufacturers could provide the same capacity, with a smaller battery, where all the cells are charged to the full voltage, but it would not last so long.

Voltage	Cycles	Capacity
4.25	200–350	105–110%
4.20	300–500	100%
4.15	400–700	90–95%
4.10	600–1,000	85–90%
4.05	850–1,500	80–85%
4.00	1,200–2,000	70–75%
3.90	2,400–4,000	60–65%
3.80	See note	35–40%
3.70	See note	30% and less

Controlled Rate of Discharge

We also need to look at discharging. Where a lithium cell has a nominal capacity of say 1500 mAh, that capacity could be provided by giving 1500 mA for an hour, 750 mA for two hours, or 375 mA for four hours. If we call 1500 mA in one hour “1C,” then 750 mA would be “0.5 C,” and 375 mA would be “0.25C.” Where the batteries are never discharged at a rate of more than 0.25C, they will last much longer than if they are drained at their full capacity.

This is a secret of EV battery packs, how the control system ensures discharge rates are never excessive. This is another reason for EV battery packs being so big and heavy, not just for the sake of range but for the sake of minimizing the rate of discharge, and so, further extending the battery life.

A Tale of Two Batteries

So, it is as if you have two batteries in your car: one is the physical battery, and the other is a virtual battery as presented to you by your instrumentation and as created by the battery management system.

Your physical battery, if it was all to be made available to you, would be much bigger and more powerful than it appears in reality.

The virtual battery, created for you to use by the battery management system and which you see through your dashboard displays, is smaller and less powerful but longer lasting. All these secret techniques that go on stealthily in the background, and probably unknown to you, are what constitutes the secret life of your electric vehicle battery.

Don’t Leave Your Battery in a Locked Car

That subheading normally relates to dogs, and is a cryptic clue to one further secret. Although nothing to do with charging and discharging or even running the electric vehicle at all, the following is also something to be aware of.

When a lithium battery is charged to 100% and then left stored, unused, but at a temperature above 25°C, then degradation will occur without using the battery at all. So, if you are living somewhere hot, where your garage reaches high temperatures, or even if high temperatures exist on your driveway, it is not such a good idea to leave your electric vehicle fully charged up for long periods of time unused. It might even be worth having a dedicated solar panel on the garage roof to power air conditioning in your garage during the heat of the day to keep your battery cool when not in use.

°C	Capacity after 1 year stored at 40% charge	Capacity after 1 year stored at 100% charge
0	98%	94%
25	96%	80%
40	85%	65%
60	75%	60% (after 3 months)

Limitations on Fast Charging

Despite people’s impatience about waiting to charge up, there are limits to safe charging currents, and good charging practices to follow if you do not want to damage your battery.

The power level of only 3kW can produce enough heat to warm up an entire room. So, as you can imagine, 50kW is a huge amount of power to put into any electrical system. Impatient or not, regardless of what people “want”, there are limits to the amount of power you can safely put into an EV battery. Ideally, the fastest charge rate for a 50kWh battery is 50kW over a period of 1 hour, because that exactly matches the characteristics of the battery. However, charging from 30% to 80% (a 50% charge), the rate could be 50kW over a period of half an hour. To bring charging time down to 15 minutes would require 100kW, which is double the ideal. The combination of false capacity percentages and lower charge voltages, plus over-sizing of the battery and carefully designed battery chemistry, all help to make faster charging possible, but there are limits.

People should not expect charging times to come down much below 20 mins, or ever be equivalent to filling a tank with fuel. Fifty liters of fuel represents 600kWh of energy. A 600kWh battery would weigh 6 tons today. A 50kWh battery weighs half a ton — do you really want to be carrying around more than half a ton on all the short journeys you take every day, just to save a few charging stops on the occasional long run? People are just going to have to learn to be more patient if humanity is to survive much longer.

Further Information

One thing I have not included in this article is any specific reference to any specific manufacturer or car.  I have emailed Nissan and Tesla about their battery management systems, but have received no reply at the time of publishing.

I did find the following statement about batteries on the Tesla site:

factors affecting cycle life are tied to how the cell is used. In particular:

1. Avoiding very high and very low states of charge. Voltages over 4.15V/cell (about 95 percent state of charge [SOC]) and voltages below 3.00V/cell (about 2 percent SOC) cause more stress on the insides of the cell (both physical and electrical). Avoiding very high charge rates. Charging faster than about C/2 (two hour charge) can reduce the cell’s life.
2. Avoiding charging at temperatures below 0° C. (Our design heats the pack before charging at cold temperatures.)
3. Avoiding very high discharge rates. (Our pack has been designed such that even at maximum discharge rate, the current required from each cell is not excessive.)

There is a huge difference in cycle life between a 4.2V/cell charge (defined by the manufacturers as “fully charged”) and a 4.15V/cell charge. 4.15 volts represents a charge of about 95 percent. For this reduction of initial capacity (5 percent), the batteries last a whole lot longer. Unfortunately, further reduction of charge has a much smaller benefit on cycle life. Understanding this trade-off, Tesla Motors has decided to limit the maximum charge of its cells to 4.15 volts, taking an initial 5 percent range hit to maximize lifetime of the pack. We also limit discharge of our battery pack to 3.0V/cell and will shut down the car when the batteries reach this level.

The information about batteries was obtained from BatteryUniversity.com, which is a very useful site for technical information on batteries of all kinds.