The Secret Life of EV Batteries

Previously published on CleanTechnica.com

You are probably familiar with the lithium batteries in your smart phone, 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 everyday, 365 days in a year, a 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.

Of course, you and I know that an EV battery will last at least eight years, possibly 10 years, or more, depending on how it is treated. I have had my electric vehicle for three 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, and yet this seems to conflict with our own experience, of smart-phone batteries, and the like, which we know do not last very long.

The Puzzle

I did not like that puzzle, because it made me realise 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, and so could not explain much about the longevity of any existing EV battery. I had also read, that when Tesla chose cells for the battery pack in the original Tesla roadster, they chose a cell, the “18650” cell, that was already very commonly in use, and so, very cheap to buy, readily available, and reliable, but 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 life cycle 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 five times as long. There is only one way to solve such problems, and less like Alice, by hitting the bottle, and more like Hermione Granger, by a visit to the library. Well, not the library as such, but the modern-day version, with a delve 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 100%, and discharging to close to nil%. Lithium batteries should never be entirely discharged as this drastically shortens their life. There are a number of possibilities, such as charging from 100% down to 50%, from 85% down to 25%, or from 50% down to 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%	2010
100% – 40%	2800
100% – 50%	2800
85% – 25%	4500
75% – 25%	7100
75% – 45%	10000
75% – 65%	12000

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, to prevent damage to your battery. Your electric vehicle will behave as if the battery were completely flat, when it is not, and might tell you it is down to zero, 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, and arrived on my drive, under full battery power. 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. So, 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 dash board, or your touch screen. What I am saying is that there is not just a battery, and 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% to zero, with a range of, say, 100 miles, (160kM), then, as the battery lost capacity, the system charged it up to, say, 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% to 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 how ever 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 ageing 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, but I have read about this idea in general as if it were a common practice.

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, which 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 this following chart 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, or 30kW for an hour.
Where a lithium battery cell has a nominal voltage of 4.2 V, it can be charged up to slightly more than 4.2 V, or slightly less than 4.2 V. The difference between charging to only 3.9 V and 4.2 V 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. They 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, 0.2 5C. Where the batteries are never discharged at a rate of more than 0.2 5C, they will last much longer than if they are drained at their full capacity. This is a secret of EV battery packs, where 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 minimising 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, were it all to be made available to you, would be much bigger and more powerful than it appears. The virtual battery, created for you to use by the battery management system, that you see through your dashboard displays, is smaller, 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 subtitle normally relates to dogs, and is a cryptic clue to one further secret, which although nothing to do with charging and discharging or even running the electric vehicle at all, is something to be aware of. Where 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 high temperatures exist on your drive, it is not such a good idea to leave your electric vehicle fully charged up for long periods, 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, if you do not want to damage your battery. A power level of only 3kW can produce enough heat to warm up an entire room. A power of 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%, where 80 less 30 is 50%, representing only half the battery, the rate could be 50kW over a period of half an hour. To bring charging time down to 15 mins, 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 helps 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 litres of fuel represents 600kWh of energy. A 600kWh battery would weigh 6 tons. That makes a 50kWh battery weigh half a ton, so do you really want to be carrying around more than half a ton, on all short journeys 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 e-mailed 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.