Let’s see if you can answer this pop quiz: How much capacity fade can you tolerate in your battery before you consider it “dead”?
The answer (as usual) depends on the application.
If you have a laptop and it is anything like mine, then “dead” is when you get 10 mins of run time and you decide to bite the bullet and get a new battery. With some laptops (the one that comes to mind is made by a company whose name starts with a D and ends with a L and has four letters) getting one year from your battery is considered very good.
But laptop batteries are $50. And most customers are probably going to go back and get their next laptop based on price anyway, so there is no long-term effect of giving someone a bad battery. And if your laptop battery dies you can use it plugged in (i.e., as a desktop).
But if you had an EV which run’s 200 miles and if in, say, 3 years you start to get 160 miles, you can be confident that you will sitting in the dealership demanding that they change the battery for you. At $10-$30k a pop you are not about to pay for another battery.
And running your car with a long cord connected to an outlet can be a bit problematic.
But, as definitions go, most manufacturers will consider a battery to be at the end-of-life if it has lost 20-25% of its capacity. In some applications (e.g., HEVs) 20% loss of power is considered end-of-life. The less expensive the battery, the higher the loss that is tolerated and vice versa.
What this means is that when your EV battery is considered dead, it still has 80% of its capacity left.
Despite the fact that you are reading this blog, I’m going to go out on a limb here and assume that you are a smart person. Right about now, you must be thinking to yourself that it is stupid to call something dead when there is 80% of it left.
Do you get new brakes or tires when they have depleted by 20%?
No! You wait till your tires are bald and you skid into your auto repair shop and stop the car by crashing into something soft.
By now you are, hopefully, having this epiphany that there may be a hidden business idea here. One that involves taking these “dead” EV/PHEV batteries and using it someplace else.
Maybe ship it to some other country where they may not be as picky about the number of miles you can get?
Or maybe if you can find an application where energy density were not important, you can repurpose these batteries to sell it for these applications.
While you chew on that, let me change subjects.
Did you know that as states like California decide to enact renewable standards, utilities companies are going to have a problem? California promises that by 2020, 33% of the state’s electricity will come from renewable sources. So let’s say that to satisfy the mandate, the utilities decide to build a giant solar farm in the middle-of-nowhere (Tracy?). But even in the middle-of-nowhere the sun does not shine at night.
So enter storage as the savior. You charge the battery during the day and discharge it to power the cities at night. Transmission allowed us to space-shift the electricity (generate in point A, use it at Point B); storage allows us to time-shift. Its the DVR (or TiVo) of the grid.
And how much electricity are we talking about storing? California uses electricity in the range of giga watt hours (GWh). So a lot!
Some of you may have heard of AB2514. Its a legislation in the state of California that requires utilities to incorporate storage in the grid. The utilities will need to store 2.25% of the daytime peak power by 2014 and 5% by 2020. This is probably the first explicit mandate calling for storage to be a part of the grid.
For all you veteran battery folks who thought the community did not know how to lobby, take heart. We may be nowhere close to what the hydrogen guys have done, but it is a start!
But let’s back to the storage issue.
So storage is the DVR of the grid. And because your DVR is not moving, it can be big (and ugly if it is like my DVR).
But DVRs cost extra money and they die every few years. Batteries are a lot more like your DVR than you can imagine!
Meaning, the energy density of the battery is not that important for this application. The great thing about middle-of-nowhere is that there is a lot of it out there. What is important, however, is the cost.
Hence the interest in taking “used” vehicle batteries and using it in grid applications. A second life, if you will.
Think of the possibilities: If your battery lasts say 15 years instead of 7 years, then you just doubled the time to amortize the upfront cost. You can ask the utilities to buy the car batteries and lease it to the consumer. Once the battery is “dead”, the utility moves it to grid applications and starts its second life. The consumer does not have to worry about paying $15,000 extra for the battery or worry about it dying on them. Someone else owns it; all I need to worry about is making sure I don’t get into a crash and ruin my warranty.
There are other themes on this business plan, but you get the idea.
All this is sounding so great that I’m contemplating sending an email to my bosses telling them what I really think about my job.
But wait a second. This whole blog survives only because we hate our energy storage devices. How many examples can we think of for our batteries lasted more than a few years? My car battery, which died after 7 years, has been the only battery that has done me proud. And I was lucky to get anything close to that!
We all spend an inordinate amount of time babying our batteries and asking how we can extend the life, and here I’m claiming that we can get a second life from our batteries. There must be a reason why my post “pull the plug, your battery will thank you” has the highest hit among all the posts.
What exactly are we missing?
Turns out that we are not missing anything. The second life concept is being pushed by business-types. For a biz. dev. guy this makes perfect sense. But the real question is: Does this make sense for the tech guy in the back actually doing the testing?
Let’s look at this in detail.
If you plot the capacity of a battery versus cycle number you will see that different batteries fade differently. Some batteries fade rapidly in the 1st few cycles and then the capacity stabilizes. Others increase in capacity in the 1st few cycles then level off and then starts a linear fade.
And in some batteries, as you keep cycling them there is a point where the curve starts to “roll over”. In other words, what began as a linear fade starts to accelerate. When this occurs, you are a few cycles away from a complete dead battery (i.e., you can’t even power your watch with it!).
Lets talk a bit more technical for a minute. Those not interested, move a few paragraphs over.
Let's take a Li-ion batteries with a NCM cathode with 14% 1st cycle irreversible loss and a graphite anode with 8% 1st cycle loss. After formation, the battery discharge is dictated by the voltage of the cathode when the battery reaching the cutoff voltage. There is still lithium in the anode. Think of this as an anode-discharge buffer.
But then let us assume that the cathode works very well. The anode, as anodes tend to do, still has a small side reaction because the SEI (which is a passive layer on the anode expected to protect it) is not quite as protecting as we hoped for.
As each cycle proceeds, the lithium comes out of the cathode and a small part goes into making a new SEI and does not intercalate into the anode. As you discharge, you start to slowly deplete this anode buffer. Each cycle slowly pushes the anode potential higher and higher. At some point, all the buffer is fully depleted.
Before this buffer was depleted, if you looked at the graph of capacity vs cycle number you would not have seen any capacity fade. When you hit this point, you will start to see the fade and what you observe is like a roll over. The slope of the capacity fade curve changed.
In some cases, this change starts to accelerate the fade because of the placement of the cutoff potential in the battery.
Ergo... battery fade is nonlinear and just because you have a particular kind of fade in the first five hundred cycles does not mean the fade can be predicted over the next five hundred cycles.
All right, so you think the battery you are making is different. It has a cathode that has less irreversible loss compared to the cathode. So you don’t have the same chemical problem.
But maybe you have a mechanical degradation problem.
All batteries “breath” as they charge and discharge. As each cycle proceeds, you are slowly swinging the volume of the battery. The whole electrode is under stress (compressive in one direction and tensile on the other). Fatigue starts to set in. After repeated cycling, at some point cracking and breaking start to become a problem. Now we have a definite capacity fade.
As some of the particles break and stop participating in the reaction, the rest of the particles have to take the load and these particles are stressed further and this accelerates their fade.
And these effects depend on the kind of cycling that has been conducted. A 3 hour discharge (like that in a EV) will have a different fade characteristic compared to a 1/2 hour discharge (like in a 10 mile PHEV).
Let us try a third case. Let’s say you have a battery pack consisting of 100’s of cells (or 8000 cells if you are at Tesla). Now, its kind of hard to make all the batteries exactly the same. When you manufacture batteries the yield is not that great and companies typically wait three to four weeks before binning (sorting) the batteries. But batteries are not binned for consistent life, because there is no way to do that. Instead they are binned for self-discharge, which does not tell us much about the life.
So let’s say that as the pack is cycled, you start having a few cells fading at a faster rate. Then the cells that are good are going to work that much harder and so they will start to fade more. This will also result in an acceleration of the fade. And because its not possible to predict a priori at what rate each cells will be fading, its impossible to predict how long the whole pack will last.
If that sounded complicated it was a desperate attempt to hide my insecurities by trying to convince you that I know something about batteries.
The short summary: Battery fade is complicated and difficult to predict.
Now, not all batteries are going to do this. Some battery chemistries are better than others. And some companies (that can make consistent cells) will be better than others. But the question remains: How do we predict what is going to happen in 15 years, when the weather report seems so far off for the next day?
But why predict. Why not just cycle these batteries; wait for them to die; and use this data to find out how to cost the battery and amortize it; and then sit back and watch the moolah pile up.
Because it takes.... 15 years to get this information and we don’t have another 15 years for this business plan to come into effect. To be fair, we have been testing these batteries for a few years now and have some data. But this is nowhere close to the number of years of data needed and so we still can’t say for sure how many more cycles/years the batteries will last.
So why not do accelerated testing, you ask? Because one is not sure if the method used to accelerate the fade (e.g. increasing the temperature) results in the different fade mechanism becoming dominant. Chemistry has this nasty habit of being hard!
So all the biz dev types have a problem: If you can’t predict how many cycles/years the battery will last how do you price the batteries today?
And the tech guy in the back of the room doing all the testing is sweating because he/she knows the complexity and knows that it is hard to predict how the battery is going to behave so far in the future.
Personally I think the concept is great and in time we will know how to make this a profitable business plan.
But for now, I’m not going to send that email to my bosses. I’m thinking about buying a house and I need the paycheck!
Venkat
