Did I say “Pull the Plug”? Meant to say “DO NOT pull the plug”

This could be a mea culpa post. It is rare that I’m wrong; it is even rarer that I admit it! So listen up folks.

In the early days of this blog, one of my dedicated (?) readers had asked me about the urban myth about not keeping the laptop plugged in to extend the life of the battery. In response, I had written a post titled “Pull the plug, your battery will thank you”. This post is the single most popular post on this blog. Almost a year after the post was made, it still gets the most hits.

The logic behind doing this is very sound. As you can read from that post, it has to do with side reactions that occur in the battery at the top of charge. Letting the battery discharge a bit is good for life because the rate of these side reactions decreases with decrease in the voltage. Suffice to say that I recommended you wait for the battery to charge and then you pull the plug and let it self discharge. This way you can extend its life.

I follow this rule pretty diligently. And I thought it had worked well for me. I have one laptop that is 2 years old, has had 297 cycles and has lost 4% of its initial capacity. Not bad. This is my workhorse. I use it every day and although I pull the plug diligently, my usage is such that I keep it pretty close to fully charged. So over the last 2 years, it has spent its time at close to, say, 4 V.

I have another laptop which is 3 years old. It is my personal laptop which we (my wife and I) use typically only over the weekend. We pull the plug diligently, but then the computer sleeps all week; self discharges; and by the end of the week is pretty much discharged. This battery, as of last week, had not lost any appreciable capacity even after 350 cycles.

These two data points tell you something about batteries. The cell with more cycles and with more time is cycling better! No magic. Just a simple fact that the battery was sitting at a lower state of charge and so the side reactions were not as worse. Ergo, better life.

Did I mention that both these are Macs? I have a third laptop given to me by a startup where I spend some of my time. This is a PC assembled by a company whose name starts with a D and ends with an L and has 4 letters to it. That computer is on its 4th battery in 2.5 years. After I lost my first battery I spent significant time trying to understand why my rules were not working and trying to tweak the rules. Soon, I came to the conclusion that with some batteries there really is no point trying to find ways to extend life. They are beyond help.

Actually, these rules have been helping this battery also. But different battery companies make batteries with different quality (achieving tightly-bound quality metrics has been a challenge in the manufacturing of batteries). So when you start with a battery with bad quality, there is only so much you can do.

But let us get back to my Mac.

Well... last weekend, my 3 year old Mac with no capacity fade suddenly appeared to have a dead battery. Not a battery with some loss in capacity; or one with 20% loss in capacity (which is considered dead). It was just plain dead. No charge; pull the plug and it would shutdown. It was on life support, literally!

The only way this battery would have a second life was if it were a Hindu and had not attained enlightenment and so was eligible (I suppose doomed is a better word) to be reborn. Somehow it seemed like even with the 1000 (or is it 10,000) Hindu gods there was no way to get this battery back up.

My first reaction was one of disbelief. There was no way a battery can go from no fade to completely dead in a matter of 1 week of self discharge. It had to be a software glitch that was not allowing the battery to be used.

Two hours, a bit of heartburn, and a detailed scouring of the world wide web later, I found various tricks to reset the battery management software and a software to measure the voltage of the battery and came to the conclusion that it was indeed dead. The (average) individual cell potential appeared to be close to 1.5 V! The typical cutoff potential of these cells is around 2.5 V.

So I pulled the plug and my battery died!!

A call to Apple confirmed that I was out of warranty and was told to go to the Apple store for a “detailed diagnostics”.

So I dragged myself to see the “genius” at the store (I’m not being condescending here; they really call the tech support guys genius’. Apparently if Einstein were alive today he would be working at the Palo Alto Apple store!).

Albert plugged a USB stick into my laptop; my screen turned into a series of numbers. Albert then turned and says that my battery is dead. Clearly he was on his way to writing a paper on the unified field theory.

I apologetically told him something like “I understand a bit about batteries and I don’t expect them to fail like this” and he said “I would not expect them to do that early on, but after 3 years I fully expect this”. Not “its possible”, but “fully expect”!!!

I debated giving up versus trying to argue on the finer points of battery chemistry but it seemed like a lost cause. I’ve been very reluctant using my celebrity status as the author of TWiB, and I have to say that it is intimidating arguing with a “genius”!

So I shelled out $130 for a new battery; thanked the guy for his help; and left.

I drove back re-examining my whole life and everything I know. I always thought there was some merit to the George Costanza (of Seinfeld fame) principle of doing the opposite of what our instincts tell us. Maybe I had it all wrong. Maybe you should not be pulling the plug. Maybe my jingle on battery rules needed to be rewritten.

A couple of days later my confidence started to return. I decided to do what anyone looking for credible information does: perform a google search to see if plugging in your laptop battery is bad. I came across my original blog post on this topic.  I sounded so convincing in the post that I started to get re-convinced that I was right about my rules.

So what is going on? How can a battery die when it is self discharging on sleep?

Here is my take.

All you PC folks are familiar with the hibernation mode that you can either force your computer to enter, or set it such that the power management utility moves the computer to hibernation after a while of being at sleep.

In sleep the computer stops many of the processes from running and thereby decreases the processing needs and hence drains the battery slowly. In hibernation, the computer (presumably) stops pretty much everything; stores the state in memory; and basically shuts down. This means there is very little drain on the battery.

In a Mac there is a sleep option, but there is no hibernation option. However, if you are in sleep and if your battery drains down to some small state of charge (say 5%), then it automatically moves into a “hibernation” mode; freezes the state and stops all the processes.

One thing we had noticed in the dead Mac (before it died) was that when we opened it over the weekend it was pretty much in hibernation with little juice left in the battery.

Ideally hibernation in a low state of charge is a good thing. Remember the rule “don't charge them too high”? Higher the voltage of the battery, worse will be its capacity fade. So storing it at a low state of charge (or low voltage) is actually good for the battery.

Did I mention that keeping the voltage way too low (i.e., over-discharging) is a bad thing?

This is because many cathode materials can get irreversibly damaged on over-discharge. More importantly, if an anode is over-discharged you can start dissolving the current collector (copper).

When you discharge the battery and it reaches its end of discharge voltage, depending on the battery chemistry (i.e., the anode and cathode that it uses) and on the design of the cell, the battery is limited by either the anode not having any lithium left, or the cathode not being able to accept any more lithium.

As you cycle this battery there are side reactions in both the electrodes. The extent of these side reactions depends on the design of the cell, the chemistry of the electrodes, the composition of the electrolyte, the level of impurities in the manufacturing, the way the battery is formed etc.

In other words, the side reactions are pretty complicated.

However, what we need to understand is that these side reactions can actually change the way the electrode reaches the end of discharge. They can even change which electrode limits the end of discharge.

So here is my take on what happened to the Mac battery.

The battery management system had a methodology of estimating the state of the battery and deciding if its needs to jump from sleep (the usual mode) to hibernation. This estimation was probably pretty accurate at the beginning of the life of the battery.

But years pass (3 in my case); the side reactions chug along; and they start changing the shape of the voltage curves, especially at the end of the discharge. Slowly, but surely, the management system was making errors in its estimation on the remaining charge.

The battery was not fading appreciably. Instead it was becoming harder to predict the time it would take to go from, say, 5% SOC to being fully-discharged.

I think that as my battery kept fading, the transition from sleep to hibernation was not getting triggered correctly, the battery over-discharged.

This is why when I checked the battery voltage it was sitting at 1.5 V.

I have a sneaking suspicion that my battery may actually come back to life if charged but that the power management software is not allowing any charge to reach the battery because the battery voltage is so low. I should have asked for my old,dead battery to try to resuscitate it myself!

If this sounds like an easy explanation considering how complicated all this is, its because it is the only plausible explanation I can come up with. If Steve Jobs would like to disagree, I’m listening.

So I still believe that if you “pull the plug, your battery will thank you”. I am glad I don’t have to go back and change 7 of my posts and apologize to my regular readers (all 7 of them!)

So what can one do about all this? Download the desktop hibernation widget at http://deepsleep.free.fr/ This gives you a way to move your Mac directly into hibernation instead of to sleep. This is probably a good thing anyway to conserve battery on long trips etc.

Or you could buy one of the new Macbook air computers which comes standard with hibernation.

In the meantime, my rule stands: Pull the plug, and your battery WILL thank you.

When Albert at the Apple store told me he “fully expected” my battery to behave this way, maybe, just maybe, he actually knew all this. After all, he is a “genius”.

Venkat

If your cat has nine lives, shouldn't your battery have at least two?

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