Sunday, February 28, 2010

Battery Rules.

I’ve been reading Michael Pollan’s new book titled “Food Rules” and thinking about his Haiku on food which goes: “Eat food. Not too much. Mostly plants.”

Succinct, but loaded with a lot of complexity. Inspired by this, I decided to try my hand at a (kind of) poem for battery health. But first, the way to extend the life of a battery depends on the battery type. Lets start with the battery we use everyday in our laptops and cell phone- the lithium-ion battery. The rules read,

Don’t charge them too high

Don’t swing them too wide

Keep the temperature low to extend their life.

Lithium ion batteries are not all equal. A battery from A123 will be different from one from Panasonic. So use these rules only for your cell phone and laptop batteries.

In the previous blog post, I told you why you don’t want to charge them too high in voltage (answer: side reactions). Higher the voltage, higher the fade. Turns out that if you swing the state of charge too much (i.e., charge and discharge the battery completely each time) the life decreases significantly.

This is because the battery materials expand and contract on charge/discharge (by as much as 10%). This constant “breathing” results in the particles cracking. As a matter of fact there there is data that shows that if you swing the battery to, say 3% (like in a HEV) you can get 300,000 cycles (yes, you read that right). But if you swing them all the way, you only get 300-1000 cycles. So you can charge and discharge them a lot, but you cant let them swing too wide.

Turns out that while a small swing in the state of charge is good for life, its wrecks havoc when trying to estimate the state of the battery. So as a good rule of thumb, every so often (say 2 months), discharge the batteries completely and then recharge them back up to make sure the software can reset the battery capacity and predict run-time better.

Finally temperature. Temperature is a boon if you want to make things faster (reaction rates increase with temperature). But remember those side reactions? The rate of these reactions also increases with temperature and they accelerate the capacity fade. Hence the recommendation to keep the temperature low.

As a matter of fact, I’ve seen recommendations that ask you not to keep your cell phone in your pants such that the battery is close to your skin (which would be, assuming you are normal, at 37 C). The 15 C higher temperature compared to the ambient (if you are in California) will kill the battery fast. Folks around the equator- sorry.

Turns out that if you go down to freezing temperatures, you get other problems with lithium batteries, but that for another blog post.

This is it. Three simple rules for a long lasting lithium-ion battery. Forget all the rest of the stuff that you hear about keeping it on the top of charge, get the juices flowing e.t.c.

Question is, where do these myths come from? Like I said before, the rules change when you change the battery chemistry. So rules from one battery chemistry get applied to another and what you have is confusion.

So here is a second (kind of) Haiku that tries to capture the different batteries we typically encounter or have encountered. This reads,

Keep your lead-acid’s charged

Let the Ni-Cad’s completely discharge

Lithium-ion and Ni-MH? Somewhere between

Lets get to the first rule. For a lead-acid car battery, the failure mechanism is called sulfation, where the discharged material undergoes a phase transformation after which it can’t recharge. Remember the time you left the glove compartment light on, the battery died, you got it jumped a couple of days later, and was told to buy a new battery? Yup, that was sulfation. Happens everytime the lead-acid battery discharges. Hence the rule that we should keep the lead-acid battery charged.

But, keeping a lead acid completely charged also leads to other problems (like grid corrosion) which would be lessened if you let the voltage decrease a bit. Moreover sulfation takes a few days. So one could do something complicated like let the battery charge, and then let it discharge a bit, but come back the next day and charge it back up before sulfation kicks in.

But let’s not make life very complicated, shall we. Just keep the battery charged, it will last 6 years, then get a new one. If you are an enthusiast, contact me and we can talk about an optimal charging scheme. And if you are one of those using the lead-acid battery for deep discharge-cycling then you are in real trouble. This rule is not going to help you. Some companies are now claiming to have solved this problem. Maybe they can help you.

On to the next rule.

Remember the mythical memory effect? Picture this (true) scenario: father calls his electrochemist son asking why the cordless phone battery was not holding charge; son admonishes father to get off his cheap lifestyle and buy a new battery already; father is convinced that son doesn't know what he’s talking about; father contacts second son who decided to use google and discoveries memory effect; second son asks father to discharge battery completely and try using it; father reports success and decides to disown electrochemist son. Amateur battery enthusiasts are the bane of my life (especially if they are family).

That was a Ni-Cad battery. if you don’t discharge these batteries completely and charged them back up, they seem to not remember that there was actually some capacity left. I have never experienced this myself, but there are many reports on this subject. Hence the oft repeated mantra- let the battery discharge completely before you recharge. Its specific to Ni-Cad batteries. See how different batteries are different?

Remember when the nickel-metal hydride (Ni-MH) battery came out and everyone said there was no memory effect? Turns out they jumped the gun. Ni-MH also appears to have a memory, albeit not as bad as Ni-Cd. There is some controversy as to why it actually happens and there is a thinking that overcharging these batteries causes memory. I used to work on this system and have overcharged these batteries quite a bit, and have not seen the memory effect, but... maybe I have a golden touch. Without getting into details, its best to pull the plug in these batteries when they are charged. Even if you don’t believe in memory, this also helps prevent drying up of the battery because of hydrogen evolution and venting.

As for discharge: completely charging and discharging a Ni-MH battery is not a good idea (the metal hydride particles also expand/contract and can crack if you do that), but then again, there is some data that suggests that there can be a memory-type effect if you don't discharge them completely.

So what should you do? I’m going to recommend that you don't worry about memory and try not to swing the state of charge too much. Every so often (say 2 months) discharge the batteries completely. May be a bit hard to do in practice, but one can try. Remember that the Toyota Prius has Ni-MH batteries and you never completely discharge them. They work fine for 10 years without any problems from the memory effect.

Hence the rule to keep the state somewhere between the lead-acid and the Ni-Cad for the Ni-MH battery. In many ways this is the same as the lithium-ion battery.

Lets hope this post does not end up adding to the myths that are already out there!

So here’s to Haiku-ing your way to better battery life.

Monday, February 22, 2010

Pull the plug. Your battery will thank you.

One question that I have been asked frequently is “do laptop batteries fade faster if they are continuously plugged in?” A reader asked this question in the blog and I thought it would be good to get into battery failure using this specific question.

Batteries failure depends on the chemistry and can be broadly classified as mechanical failure and chemical failure. When I say mechanical, think cracking, breaking, and shedding of the electrode. Chemical means reactions, like corrosion, that alter the state of the battery for the worse. In lithium batteries both kinds of failure can happen. For example, people have shown that the electrode particles can break, especially when you fast charge the battery.

But the question that was posed regarding failure when you plug-in the battery is specifically chemical in nature.

First some basics. For chemical stability, the battery should be operated within the stability window of the electrolyte. For water-based batteries, the stability window is 1.2V. Go above this window and you split water and make hydrogen gas and oxygen gas. This is about the time you should be wondering how lead acid batteries even work considering that their voltage is ~2 V, but that is outside of the scope of this post.

But getting back to our laptop, the stability window is ~3.2V. Meaning that when you operate the battery above this the electrolyte is oxidized on the positive electrode and reduced on the negative electrode. Remember that we only want to oxidize and reduce the “active” materials and don’t want to do anything else. All these reactions other than the ones we want are called “side reactions” and these are really bad for the battery. The nominal voltage of a laptop battery is 3.7 V which means that something bad wants to happen as we use the battery. Just because things
want to happen does not mean that they actually do (for example, I want to buy a Tesla or a Volt, but...).

So long story short, stuff (e.g., passive layers and poor kinetics of reactions) happens and things are not as bad as they seem and you can increase the voltage up to 4.2V without bad things really happening. All chargers for Li-ion cells today cut the battery off when it reaches 4.2V. What you have to realize is that at 4.2V, these side reactions are present in finite amounts and start to chemically kill the battery, but its not that dramatic.

Operating to 4.1V makes things better and extends the life, 4.0 V is even better and so on. So why don’t battery manufacturers cut the voltage off at, say, 4 V to get better battery life? Because every time you cut this voltage down you decrease the capacity of the battery and its run time. The 4.2V cutoff is a compromise between good run time and decent (read “not pathetic”) life.

Were you supposed to understand all that? Not really, I just wanted you to know that I’ve really thought about these things. What you
do need to know is that if you keep your laptop plugged in, you force your battery to remain at 4.2V continuously and these side reactions continue to happen and slowly kill the battery.

On the other hand, if you charge the battery and then pull the plug (so to speak), the battery discharges some, the voltage drops, and these reactions become less of a problem and your battery life goes up. So the best things you can do is to charge the laptop (or cell phone, camera etc.) and once its charged, pull the plug. Your battery will thank you for it.

As a matter of fact, if you own a Lenovo Thinkpad, you can actually change the state of charge to which you charge the battery using the “Battery Maintenance” utility. You can change this from charging to 100% state (where the voltage is 4.2V) to 90% so that your voltage is less. You lose some energy is doing that, but atleast you can change it to 100% when you need battery power and put it back down to 90% when you can plug in. I wish my Mac has the same feature.

This problem has implications for PHEVs and EVs. Lets say you have a 15 kWh PHEV pack. You come home after a 40 mile commute and you plug it in at 6:00 PM. Let’s say you have a 120V, 15A outlet, so that you can put out 1.8 kW of power. So the battery is going to charge in 8 hours.

By 2 AM you have a fully charged battery. If you leave your house at 8:00 AM, your battery is going to be sitting at 4.2V for 6 hours in any 24 hour period. This is not going to be good for the battery. It gets worse if you decide to bump the amp/volts on your house to charge it faster. So we need to get these batteries charged faster, but we also want to make sure to have smart chargers that don’t do what I’ve described above. Something to think about.

What does this mean for researchers? If someone can find an electrolyte that has a wide voltage window of stability, then this problem goes away. Or you can try to use materials that work within this window (For example
A123 Systems battery does this on the positive side). But this means the battery has a lower voltage, which means it has lower energy and less run time. We don't want that, do we? Finally, we can try to isolate the electrode and the electrolyte and see if we can kinetically hinder these reactions. In the Battery Program at Berkeley we are actively working on this problem so that we can get more energy and better life.

In the mean time, remember to pull the plug.


Monday, February 15, 2010

A Moore’s law for batteries? Maybe not.

Its 6:00 p.m. on a Monday evening and the traffic seems horrendous. You decide to whip out your iPhone and log on to Google Maps to check out alternate routes when you see the dreaded warning that your battery is out of ‘juice.’ You curse the guys who work on batteries for not doing their jobs better and wonder if batteries have actually improved in your lifetime. Sound familiar?

The problem of what many consider to be the poor improvement in battery performance is the bane of all professional battery folks—especially in the Bay Area, where (sadly) people know Moore's “law” better than they know Faraday's law (no prizes for guessing which one is a real physical law).  So where are the innovative solutions that are the battery equivalents of iPods, iPhones, and [maybe] iPads?

First of, Moore’s law is an observation on the ability to pack more transistors into the same space at optimal cost. In devices where packing more means more performance, this translates to continuous improvement. While there is quite a bit of “packing things into a smaller footprint” that goes on in the battery space, fundamentally, this is not how batteries improve. Let me elaborate.

If you take your cell phone lithium-ion battery, it has a lithium cobalt oxide positive electrode and a graphite negative electrode. The theoretical energy density of this battery (i.e., the best you can hope to achieve) is ~360 watt-hours per kilogram (Wh/kg). The battery you buy is probably ~180 Wh/kg; only one half of the theoretical max.

Where does the other half go? It goes toward making an electrode with the lithium cobalt oxide and the graphite; in putting a current collector in the cell; in the electrolyte that is added to get the reactions to occur; and, in the packaging of these components in a container. Only the lithium cobalt oxide and graphite are “active”—the rest is wasted space and weight.

Can we remove this unwanted weight and make things better? Sure, but it’s not easy. Battery companies have actually done quite a bit of this already. Since this chemistry was first commercialized in the early 90’s, the energy density of lithium-ion batteries has doubled. But when you start by being at a factor of four from the ultimate, it’s hard to make dramatic advances. In contrast, for integrated circuits, technology has moved from the 350 nanometer node in 1995 to the 32 nanometer node this year. Dramatic changes in the same time period, but then again, the initial size was far away from the ultimate limit (my cursory reading of the Web suggests that 7-9 nm may be the limit with silicon).

So how do we make a better battery if we cannot depend on packing them tighter? We change the positive and negative electrodes to something that has more energy. This is already happening; many devices are now sold with a different positive electrode, with theoretical energy of ~450 Wh/kg for these cells. And there are some amazing things going on at Berkeley via the Batteries for Advanced Transportation Technologies (BATT) program that will surely make things better in the very near future.

This fundamentally is the problem in the comparison of batteries to semiconductor: in the case of transistors, by using better lithography tools, the industry has made dramatic advances on one material, namely, silicon. Sooner or later, they are going to start butting up against a material problem. I’m told that this is already happening. When this occurs, the industry is going to start moving toward other materials (such as III-V semiconductors, graphene, carbon nanotubes and nanowires).

This will not be an easy change and would have required many years of research. In batteries, changes in materials are pretty much the only way to improve energy; engineering is getting harder and harder to do. To expect that a new material is going to show up every two years is a bit unrealistic considering that in the battery space we are optimizing not just on getting better energy density, but on achieving long life, excellent safety, reasonable power, and low cost.

And there is this tiny little problem that if we actually do succeed in, say, doubling the energy density every two years, we may be violating a few laws (as we know them today) in about eight years! Electrochemists have not yet recovered from the trauma of cold fusion; let’s wait for a bit before we trigger another one of those episodes, shall we? The ultimate energy that one can expect from a battery is an interesting concept. I will address this in the very near future.

So what can we do to make things better? Others have observed that, unlike the semiconductor industry, there is little collaboration among the various battery industries and the same sort of ecosystem does not exist for the adoption of new technology. The Department of Energy national labs could be the key to establishing a new ecosystem where companies can participate on a pre-competitive basis.

In the meantime, every time your battery fails, stop blaming your friendly neighborhood electrochemist, and think back to a few years ago when you could not get on the Web with your mobile device. Now keep repeating to yourself that that was a better time, and before you know it, you will be home.


Wednesday, February 10, 2010

Should I get ready to buy a plug-in hybrid?

I work in Berkeley, CA, the tree-hugging, vegan-eating, Toyota Prius-driving, environmental sensitivity capital of the world. I live in Fremont, CA. This means I must drive 35 miles each way to work. Yes, you read that correctly—I drive 35 miles each way.

Every time I hear about a plug-in electric hybrid vehicle (PHEV) capable of providing 40 miles of driving on its battery before the combustion engine kicks in, I salivate at the prospect of not having to apologize to all the classic “Berkeley-types” for my gas-guzzling lifestyle. So, am I going to be first in line buying a Chevy Volt? I don’t think so and here’s why: it will be expensive!

Other thing you learn when you are around Berkeley a lot is that nearly everyone has seen the documentary “Who killed the electric car?” and believes that Detroit can easily make cheap PHEVs, but they don’t want to because of [insert favorite conspiracy here]. I shall steer clear from any such controversy and stick to some numbers for costs of batteries, and leave you to come to your own conclusions. (I shall conveniently sidestep the fact that battery costs themselves are controversial!)

Most people agree that for transportation applications, Lithium-ion batteries are the best option (more on this in a later post). Laptop Li-ion batteries cost ~$200 per kilowatt-hour (kWh), the most common measure of battery performance. This is for a single battery cell. We have to take a bunch of these cells to make a vehicle battery pack. Estimates suggest that the pack costs will be ~$450/kWh. Remember, these are laptop batteries, and if your laptop is anything like mine, this battery will last two years or less! But, let’s be optimistic and say that a PHEV battery will last four years.

To drive 40 miles on the battery pack, you need 16 kWh (give or take). This means you have to buy a battery pack for the car costing about $7,200 (16 times 450). And you still need to buy the engine, brakes, steering, seats…you get the point. And every four years, at least, you will be visiting the dealership, waiting to change the battery and shelling out another $7,200. I wonder how many Berkeley-types will be willing, or able, to do this.

Now, the batteries that General Motors will use will be much better than laptop batteries and will last a lot longer. But this also means that they will cost more. Now we start to delve into the unknown, but experts say that batteries capable of meeting the long-life requirement cost ~$1,000/kWh today. Given that, the same 40-mile battery will cost a whopping $16,000! Add on the cost of the car (~$25,000) and you get a ~$40,000 PHEV. News reports suggest that, indeed, GM is planning of a $40,000 launch for the Volt (note that government subsidies will get this cost down a bit, atleast for the early adopters). There is no way I can afford a $40,000 car with my LBL job (I will try this argument on my performance appraisal and see if I can get a pay hike). So no Volt for me.

But here is the optimistic part. Experts also think that with mass manufacturing, it will be possible to reduce the costs of these long-life batteries to be on par with laptop batteries. Which means that we can expect to see the costs coming down to, say $8,000 per PHEV pack in the future. This would lower the cost of the car to about $33,000—still expensive, but closer to what I would be willing to pay. And remember that these cars will cost less to run (electricity is cheaper than gas) and, hopefully, cheaper to maintain (less wear and tear).

But to really have a big impact, especially in getting widespread penetration in countries with large emerging economies such as India, these costs have to come down significantly, maybe by as much as a factor of two or more. And if we want to ever get to a pure electric vehicle, we will need a lot more energy (55 kWh instead of 16 kWh) and these costs have to come way down for these cars to be widely affordable. Fundamental research into battery energy, safety, and lifetimes are the only way we are going to achieve this. More on this in future posts.

What does this all mean? There is a PHEV in my future. What I don’t know is how far in the future! In the mean time, I will continue to come up with new excuses for why I need to drive, and not take pubic transportation or move. If you have any smart suggestions, let me know.

Tuesday, February 9, 2010

Welcome to TWIB

Welcome to "This Week in Batteries" (TWIB for short). Electrochemical energy storage, or batteries, if you prefer, seems to be all over the news lately, and for good reason. Without getting into any controversy about global warming, the Intergovernmental Panel on Climate Change, or the wisdom of debating complex science by email, I think we can all agree that humans would be better off if we cut back the amount of CO2 that we put into Earth’s atmosphere, and that reducing our dependence on oil is one big step toward that. (I suppose you may disagree if you work for an oil company!)

Improved batteries are a fundamental element of enabling this, whether for transportation or for power grid storage.

But with interest comes hype, and misinformation. And there is a lot of it when it comes to batteries! My purpose in starting this blog is to be an (arguably) independent expert evaluating the various news items that come across my virtual, desk and provide some objective analysis to help you separate the truth from the hype.

The name TWIB is a take on TWIT, which stands for "This Week in Tech", a podcast by Leo Laporte (see TWIT is one of the top-rated tech podcasts out there and has spawned a whole lot of "This Week in.." podcasts, including "This Week in Law", " Google" etc. A combination of an iPhone and TWIT is guaranteed to make sure you are not bored for the hour you spend on the treadmill everyday, but that's for another blog.

Leo appears to be running a grand franchise with sponsors, etc. I do not have any sponsors (but would love some. Call me), and have rent to pay. So I have a day job as a Staff Scientist at the Lawrence Berkeley National Laboratory, and this blog's upkeep will be subject to me hanging on to that day job. The "This Week" portion of TWIB may be a tad optimistic, but, hey, one can hope. I promise that I will be online everytime there is a newsworthy event.

I will start the blog with a few generic postings to set the stage for what is possible with batteries. We will start by talking about limits of energy density of batteries and ask why the development of batteries has not proceeded as well as in, say, semiconductors (short answer: moving electrons is one thing, reacting them is a whole other story). I'll also say something about the problems in getting PHEVs/EVs on the road (in a word: cost) and ask if nanotechnology is the answer to all our problems (not really, but it has its uses). I will also deviate a bit from my comfort zone, go crazy, and say a few things about making money in the battery business (Anyone thinking about making money in the battery business is obviously crazy!)

So let’s hope you have fun reading this blog (and let’s hope I have fun writing it).