Tips on extending battery life; from a battery guru.

Probably best to get this out of the way right upfront: The battery guru referred to is yours faithfully.  Sorry to disappoint.  

Regular readers of this blog (all seven of you) know my rather famous (!) blog post titled “Battery rules”, where, inspired by Michael Pollan, I penned a poem that went,

Don’t charge them too high

Don’t swing them too wide

Keep the temperature low to extend their life.

This post was a follow on to my other tremendously popular (read:  eight total views) post titled “Pull the plug.  Your battery will thank you.”  The posts were meant to provide an understanding on why Li-ion batteries fail and how one can use this understanding to extend the life of batteries.  

Just to be clear: These rules are for Li-ion batteries; for phones, laptops, cars etc.  Not for other kinds of batteries.  And not for every Li-ion battery known either; but the most popular ones.  

The reason’s for these rules are simple.  Li-ion batteries don’t like to be at the top of charge (because of side reactions that consume lithium).  They don’t like being charged and discharged completely (because of the volume change associated with moving a lot of lithium back and forth and the associated stresses).  And higher the temperature, more the side reactions that impact battery life. 

When the blog post was written, the whole fast charging of batteries was not a big deal.  But now, we are seeing more and more emphasis on this.  Fast charging can also be bad.  More on this at the end of the post. 

The simple statement “Pull the plug” was really a way to implement the 1^st rule “Don’t charge them too high” without much thought.  In other words, don’t use your laptop like a desktop and keep it plugged in.  All. The. Time.  This only makes the side reactions worse.  Stop charging.  Let the battery discharge a bit. 

Every time I walk into a meeting and see the inevitable dive under the table to find the charging plug, followed by attaching the charging cable to the Mac laptop, only to see that the charging light is green, my blood pressure increases.  I then go thru the sermon explaining why they need to pull the plug. 

They comply.  For 15 mins. After which they are back to plugging it in.  Blood pressure increases.  Cycle repeats.

And No.  No real breakthroughs have occurred since the post that makes the rules obsolete. 

Clearly, the rules are not working.  I’m sure it is not the messenger (?).  I’m sure it is not the message.  So it must be the way the message is delivered. 

So this post is battery rules redux.  I will explain how I charge and use my battery.  Hopefully, this gives folks a sense for how they can maximize their battery. 

My track record:  I have a computer that lost 4% capacity in 2 years.  I have a phone that lost 5% capacity in 2 years.

In this post, I will focus on the phone and how I baby the battery.

First some basics:  On any typical day I only discharge my battery around 30-40%. There may be an occasional day when I will discharge the battery by 70%, but those are rare.  In other words, I could use my battery for two days without needing to recharge it. 

If my usage seems rather minimal, it is not (although I never got into the pokemon revolution).  Rather it is because of two attributes that many of you probably share. 

First, I have an iPhone 6 plus (the YUGE one that does not fit in any normal size pocket).  This means that the phone has a pretty large battery.  And although the screen size is larger, the bigger battery more than compensates for the power draw of the screen.

Second, the operating system on the phone, iOS, is very battery friendly.  I moved to the 6 plus from an Android phone (with, I believe, the KitKat version).  That phone had to be charged after an eleven hour day, despite having a reasonably large battery.  It would drain 5% of the battery capacity every hour, even when the phone was, supposedly, idle.  iOS appears to lead to less battery drain, especially during rest.

Based on my usage profile and the attributes of my phone, here is what I do:

- I charge my phone in the evening close to when I’m about to go to bed.  My battery is around 40% state of charge (SOC).  I charge it to around 80-85% SOC.  Takes me ½ hour or so.  I don’t agonize over the exact SOC, I pull the plug when I remember to. 

- I sleep and when I wake, the battery has lost maybe 3% of so of its capacity (No. I don’t sleep for 1 h.  I sleep the normal 7 h).  I don’t plug it in. Rather, I go about my day, make calls, check email, listen to podcasts, etc.   I use the phone as much as the next person.  When I get back home I’m at approximately 50% SOC and a couple of hours later, when I’m ready for bed, at 40%.  I repeat the charging cycle.

In effect, I don’t let the battery charge too high.  I don’t let it swing too wide.  In other words, I follow my rules.  I don’t leave it charging all night (i.e., where the battery is fully charged a couple of hours after we hit the bed and is sitting at the top of charge for the next 5-6 hours).  I don’t discharge it all the way down each time before charging it.

- When I feel like my day may be particular long with a lot of phone use, which for me typically means I’m on travel, I charge the phone upto 100% SOC, typically in the morning before the day starts.  Meaning, it is fully charged, but is not sitting at the top of charge for hours on end. 

- Every few months, when I’m not on travel and have a predictable week, I fully charge the battery and drain it down all the way before recharging it.  This should allow the phone to calibrate the SOC by performing a discharge capacity check. 

This may sound counter intuitive:  I have a battery that should last 2 days.  But, I choose to charge it every day.  In effect I’m racking in cycles, twice as fast as I need to.  Is that not bad? 

I would posit that it is not.  The number of times the battery is cycled has to be put in context of the SOC that the battery swings, and how much time it spends at places where side reactions can dominate.  We know that batteries can cycle 300,000 times (yes.  Three hundred thousand!) when the SOC swing is kept rather small (say 3%, like what occurs in a hybrid electric vehicle).  In these situations, the calendar life of the battery (i.e., how many years it lasts) is more critical than the number of cycles.

As we widen the SOC swing, the battery life decreases.  The worst is fully charging and discharging the battery.  Even worse is going to the top of charge where we have side reactions.  My scheme minimizes the time spent at the top of charge, without overly swinging the battery. 

And it keeps me on a reasonably manageable daily schedule for charging without having to think too deeply about it.

Frankly, it would be better for me to charge the battery upto say 50-60% SOC and let us drain down to 10-20% each day.  But, that, to me, is living dangerously.  Having that extra 20%-30%  of buffer capacity gives me peace of mind (What if I get lured into Pokemon Go on my way home?).

Irrespective of what phone you have, and how you use it, the two rules (don’t charge them too high and don’t swing the too wide) will help you.

As far as the rule to keep the temperature low, I don’t keep my phone on my car dash where it is hot. My phone happens to be sitting somewhere in front of the air vents, which helps during summer.  I don’t keep my phone on hot metal tables in summer. 

I have been known to go so far as to contemplate living in frigid conditions just to avoid the high temperatures in some parts of the world.  Talk about taking ones job seriously… 

Now for the whole fast charge issue. 

In my blog post titled “The Hero with four faces: Part 1” I explored the question of fast charge and the issues surrounding it.

Short summary:  Lithium can plate (which makes the battery go boom).   If you swing the SOC too much and too fast, you can break the particles (although this may not be as big a deal as folks think it is).  And the battery tends to get hot when charged fast , violating the rule to keep temperatures low. 

Turns out that one does not want to go too low in temperature either.  We know that lithium plating is much more of a problem at low temperatures; i.e., Golidlocks effect. 

Of these, lithium plating is the one to avoid at all cost; the rest only degrade the battery and are not dangerous (unless the battery really starts to cook).  All it does it kill the battery capacity rapidly, which, for a device that one changes every 2 years, is not really a big consideration.

Having said all this, the phone maker meters in the charge from the charger.  Meaning, just because you get yourself a beefier charger does not mean that your phone is going to accept the charge.  The phone maker is supposed to watch out for the battery. 

But there is a push to use fast charge as a differentiator in consumer electronic devices.  This means is that unless the company is performing some sort of smart charging to avoid the problems above, the battery is probably not going to do well. 

So, unless you hear it from this blog, best to avoid fast charging your phone. Assuming your phone maker gives you such an option. 

In the meantime, assuming poems are not your thing, can I remind you all that that friends don’t let friends keep their batteries plugged in.  So, be a nice buddy, and ask your neighbor to pull the plug. 

Venkat

The Hero with four faces: Part 1

A few years ago, when I visited a battery company with a few colleagues, I saw data from the company that was pretty impressive.  Slide 1 showed the rate capability of the battery, which was better than the state-of-the-art; Slide 2 was a calculation of the energy density, which was better than the baseline; slide 3 was cycle life and this too looked impressive; slide 4 was…

You get the drill. 

Suitably impressed, I came out wondering what I needed to polish in my resume to land a job at the company before they went public… when my travelling companion wondered aloud if each slide was from a different battery!

 My colleague had a point.  Most (not all) battery chemistries can be made to perform well for a particular metric.  The trick is getting all the metrics to work out for the same battery. 

Make the electrodes thin to get power; make them thick to increase energy (but at the loss of power).  Increase the voltage to increase the energy, but at the loss of cycle life.  The endless games one can play.  This dependence of battery design, cycling conditions, voltage of operation, etc. on performance is the reason battery companies have been getting away with obfuscation. 

But is it really obfuscation?  A proof-of-concept can be a useful learning tool.  If going to high voltages yields more energy but kills the cycle life, while we know that the voltages being accessed should be possible, then it gives us hope that the problem is not fundamental and that a solution exists.  Given time, maybe we will find it. 

Then again, we have been looking for a cure for baldness for ages.  There is proof that baldness is not fundamental (after all, some are oh so lucky), but that does not mean we will find the answer to that problem either.    

But there is merit in learning about what the best possible system is and understanding why it is the best.  To this end, I thought I would list out the best-known (at least to me) battery chemistry for each of four metrics of importance:  energy, charge time, life, and cost.  Each would be a “Hero” battery; a term borrowed from other technology areas to denote a proof-on-concept that something amazing is possible for that metric.  While each metric has a different “Hero” battery there are probably lessons we can learn from them.  These lessons, I summarize in the end.

The energy-density Hero:  All seven readers of my blog know that Li-ion batteries are the highest energy density secondary batteries one can buy, with energy density in the range of 250 Wh/kg.  But the Hero is actually the Li-thionyl chloride battery, which has an energy density of 550 Wh/kg and is, more importantly, commercially available.  The catch is that it is a primary battery (i.e., not rechargeable). 

What gives?

Typical Li-ion cells have a graphite anode.  Moving from graphite to Li metal (which holds ten times the charge and results in a slightly higher voltage) bumps the energy by 50% without changing anything.  On top of that, the thionly chloride cathode has a capacity of 450 mAh/g compared to 180 mAh/g for typical Li-ion cathodes.  Combine the two and you get something much better than Li-ion.

Imagine a battery with 2x the energy density.  Maybe we double the range of a Nissan Leaf, making it a car that is actually useful (joking, joking, all you Leaf lovers) for the same cost.  Or cut the cost of the Tesla battery by half without sacrificing range, bringing it tantalizing closer to the point where everyone can continue to not be able to afford it. 

Unfortunately, all these will not come to pass because the Li anode does not recharge gracefully while the thionly chloride cathode is not at all rechargeable.  If you want to know why, you have to read my blog post titled “A Brief History of Batteries- Part 1” and “A Brief History of Batteries- Part 2”.  Frankly, everything you need to know about batteries is probably hidden somewhere in these pages. 

To make it rechargeable, you just need to change the anode, the cathode, and the electrolyte (i.e., all the three components that make a battery).  When you do that, the battery now cycles a lot better, but at the loss of capacity. 

So is 550 Wh/kg the ceiling?  No.  There is a lab demonstration of a 750 Wh/kg Li-air primary battery.  This would be a record for energy density.  But as I have alluded to in the past, a one-off lab demonstration does not a product make.  So, for now, we shall call 550 Wh/kg the Hero for energy density.  As an aside, the Li-air battery also does not cycle. 

Reversibility restricts us to certain materials; a constraint when removed, allows high energy density batteries to be made. 

An interesting question to ponder: Is it possible to get batteries that are rechargeable and at least as high in energy as these Hero’s?  

The fast-charge Hero:  One of my funnier blog post was titled  “I’ll be back…in 8 hours”.  That post had nothing to do with charging times (then again, this blog is like Seinfeld; its about nothing, as far as I can tell) but the title articulates the problem:  We tend to want to charge our batteries very slowly.

Charge them too fast and we have unwanted side reactions (lithium plating and electrolyte breakdown) that can degrade the battery.  1h charge is doable, 30 mins makes it degrade a bit; 10 mins a lot more; 1 min would basically kill the battery (and kill you from the fire that is creates). 

This is true for most batteries, but it is not fundamental.  Meaning, there is no law that says that we cannot charge a battery fast.  One can design the battery for fast charge.  But the compromise is loss of energy. 

How fast do we really need to charge?  For an electric car, a really smart person (who shall not be named) told me that we needed to charge within the time frame of a restroom break.  Another smart person told me that we needed the battery to charge and discharge fully within the time frame of clouds covering a solar panel.  After a few experiments timing myself on restroom breaks and watching clouds move (I plan to watch grass grow next) we can approximate the charging time needs as 5 minutes.  What can I say: I was having an Austin-Powers-just-got-out-of-hibernation moment. 

The question of the grid actually handling this kind of electricity load is a whole other area of debate, but let us focus on the battery for a second.

Electrochemical capacitors can easily charge this fast.  But they have no energy.  Question is: Is there a battery chemistry that can mimic a capacitor’s charge rate?

There was one system that was kind-of-sort of commercialized that I would consider the Hero in this regard.  This was the Toshiba Super Charge battery, which is rated to charge to 80% capacity in 6 minutes and more than 95% in 10 mins!

Here the anode (lithium titanate) operates at a higher voltage than the typically used graphite anode. This helps because the potential of the anode is far away from the lithium plating potential.  This makes it much easier to charge fast and not worry about plating lithium, shorting, and the ensuing degradation and possible fires.   

But the downside is that the higher anode voltage decreases the overall cell voltage, which in-turn deceases the energy.  The battery has a third of the energy density of a typical Li-ion cell (so a third the driving range).  At 90 Wh/kg and 177 Wh/l at the cell level, it is far, far lower than most Li-ion batteries.  Cost of these devices scales with the energy:  this battery will probably cost three times a typical Li-ion cell!

So… an ideal EV battery for the super-rich-with-overactive-bladder demographic?  

Question is: can we get both high energy and fast charge?

If we want to get there I believe we cannot use thick porous electrodes.   While they are great to spread the current, it seems impossible to get away from the electrolyte losses of these highly-resistive organic electrolytes.  Which means that we cannot have thick bulky current collectors and separators.  But then how do we collect the current (especially if the currents are high, which seems likely with fast charge batteries)?

Should we move to electrodes that are not porous?  This limits the useable energy, unless we use electrodes that undergo deposition. One can continue to deposit the metal on top of itself and so “build” capacity, without the added losses from the porous structure.  This may be an avenue. 

My suspicion is that even if we find ways for the electrodes to accept the charge at that rate, the electrolytes in lithium-based batteries will not have the ability to move ions from one side to the other.  Unless we move toward much thinner separators.   This topic requires some careful thinking.

But let us revisit the question: how fast do we really need to charge? 

If we can ensure we have a 400-mile range battery, this should translate to an approximately 5-6 hour driving time (at 70-80 mph).  Then we may be willing to wait a half hour to charge the battery as we make a beeline to our favorite artery-clogging fast food joint. 

For the grid, if we can use the battery to not just take care of the intermittency, but also to time-shift from peak to off-peak, we can size the battery for the time shift and use the (big) battery for handling the small 5-min intermittency.  Each 5-min charge and discharge would only require the battery to swing by a few percent: easily possible with most systems. 

The catch: both of these would require us to pay for the bigger battery!  But hey, batteries are getting so cheap, and companies are going to give us money when they hand us the battery anyway. 

The cycle/calendar-life Hero:  I know what you are thinking: there is no such thing as a cycle-life Hero!  After all, every battery we own seems to last all of 1-2 years before they crap out.   You must be thinking that our Hero must be a 3-year life battery.

What if I told you there are batteries that last 20, even 30+ years and they cycle 20,000 times (no error there, really meant to have four zero’s)?  And what if I told you that these batteries are not the hybrid car batteries that cycle 3-5% per cycle but are cycled deep, greater than 50% of the capacity per cycle?  And that they are (at least they were) being used day in and day out? 

Imagine batteries that last as long as a solar panel.  Imagine being able to cycle them once a day and make them last those full 20 years with no maintenance.

Intrigued?  Come back next week and you shall learn more.

 Venkat

p.s. the title of this post is inspired by the Joseph Campbell book “The hero with a thousand faces.”  Campbell was trying to point out that all the world’s myths, across religions, shared a Hero figure with similar characteristics.  We will see next week that in batteries there is no one Hero.  We basically have four Hero’s with four faces.