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

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.