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 1st 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.
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
