Monday, October 12, 2015

How many watch batteries does it take to drive a car?

Apparently the Apple watch has a battery in it. What the what! And here I thought that crown thingy was for winding the little sucker.  That was one embarrassing scene at the Apple store when I tried to pull the crown (on the gold one, no less) to wind it.  Skeuomorphism much.   

Jokes aside, when iFixit opened up the watch, I was fully expecting it to be 90% battery.  Surprisingly it is not.   It only occupies 50% of the space (all us battery types are the glass-half-full kind).  Obviously a smaller battery would help make this into a little less of a hulk on the wrist, but clearly Apple has made a compromise: balancing bulk, functionality, and battery life. 

We have come to expect nothing but less from our battery (!).  But why can’t the electronics guys figure out how to shrink the taptic engine, the chips, etc.  We need to get them to pull their weight. 

More than the actual volume of the cell (or the capacity) the energy density tells a more interesting story. The apple watch has an energy density of 450 Wh/l, while typical cell phone energy density is approaching 650 Wh/l.  A whooping 40% more: imagine how many more twitter messages you can get before you have to recharge the watch if they had incorporated the better battery. 

So what gives?

In any battery, the packaging takes up significant space and adds unwanted weight.  You need tabs to pull the current out; you need a pouch made of plastic with embedded metal to keep the battery in (and you so want the battery to stay in, especially when you know what goes into them!) and the moisture, oxygen etc. out.  The pouch has to be sealed and so you have a seal line that adds volume, and so on and so forth. 

As the size gets smaller, it is harder and harder to keep the ratio of the packaging to that of the actual active ingredients in the battery the same.  Meaning, there is a critical size below which one cannot get a proper seal, there is a thickness of the pouch below which one cannot guarantee that they will be impervious.  All this leads to a bigger and bigger fraction going toward the packaging, as the battery gets smaller.  Ergo, lower energy density. 

This is kind-of sort-of the problem I was talking about in my post titled “In batteries, 2+2=1. Actually more like 1/2. Well... maybe a bit less.”  In that post I was referring to the problem of needing extra weight and volume for the current collectors, separators etc., the penalty this imposes, and the difficulty of dramatically improving the energy density of batteries using new materials.  But the same issue comes to play when you go to smaller devices with the packaging being the driver. 

Now imagine going down in size even further, to, say, a contact-lens glucose-sensor that Google (or is it Alphabet now?) is developing.  How does one integrate a typical battery into that?  (make an eye patch and go for the pirate look?)

Which is why Google is going for some sort of wireless power delivery. 

As we start moving toward the world of ubiquitous sensing, the internet of things, and the “quantified self” (and other buzz words that I may have forgotten) where we will measure, and broadcast, how many times we cough in a day, what germs we eject when we do, how many of our neighbors are infected by those germs, and when and where the CDC should come pick us up when the germs reach epidemic proportions, the problem of smaller batteries with more energy density is only going to get more serious. 

So how can we change this? 

Clearly we need to think about other form-factors and chemistries that don’t have the same strong dependence of size on energy density.  Thin film and solid state are buzzwords you probably have heard.  These could be applicable.  One can imagine thinner packaging materials, encapsulated cells versus cells with thick seal lines, stacked cells versus jellyrolls etc.  There are few ideas like that out there that look interesting.  But they are still in early stages and a lot of data is needed for them to be proven out.  This area is ripe for innovation and we are going to see some cool things in the next 2-5 years. 

The problem is that what we see is typically an one-off demonstration that show that one can make a small device, but there is no way to know if they can be mass-manufactured at a scale where they will have an impact and at a cost that someone will be willing to pay for it. 

Solid-state batteries have been around for decades.  But they have never been cost effective to manufacture using the expensive deposition techniques that only make sense as a demonstration of what is possible.  The capex would be massive and the yield unproven.  Not a good sign for a mass-market product like a watch. 

Which brings me to the next interesting aspect about the Apple Watch: the cost. 

No. Not the cost of the gold one (although, the one on display at the Stoneridge Mall with the slightly deformed crown may be available on the cheap). 

We can all agree that when it comes to Apple, cost and price are two very different things.  There have been teardowns of the Apple Watch that suggest that it costs $84 to make.  Others disagree, and argue that for new technologies the cost will be higher.  I don’t have a view one way or another. 

The cost of the battery in the watch, according to these teardowns, is 80 cents.  Yes, you read that right, 80 cents!  (Except for the gold watch where the whole battery is made of gold?  Incidentally, turns out that gold can be used as an expensive, and pretty bad, anode material.)

The energy of the 38 mm watch is 0.78 Wh.  At a cost of $0.80 a pop this comes to a $1000/kWh battery.  Kind of expensive compared to most cell phone batteries, but inline with what one would expect for these small devices made for a niche low-volume application. 

And oh boy!  Are those volumes low!  Rumor has it that Apple sold 3 million watches in 3 months.  Even assuming all of these are the large 42 mm with a 0.94 Wh battery, this is a total of 2800 kWh.  Which would be 33 Tesla Model S, 85kWh cars.   

Sold.  Over.  Three.  Months.   

For comparison Tesla sells 2000 cars a month.  If you work for a battery supplier, which one would you rather sell to?

Turns out, probably to Apple.   

Wait, what?

Ok.  I would much rather have the volumes of Tesla.  Or sell to the MWh grid scale installations that are popping up everywhere (I use the word “everywhere” rather generously).  But when you are selling something that big (read, expensive) margins are going to be pretty thin.  At $350/kWh for an 85 kWh battery, the Tesla battery probably costs $30k.  How much more can a battery company charge? 

But at $.80 per watch, one can afford to ask for a bit more money for something that has, say, much higher energy density.  What is an extra dollar or two between friends when you are shelling out $500 on a timepiece? (or, if you are like me, $10k).

This is the advantage of going after a market where the total cost is small and the margins high.  It provides a path of first entry to the market.   But the scale dwarfs in comparison to the EV or the grid market. 

So can one make an amazing new battery for the watch that has, say, three times the energy even if it costs thrice as much and then slowly make the size larger and larger till one day it drives our cars?


After all, the Li-ion batteries powering every electrified car and most of the recently announced grid installations all started as small consumer electronic cells.  Companies learnt how to make them more reliable, get tighter tolerances, and make them safer so that one day, after 15 years, they were able to move them to vehicles. 

Took a long time, but it did happen.  So there is hope that some new idea would first get implemented in small devices, but would cost more.  In time, the cost will come down and the larger markets, which are more price-sensitive, would become accessible.  Maybe…

Or… one can borrow a page from Tesla, string a bunch of watch batteries together and make an electric car battery with it.  Instead of eight thousand 18650 cells, all one needs to do is take 10 times that number.

Feel free to run with that idea.  I won’t even ask for credit. 


P.S.  My ardent followers (all seven of them) have, for a long time, expressed their overwhelming desire (!) for me to get on Twitter.  Well, ask and you shall receive.  Join me on Twitter on Thursday, October 22nd, at 10 a.m. PT when I will talk about the future of batteries for EVs & grid storage.  I think this would be described as a Twitter chat.  The hashtag is #BattChat 

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