Fast charging

01Jul 2016

Sleep is an essential function of life. Tissue in living creatures regenerate during deep sleep. We, humans, get very cranky with sleep deprivation. And cranky we do get when our battery gets depleted because we did not give our mobile device sufficient “sleep time.”

I explained in a prior post the power needs in a smartphone, including the display, the radio functions…etc. If all these functions are constantly operating, the battery in a smartphone would last at most a couple of hours. So the key to having a smartphone battery last all day is having down time. So by now, you have hopefully noticed how the industry uses “sleep” terminology to describe these periods of time when the smartphone is nominally not active.

So what happens deep inside the mobile device during these periods of inactivity, often referred to as standby time? Sleep. That’s right. Not only sleep, but also deep sleep. This is the state of the electronic components, such as the processor and graphics chips, when they reduce their power demand. If we are not watching a video or the screen is actually turned off, there is no need for the graphics processor to be running. So the chip’s major functions are turned off, and the chip is put in a state of low power during which it draws very little from the battery. Bingo, sleep equals more battery life available to you when you need it.

Two key questions come to mind: When and how does the device go to sleep? and when and how does it wake up?

One primary function of the operating system (OS) is to decide when to go to sleep; this is the function of iOS for Apple devices, and Android OS for Android-based devices. The OS monitors the activity of the user, you, then makes some decisions. For example, if the OS detects that the smartphone has been lying on your desk for some considerable time and the screen has been off, then it will command the electronics to reduce their power demand and go to sleep.

This is similar to what happens in a car with a driver. You, the driver, gets to make decisions all the time when to turn the engine off, or put it in idle, or accelerate on the gas pedal. Each of these conditions changes the amount of fuel you draw from the fuel tank. In a smartphone, the OS is akin to the driver; the electronics replace the engine; and the fuel tank is like the battery. You get the picture. While this is colloquially referred to as managing the battery, in reality you are managing the “engine” and the power it consumes. This is why some drivers might get better mileage (mpg) than others. It is really about power management and has very little to do with true battery management.  Battery management is when one addresses the battery itself, for example how to charge it, how to maintain its health…etc. car-engine

The degree of sleep varies substantially and determines how much overall power is being used. Some electronic parts may be sleeping and others may be fully awake and active. For example, let’s say you are traveling and your device is set to airplane mode, but you are playing your favorite game. The OS will make sure that the radios chips, that’s the LTE radio, the WiFi, GPS chip, and all chips that have a wireless signal associated with them, go to deep sleep. But your processor and graphics chips will be running. With the radios off, your battery will last you the entire flight while playing Angry Birds.

The degree of sleep determines how much total power is being drawn from the battery, and hence, whether your standby time is a few hours or a lot more. A smart OS needs to awaken just the right number of electronic components for just the right amount of time. Anything more than that is a waste of battery, and loss of battery life. The battery is a precious resource and needs to be conserved when not needed.

Both iOS and Android have gotten much smarter over the past years in making these decisions. Earlier versions of Android were lacking the proper intelligence to optimize battery usage. Android Marshmallow introduced a new feature called Doze that adds more intelligence to this decision making process. Nextbit recently announced yet more intelligence to be layered on top of Android. This intelligence revolves around understanding the user behavior and accurately estimating what parts need to be sleeping, yet without impacting the overall responsiveness of the device.

The next question is who gets to wake up the chips that are sleeping? This is where things get tricky. In a car, you, the driver, gets to make decisions on how to run the engine. But imagine for a moment that the front passenger gets to also press the gas pedal. You can immediately see how this can be a recipe for chaos. In a smartphone, every app gets to access the electronics and arbitrarily wake up whatever was sleeping. An overzealous app developer might have his app pinging the GPS location chip constantly which will guarantee that this chip never goes to sleep — causing rapid battery loss of life. Early versions of Facebook and Twitter apps were guilty of constantly pinging the radio chips to refresh the social data in the background — even when you put your device down and thought it was inactive.  iOS and Android offer the user the ability to limit what these apps can do in the background; you can restrict their background refresh or limit their access to your GPS location. But many users do not take advantage of these power saving measures. If you haven’t done so, do yourself a favor and restrict background refresh on your device, and you will gain a few extra hours of battery life. You can find a few additional tidbits in this earlier post.

App designers have gotten somewhat more disciplined about power usage, but not entirely. Still too many apps are poorly written, or intentionally ignore the limited available power available. Just like in camp when many are sharing water, it takes one inconsiderate individual to ruin the experience. It takes one rogue app to ruin the battery experience in a smartphone. And when that happens, the user often blames the battery, not the rogue app. It’s like the campers blaming the water tank in camp instead of blaming the inconsiderate camper. Enforcement of power usage is improving with every iteration of operating systems, but the reality is that enforcement is not an easy task. There is no escaping the fact that the user experience is best improved by increasing the battery capacity (i.e., a bigger battery) and using faster charging. Managing a limited resource is essential but nothing makes the user happier than making that resource more abundant….and that, ladies and gentlemen, is what true battery management does. If power management is about making the engine more efficient, then battery management is about making the fuel tank bigger and better.

17May 2016

A young woman, Anna Crail, was flying on 19 March of this year on an Alaska Airlines flight from Seattle to Honolulu. About 90 minutes prior to landing, her iPhone 6 suddenly broke out in flames causing panic in mid flight. The fire was rapidly extinguished by the flight attendants, but not without leaving the airline and the FAA searching for answers. This is one of several safety-related battery incidents that are becoming increasingly common. There are countless reports on hoverboards that are catching fire. While safety-related incidents involving Apple iPhones appear to be sparse in the media, there are increasing reports of Android-type mobile devices posing serious safety hazards, especially in Asian geographies. This post provides first insights on the factors that impact the safety of the lithium-ion battery in mobile devices.

First, let’s start with some background material. Three key design factors heavily influence the lithium-ion battery in a smartphone: i) Higher charge capacity, i.e., more mAh; ii) Faster charging ; and lastly iii) thin profiles.

Higher capacity is driving an increase in energy density at the rate of about 5 to 7% per year. The energy density of lithium-ion batteries when the first iPhone was launched in 2007 was near 400 Wh/l. Today’s state-of-the-art mobile batteries are in the range of 600 to 700 Wh/l. These higher energy densities are associated with two important physical parameters: higher terminal voltage (near 4.4V, up from 4.2V ten years ago) and significantly higher current flux inside the batteries (a lot more ions are making the journey between the electrodes) elevating the risk of damage within the battery.

Fast charging is now an expected feature in mobile devices, at least those on the higher end of the spectrum.  A charge rate of 1C corresponds to a 50% charge in 30 minutes…1C or faster is becoming the norm. Faster charging also means a lot more ions are making the journey within the battery between the two opposite electrodes. Again, faster charging = higher risk of battery damage.

Lastly, thin profiles of smartphones are pushing the battery to ever thinner dimensions. The next generation of smartphones are employing batteries that are a mere 3 to 4 mm thick, with this figure being pushed down even further where possible. Thin batteries create a slew of headaches for engineers…their performance tends to be inconsistent; manufacturing non-uniformities are amplified; and the current flux (and corresponding ion density) within the battery is also pushed to higher levels.

So what do these really mean? they mean that the perfect safety storm is brewing if proper care, battery intelligence, and diagnostics are not implemented.  One of the first consequences of higher energy density (especially higher operating voltages near 4.4V) is the increased risk of formation of lithium metal on the carbon anode (the negative electrode in the battery during charging). This is called lithium plating. Combine high energy density with fast charging and thin batteries, and the risk of lithium plating becomes dangerously significant. But lithium metal (i.e, in its molecular form, not in its ionic form) is highly flammable especially in the presence of oxidants. Additionally, spurs of lithium metal can cause electrical shorts within the battery…both of these mechanisms have seriously hazardous consequences.

The next figure illustrates the safety risk and its relationship to charge rate and energy density. Right around 1C and 600 Wh/l, the battery may become a safety hazard, especially in the absence of proper and diligent designs. Some battery manufacturers are better than others, with batteries made by Chinese manufacturers being the most prone to increased safety risks. Some device manufacturers (OEM) choose to sacrifice some battery specifications to gain a little safety margin. For example, some OEMs reduce the operating voltage of the battery as it ages, for example, from 4.4V down to 4.35V or less. This means that you will be robbed of mAh without being told. Your smartphone may have a great battery (say for example, 3000 mAh) at the beginning of its life (and when it operated at 4.4V), but a few months in its operation, the maximum voltage is intentionally reduced to 4.35V thereby reducing the capacity by 150 – 200 mAh; in other words, your battery is now only about 2800 mAh. Ouch! That’s not good, especially when you are not aware of it.


The safety risk also depends on temperature, rising rapidly with lower temperatures…and by low temperatures, I really don’t mean sub-freezing temperatures. The vast majority of battery safety tests are conducted at room temperature, usually near 25 °C (77 °F). What is considered as “low temperature” for a battery is 10 °C (50 °F) or lower. Right around this temperature range, the probability of plating of lithium metal soars creating serious hazards.

For the time being, batteries catching fire have been mostly limited in frequency and consequences. But with rising energy density and charge rates, the safety hazard is slated to become a lot more serious in the near future. Look for smartphone OEMs that are investing in the proper solutions to give you an excellent battery experience AND a safe one too.

15Apr 2016

I discussed in a prior post the charging of the 5.5-in Samsung Galaxy S7 Edge. In this post, we will look at its sister device, the 5.1-in Samsung Galaxy S7, specifically the US version (model G930) using the Qualcomm Snapdragon 820 chipset, also known as the 8996. The battery specifications on the S7 include a polymer cell rated at 3,000 mAh, equivalent to 11.55 Wh. The teardown on iFixit shows a cell that is manufactured by ATL  rated to 4.4 V. Once again, the choice of battery manufacturer is surprising given that Samsung Electronics for years sourced the vast majority of their batteries from their sister company Samsung SDI.

Samsung S7 battery

I charged the Galaxy S7 using the Samsung-supplied AC adapter and USB cable, with the device in airplane mode and the screen turned off. The charging data is next.

S7-charge curve

Let’s make a few observations. The measured battery capacity is 2,940 mAh at a termination current of 300 mA (C/10). This is consistent with Samsung’s claim of 3,000 mAh, usually measured in the laboratory at a termination current of C/20 or 150 mA.

The device reaches 50% after 31 minutes of charging, corresponding to a charge rate of 1C, i.e., a charging current into the battery of 3 A. The supplied AC adapter is rated at 5 V/2 A and 9 V/1.67 A and uses Samsung’s own version of Qualcomm’s Quick Charge technology for handshaking between the AC adapter and the smartphone. The device displays that charging is complete (the fuel gauge reads 100%) after 82 minutes, however it continues to draw a charging current for an additional 20 minutes at which point the device terminates the charging after 102 minutes.

Just like the S7 Edge, the battery maximum charging voltage is only 4.35 V, not the rated 4.4 V. This means that the actual battery maximum capacity is nearly 3,180 mAh but Samsung is making only 3,000 mAh available to the user. This further raises the likelihood that Samsung opted to lower the voltage (and sacrifice available charge capacity) in order to increase the battery’s longevity (cycle life) or decrease the battery swelling at the high charge rate of 1C, or perhaps both.

All in all, this appears to be a well-designed battery providing ample capacity to the user to last a full day with sufficiently fast charging. What is unknown is the battery’s longevity (i.e., how many days and cycles of use) and whether it was compromised in the process. Given that Samsung’s track record in providing battery longevity is not exemplary, that will remain a very important question and left to be answered in a future post.

02Apr 2016

For a seemingly simple device with only two electrical connections to it, a battery is deceivingly misunderstood by the broad population, especially as batteries are now a common fixture in our technology-laden daily lives. I will highlight in this post five common misconceptions about the lithium-ion battery:


Well, not literally, but the acrobatic move captures the perceived hypersensitivity of the average consumer about past or secret special recipes that can help your battery. One of the silliest one I ever heard was to store the battery in the freezer to extend its life. PLEASE, DO NOT EVER DO THIS! Another silly is to charge the battery once it drops below 50%, or 40% or 30%….Let me be clear, you can use your phone down to zero and recharge it, and it will be just fine.

It is also now common to find apps that will “optimize” your battery. The reality is they do nada! Don’t bother.  Don’t also bother with task managers; no they don’t extend your battery life. Both Android and iOS are fairly sophisticated about managing apps in the background.

Turning off WiFi, GPS and Bluetooth will not extend your battery life, at least not meaningfully. These radios use such little power that turning them off will not give you any noticeable advantage. The fact is that your cellular radio signal (e.g., LTE) and your display (specifically when the screen is on) are the two primary consumers of battery life — and turning these off render your mobile device somewhat useless.

Lastly is the question of “should I charge the battery to 100%?” Well, yes! but you don’t have to if you don’t want to or can’t. In other words, stop thinking about it. The battery is fine whether you charge it to 100% or to 80% or anything else. Sure, for those of you who are battery geeks, yes, you will get more cycle life if the battery is not charged to 100%. But to the average population, you can do whatever you like — your usage is not wrong. These are design specifications that the device manufacturer is thinking about on your behalf.


Yes, as long as the memory of a 95-year old suffering from Alzheimers!! Sarcasm aside, lithium ion batteries have zero memory effects. Now, if you are a techie intent on confusing your smartphone or mobile device, here’s a little trick. Keep your device’s battery between 30% and 70% always….this will confuse the “fuel gauge,” that little battery monitor that tells you how much juice you have left. The battery will be just fine but the fuel gauge will not report accurately. Every so often, the fuel gauge needs to hit close to zero and 100% to know what these levels truly are, otherwise the fuel gauge will not accurately report the amount of battery percentage. This is like your gas gauge in your car going kaput…it does not mean that the battery has memory or other deficiencies. Should you suspect that your fuel gauge is confused, charge your phone to 100% and discharge it down to 10% a few times. That is sufficient to recalibrate the gauge.


This one garners a lot of media interest. Every time a research lab makes a new discovery, it is headline news and makes prime time TV. The reality is that the path from discovery in the lab to commercial deployment is extremely rocky. There have been dozens such discoveries in the past 5 – 10 years, yet virtually none have made it into wide commercial deployment. History tells us it takes over $1 billion and about 10 years for a new material to begin its slow commercial adoption cycle….and for now, the pipeline is rather thin. Additionally, present lithium ion batteries continue to improve. Granted, it is not very fast progress, but there is progress that is sufficient to make great products….just think that current battery technology is powering some great electric vehicles.

Let me be more specific. Present-day lithium ion batteries are achieving over 600 Wh/l in energy density — that is nearly 10x what lead acid batteries can deliver. This is enough to put 3,000 mAh in your smartphone (sufficient for a full day of use), and 60  kWh in your electrical car (enough for 200 – 250 miles of driving range). With the proper control systems and intelligence, a mobile device battery can last 2 years or more, and an electric vehicle battery can last 10 years. Does it mean we stop here? of course not, but this sense of urgency to develop new materials or chemistries is rather misplaced. Instead, we need to keep optimizing the present batteries materials and chemistries. Just reflect on how silicon as a semiconductor material was challenged by other candidate materials in the 1980s and 1990s (do you remember Gallium Arsenide), only for it to continue its steady progress and become an amazing material platform for modern computation and communication.


What made silicon the king of semiconductor materials is its amazing cost curve, i.e., decreasing cost per performance, aka Moore’s law. Now, lithium ion batteries don’t have an equivalent to Moore’s law, but, the cost of making lithium ion batteries is dropping fast to the point they are rapidly becoming commoditized. A battery for a smartphone costs the device OEM somewhere between $1.50 and $3.00, hardly a limiting factor for making great mobile devices. GM and Tesla Motors have widely advertised that their battery manufacturing costs are approaching $100 /kWh. In other words, a battery with sufficient capacity to drive 200 miles (i.e., 50 to 60 kWh) has a manufacturing cost of $5,000 to $6,000 (excluding the electronics)…with continued room for further cost reduction. It’s not yet ready to compete with inexpensive cars with gas engines, but it sure is very competitive with mid-range luxury vehicles. If you are in the market for a BMW 3-series or equivalent, I bet you are keeping an eye on the new Tesla Model 3. Tesla Motors pre-sold nearly 200,000 Model 3 electric vehicles in the 24 hours after its announcement.  This performance at a competitive price is what makes the present lithium ion batteries (with their present materials) attractive and dominant especially vis-a-vis potentially promising or threatening new chemistries or new materials.


Why do we not worry about the immense flammability of gasoline in our vehicles? Isn’t combustion the most essential mechanism of gas-driven cars? Yet, we feel very safe in these cars. Car fires are seldom headline news. That’s because the safety of traditional combustion engine cars has evolved immensely in the past decades. For example, gas tanks are insulated and protected in the event of a car crash.

Yes, lithium is flammable under certain but well known conditions. But the safety of lithium ion batteries can be observed as religiously as car makers observe the safety of combustion engines. It is quite likely that some isolated accidents or battery recalls may occur in the future as lithium ion batteries are deployed even wider than they are today. In mobile devices, the track record on safety has been very good, certainly since the battery industry had to manage the safety recalls at the turn of the century. Is there room for progress and can we achieve an exceptional safety record with lithium ion batteries? Absolutely yes. There are no inherent reasons why it cannot be achieved, albeit it will take time, just like the automotive and airline industries have continuously improved the safety of their products.

18Mar 2016

The new Galaxy S7 and its sister smartphone the S7 Edge are receiving rave reviews. So let’s take a look at their batteries. A brief comparison table shows the new S7 and S7 Edge side-by-side with the older S6 and the Note 4.


Let’s make a few key observations starting with the charge rate and charging time. The S7 and S7 Edge use a maximum measured charging current of 3A, which works out to charging rate of 0.8 C (3/3.6) for the S7 Edge and 1.0 C for the S7. The charge times to 80% are 58 and 50 minutes respectively. That’s faster than any iPhone, present or past, that typically charge at a rate of about 0.7 C. This earlier post explains the meaning of charge rate. We didn’t measure the battery capacity of the S7 but that of the S7 Edge came in at 3,521 mAh, fairly close to the advertised capacity of 3,600 mAh — that’s a nice hefty capacity that should last at least one full day.


Charge profile for the Galaxy S7 Edge (blue line is capacity and green line is charge voltage)

The S7 and S7 Edge are using batteries with a higher nominal maximum charge voltage of 4.4V compared to the older 4.35V standard. See the photo below. Surprisingly, teardowns of the S7 and S7 Edge (US models) show that these batteries come from ATL, not from Samsung’s battery division, Samsung SDI.

Battery vendors are now increasing the maximum charge voltage to 4.4V to raise the energy density of the battery by an additional 5%. However, both S7 and S7 Edge do not charge to this maximum voltage — instead, they charge the battery to only 4.35V. The green curve in the charging chart above shows the maximum voltage of 4.35V (not 4.4V) at the end of the charge cycle. This means that the battery true capacity is actually 5% higher…in other words, the S7 Edge has an intrinsic battery capacity that is near 3,700 mAh, but Samsung is instead choosing to operate it at 3,521 mAh.

Is it confusing enough? So why would Samsung sacrifice this additional capacity and not mention it in the capacity label on the battery? There are several reasons. First, it is quite likely that the swelling of this battery is severe at 4.4V that it limits Samsung’s quest to build thin smartphones. By reducing the voltage to 4.35V, the effects of swelling are somewhat mitigated. Second, the S7 and S7 Edge are both running at a fast charge rate. We observed in a previous post that the older Galaxy Note 4 suffered from a poor cycle life problem due to fast charging — the battery died within less than a year of operation. Reducing the voltage to 4.35V mitigates some of these cycle life problems, of course at the expense of a loss in capacity.

An astute reader might say: “Well, 3,600 mAh is plenty capacity for me!” That might be true, but you are paying for a bigger battery and using a physically larger battery than you need. It’s a compromise! The point here is to illustrate these increasingly difficult compromises that Samsung had to go through when it came to the battery.

I think for now, I might want to stick with Sony Xperia X’s battery — that’s a battery I know I can rely on.