Fast charging

19Jul 2016

This post includes contributions from Robert Nalesnik. I discussed in the past how fast charging requires two components: i) power delivery – that means getting extra electrical power from the wall socket to the battery and ii) battery management – that means making sure you don’t destroy the battery’s lifespan with all the extra power.

How much more power do you need? Quite a bit more if you want to charge considerably faster. It’s like your car engine: if you want to go faster then you will consume more gas. For a typical smartphone, power levels go up from the conventional 5 Watts to 15 or even close to 20 Watts in some cases.

Delivering higher levels of power is a very active area. Qualcomm has Quick Charge, Mediatek has Pump Express, and there is the USB Power Delivery standard with support from Intel, and the Chinese manufacturer Oppo has VOOC. Not surprisingly, with so many parties trying to influence or even define the standards of power delivery, there is plenty of confusion to go around.

First, let’s refresh some basic high-school science:     Electrical power = current x voltage.

So if we want to deliver more power, we can either increase the current, the voltage or both. Increasing current is relatively easy but more current means a lot more heat…that is until something begins to melt. Not good! That usually puts an upper limit somewhere between 3 and 5 A on the charging current.

The other approach is to increase voltage, from the conventional 5 V up to 9 V, or even 12 V, and in some limited cases even more.

High current charging

High-current charging leverages the fact that modern single-cell lithium ion batteries can be charged using an inexpensive 5 V AC adapter that can be manufactured for about one dollar.

Increasing the charging current is limited by i) the maximum current rating of the USB cable between the AC adapter and the mobile device, as well as of the tiny connector in your device where the USB cable plugs in; ii) cost and iii) heat and safety.

Let’s do some math. A typical USB cable assembly can support a maximum current of 1.8 A. So 5 x 1.8 = 9 Watts max. That’s fine for standard charging but not sufficient for fast charging a smartphone. The new USB type-C cables (with symmetrical connectors that can be used in any orientation you like) can support up to 3 A, in other words, a maximum of 15 Watts. Much better! Under some very limited cases and using special cables, one can push USB type-C to 5 A, or 25 Watts. But at 5 A cost begins to skyrocket, so instead, we see designs gravitating towards 3 A, or equivalently 15 Watts.

To put this in perspective, 15 Watts can charge your typical 3,000 mAh battery at a rate of 1 C, meaning you will get 50% of your battery charge in 30 minutes, and a full charge in  over an hour.

A quick word on heat: if you remember Ohm’s law from your high-school physics, heat increases as the square of the current. That means as the charging current increases from 1.8 A to 3 A, or 1.66X, heat inside your device will increase as 1.66 x 1.66 = 2.8X. Ouch! That’s a lot of heat to remove from the device….and a great topic for a future post.


High voltage charging

Let’s pause for a moment and think about the high-voltage transmission lines that we frequently see from highways outside of urban areas. Electric utility companies transport electrical power from power-generating stations (e.g., dams) that can be hundreds of miles from a city. If they use the 120 V that you get at your outlet, then the overhead transmission lines will have to carry millions of amperes…this is not only physically impossible, but also economically just prohibitive. So the transmission lines run at a much higher voltage, anywhere up to 800,000 volts. These transmission lines naturally don’t come straight to your house. Instead, they terminate into smaller substations (hidden off main roads near your neighborhood) where the voltage is then gradually “stepped down.”

That’s the same concept used in mobile devices. The voltage from the AC adapter is now raised above 5 V. But what voltage should it be? 9 V, 12 V? more? This is decided by a “handshake” protocol between a specialized chip (usually the power management IC, also known as PMIC) inside your smartphone and the AC adapter when the USB cable is plugged in. This is the approach taken with Qualcomm’s Quick Charge and the USB Power Delivery standard, each using a different signaling mechanism. There is a saying among power supply engineers that “voltage is cheaper than current”, and indeed lower cost components and cables are a primary benefit of high voltage charging.

The USB Power Delivery (USB PD) standard allows voltages of 5, 9, 15 and 20 V and currents up to 3 A. This gives power levels of 15, 27, 45 and 60 W, respectively. Additionally, currents up to 5 A are allowed at 20V, enabling up to 100W. Qualcomm has similar predefined power levels at 5, 9, 12, and 20 V. High-voltage charging has a clear advantage of attaining power levels above 25 W, which makes it the preferred choice for laptops, ultrabooks and 2 in 1s tablets.


How will this abundance of approaches settle out in the market? From a historical perspective, Qualcomm was early to see an opportunity to define high voltage charging in a simpler and cheaper way than the USB committee. They launched Quick Charge 2.0 in 2013 and followed up with the latest 3.0 version in 2015. Qualcomm has been quite successful establishing Quick Charge as a defacto charging standard for smartphones. More recently, Intel and others are successfully driving USB PD and the Type-C connector into PC markets and Type-C is well on its way to become the standard connector across all classes of mobile devices.

In smartphones, the next few years will likely still see multiple power delivery approaches, with chipset and adapter vendors evolving to multi-standard support to bridge compatibility gaps – meaning a smartphone can support multiple protocols such Qualcomm, USB PD, Pump Express…etc. From a Qnovo perspective, we are agnostic and complementary to whatever power delivery approach our customers choose. The higher power makes greater the need for the second component of fast charging: battery management.

12Jul 2016

yes, you guessed right: they both spell BAD news for your smartphone battery.

Pokemon Go, the app that catapulted over this past weekend to the #1 download spot for both iOS and Android apps simultaneously runs the cellular radio (3G, 4G, LTE) along with the GPS locator and the screen. No wonder users are reporting dead batteries after a couple of hours, and several are carrying power bricks as they roam the streets to recharge their dying batteries.

What about Twitter? They announced today a live streaming agreement with Bloomberg with a few others in the making. The battery-friendly 140-character message cedes its place to the power-hungry, battery-challenging video stream.

These lead me to this post and what the battery of the future ought to look like. I don’t mean in 2017. I mean in 2020, or even beyond. The answer lies in the forecast for mobile trends in the coming year. For that, we turn to the annual Ericsson Mobility Report, an excellent resource on mobile traffic and usage.

I can sum up the report’s findings into two important points:

  1. A lot more data consumption, and
  2. A lot, I mean a lot, more video on your smartphone,

Ericsson forecasts that monthly data traffic on your smartphone will grow from about 1.5 GB in 2015 to nearly 9 GB in 2021, or a whopping 35% annual growth. More data usage means more radio time and that does not help the battery.


Of that 9GB of data usage, video and social networking will make up about 65% or 6 GB per month. Teens are a driving force having doubled their TV/video weekly viewing hours on their smartphone in just 4 years; from just 3 hours a week in 2011 to 6 hours a week in 2015. If you have young adults in your household, you will know what I am talking about. Video streaming requires the screen, graphics processor and radios to be all on…not enough sleep time for this device to extend the battery life.

All of that boils down to a better battery. Better means more charge capacity (and more energy density). Better means a lot faster charging. And better means a lifespan for a battery that will not fail you after a few months of use.

So is that possible? Yes. But not without some clever designs.

For that discussion, let’s look at how smartphone batteries have evolved over the past few years. I have chosen here the Samsung Galaxy S smartphones for further examination. Since the introduction of the Galaxy S3 in 2012, we have seen the battery increase in capacity from 2,100 mAh to 3,000 mAh in the most recent Galaxy S7. That works out to an average increase of 18% per year. This is great but surely not enough to match the growing data needs in the next years and satiate their power hunger. Samsung and several other Android OEMs have been aggressive in pushing fast charging (Apple, where art thou?) The 2016 Galaxy S7 charges nearly twice as fast as its earlier sibling in 2013.  Can you charge faster, pleaaaaase!



Now comes the killer challenge. With subsidies gone and having to fork out $600 or $700 for a S7 (or an equivalent iPhone or high-end smartphone), you, the user, are now keeping your device longer and opting to wait before upgrading. That spells trouble to the device maker. Here’s why.

You see, they are finding themselves in a position where they need to increase capacity, AND charge rate (faster charging), AND now, consumers are demanding more lifespan, or cycle life (the technical term for having your battery live for at least 2 or even 3 years). This is not even possible without clever battery management solutions, such as our adaptive charging software. Such solutions can extract a lot more performance from the battery, and enable more capacity, faster charging and more lifespan. The batteries in the Sony Xperia line are a living proof of what is possible.

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.