Fast charging

20Oct 2014

Samsung recently announced their new Note 4 with a claim that it charges from zero to 50% in thirty-ish minutes. What is thirty-ish? Is it 31 or 39?  Motorola also announced that their newest Moto X will provide 6 hours of mixed use after only 15 minutes of charge. Mixed use, as Motorola defines it, includes a mix of actual use time as well as standby for an average user. Hmmm….this is not very specific! Let’s see if we can decipher some of these claims in this post and shed more light on what these handsets are accomplishing.

Let’s establish a few fundamentals that relate charge current, and consequently electrical power, with charge time. I will try to keep it simple enough for many readers to understand. 

First, we need to establish the C-rate. It should be fairly intuitive that more electrical current and power from the AC wall adapter into the battery should mean faster charging. To read the rating of your AC wall adapter, look at the fine print on the adapter itself and it will tell you the wattage (e.g., 5 Watts) and the output current (e.g., 1 A at 5 V). In this particular case, the charging current is about 1 A. This charging current should be measured relative to the capacity of the battery to establish charge time. For example, a charging current of 1 A will charge a 1,000-mAh battery far faster than it will charge a 3,000-mAh battery. So we define a new relative measure called the C-rate: It is the charging current divided by the capacity of the battery. So if the charging current is 1 A and the battery capacity is 1 Ah (1,000 mAh), then the C-rate is 1 A/1 Ah = 1 C. If the charging current is 1 A but the battery capacity is 3 Ah (3,000 mAh), the the C-rate is 1 / 3 = 0.33 C. The higher the C-rate, the faster the charging. So let’s take an example of a real-life smartphone: the iPhone 5S. Its AC wall adapter has an output rating of 1 A at 5 V (which you can read between the prongs of the little adapter cube). Its battery is rated at 1.55 Ah (1,550 mAh). You can’t see this unless you open your phone back cover; alternatively, you can google the capacity of the phone. So, the C-rate of an iPhone 5S is 1/1.55 = 0.65 C. So far, this should be fairly straightforward.

Second, let’s convert C-rate into charge times. This conversion table makes it quite easy.

Approximate charge times for a lithium-ion battery for particular charging C-rates.

So one can see that the higher the C-rate, the faster the charging. The numbers may vary a little from battery to battery, but these figures give you a fairly reasonable estimate.

Now, let’s go back to the Note 4 and see if we can decipher its claims. Samsung claims zero to 50% in  thirty-ish minutes. From the table, we see that 30 minutes would be equivalent to 1 C, but 39 minutes would be closer to 0.75 C. Given that the published capacity of the Note 4 is 3,200 mAh, the range of charging current would be 2.4 A up to 3.2 A. The corresponding AC wall adapter wattage rating would be in the range of 12 W up to 16 W. I have not seen yet an AC adapter for the Note 4, but if you have one, you can confirm the wattage and the charging current, and consequently the charge time.

For the Moto X, the claim is more tricky. The Moto X has a published capacity of 2,300 mAh. Motorola’s website claims that this capacity should last the average user up to 24 hours of mixed used. Therefore, if a 15-minute charge gives the user 6 hours of mixed used, one can estimate that this quick charge provides 6 / 24 = 25% battery capacity, or consequently, it will take 30 minutes to get to 50%. Based on the table above, this corresponds to 1 C or a charging current of 2.3 A, and a wall adapter rated at or near 12 W.

19Oct 2014

Charging your mobile device’s battery was pretty much a non-issue for many years…customers seldom complained; manufacturers did not see charging as a limitation; and charging, well, was really not a problem.

That began to change in the past year or two. Several factors are converging to elevate battery charging to a whole new level of importance and complexity. So what are these factors:

  1. The rise in battery capacity has made charging painfully slow. Early cell phones and smartphones had batteries that lasted days, but the newest smartphones now sport large capacity batteries (up to 3,200 mAh) and are charging using the same 5-Watt AC wall adapter that shipped with the smaller smartphones 5 years ago. The result: It takes hours to charge the newest smartphones. In technical terms, this is called the C-rate. A 3,200 mAh packs 12.2 Watt-hours of stored electrical energy. Dividing the power of the AC adapter by the energy of the battery gives the charging C-rate, in this case, 5 divided by 12.2 equals 0.4C, which equates to 3 hours or more of charge time. In contrast, batteries from earlier generations of mobile devices charged at twice that rate, somewhere near 0.7C. So there has been a drive in the industry to increase the power of AC wall adapters and increase charging rates.
  2. The rise of energy density makes charging more complex: As I mentioned in a previous post,  increasing charge rates at these higher energy densities have a serious impact on cycle life, effectively reducing the battery warranty. In other words, the simple charging methodologies of earlier generations of mobile devices are not applicable to charging the newest generation of smartphone batteries. Therefore, charging newer mobile devices at higher charge rates (in order to reduce charge times) now involves newer designs and methodologies for charging — otherwise, you can charge fast, but your battery will not last! That is not an acceptable compromise.
  3. Consumers asking for super fast charging: End users and consumers are now beginning to realize that if batteries cannot have battery capacities that can last several days, if not weeks, then they should have the ability to charge their batteries at blazing fast speeds; certainly faster than 30 minutes, and possibly 15 minutes if possible. These kinds of charge times are equivalent to charge rates in the range of 1.5C to 3.0C, in other words, about 4 to 10X faster than presently shipping in smartphones. Naturally, at these super fast charge rates, the design of the entire battery charging circuitry and methodologies need to be conceived from scratch to allow for the extra charging power and to deliver sufficient battery cycle life (and battery warranty).

Handset manufacturers are beginning to recognize these factors and why fast charging should become an important part of the mobile experience. It is a major transformation for them — new designs for their charging platforms are required and that is not trivial. But that train has left the station, and handset makers who are not taking action will miss the boat. The first shot across the bow came from a small but promising handset maker in China called Oppo. They released in early 2014 the first smartphone that is capable of charging at 1.5C. Expect to see more mobile device makers to follow suit.

In future posts, I will talk more about the metrics of fast charging, especially as mobile handset makers will begin to make claims about charge times that may be confusing.

30Sep 2014

I covered in the prior post about the ill consequences of charging a lithium-ion battery using constant current, constant voltage (or simply CCCV). The damage incurred within the battery during charging with CCCV is attributed to a series of undesirable side reactions that effectively reduce the effectiveness of the primary energy storage reaction. In other words, during charging, one desires that all the energy goes into the ideal reaction that stores energy inside the battery. In reality, CCCV charging promotes a number of bad reactions that effectively damage the internal structure of the battery, and reduces the battery’s ability to store electrical energy.

I will not go into the details of these side reactions; they are fairly involved and can be quite complex for the average person. But they are reasonably understood by our scientists. For example, one of them is the formation of lithium metal deposits when lithium ions combine together. Others relate to the physical damage to the electrodes and the decomposition of the electrolyte solution.

A few of these undesirable side reactions, but certainly not all of them, have been shown to exhibit a dependence on voltage. Specifically, some of the damage accelerates when the voltage of the battery approaches 4.35 Volts….or in other words, when the battery is approaching 100% full charge. This is why a common tip is to charge the battery up to about 80% instead of the full 100%. 

So step charging, probably introduced several decades ago, was an early attempt to charge the battery very gently at the higher range of voltages, or when the battery is approaching full. It is simple: it means reducing, or stepping down the current, when the battery voltage reaches say 4.1 Volts, or say around 60% or 70% of its maximum charge. However, extensive tests and results over the past many years have shown that the damage reduction was at best minimal. There were indeed a few cases where step charging seemed to have helped, but these were few and far in between, and worse yet, there was not much consistency. In other words, step charging did not deliver a solution.

I will leave the discussion of better, more sophisticated, charging methodologies to another post, but let me address here why step charging fundamentally is flawed or at best, incomplete.

First, step charging is only attempting at alleviating the amount of charging when the battery is nearing its highest voltages. But the damage to the battery is not only due to high voltage. It is due to more complex reactions of which voltage is but only one parameter. Failing to recognize the relationships between all the damage elements makes step charging quite ineffective. This is particularly acute in more modern lithium-ion batteries with high energy densities (or higher capacities). 

Second, step charging, much like CCCV, is an open loop solution. In other words, it has no knowledge of the battery’s inner reactions, inner health, inner status, and consequently  has no means to measure or assess the rate at which these undesirable damaging reactions at taking place. So let’s say for the sake of example, we have two batteries from the same type and vendor, but with  manufacturing variations between them (which is very common). Let’s further say that one battery is better, and that its damage seems to occur at an onset voltage of 4.1 Volts. Let’s also say that the difference in manufacturing causes the damage in the second battery to occur at a lower voltage of say 4.0 Volts. So if step charging reduces the charging current at 4.1 Volts, then one battery will see an improvement but the other will not. And if one were to say let’s drop the charging current at 4.0 Volts to be safe and cover both batteries, then there is a serious penalty to charging times — charge times will balloon significantly.

So in a nutshell, if someone is promoting to you step charging as a solution, my advice is simple: RUN!

29Sep 2014

CCC…what? Yes, it is a mouthful and it stands for “constant current constant voltage.” It is presently the charging approach used for lithium ion batteries. As I indicated earlier, it was invented in the 19th century for charging lead-acid batteries, and somehow it became the default charging methodology for present-day lithium-ion batteries. This is what your smartphone, tablet and your electric vehicle do to charge your lithium-ion battery.

As the name implies, the electronics in the battery management system (see the earlier post on BMS) charge the battery initially with a constant amount of charging current. The higher the charging current, the faster the charging time; and consequently, the higher the power; and therefore, the bigger the AC adapter (or wall charger) to accommodate the higher power rating. That’s why a tablet adapter, typically rated around 12 Watts, is bigger than a typical smartphone adapter which is rated at or near 5 Watts. And that’s why an electric vehicle requires a far bigger charger, rated above 6,000 Watts.

As the battery is charged, its terminal voltage rises. A single lithium-ion battery starts near 3 Volts, and as it charges, its terminal voltage will rise above 4 Volts. When the voltage reaches a predefined limit, often 4.35 Volts, the charging electronics will switch from a constant current to a constant voltage — this is to ensure that the voltage across the terminals of the battery never exceed 4.35 Volts. Higher terminal voltages risk the trigger of unsafe failures. Incidentally, never charge a lithium-ion battery with a charger that was not designed specifically for lithium-ion batteries.

Now a lithium-ion battery’s internal chemistry is quite complex. When the battery is being charged, lots are happening inside the battery. As I explained in prior posts, lithium ions are traveling from one electrode to the other and inserting themselves within the electrode. This is all happening within the battery and is transparent to you, the end user. Alongside this charging process, as you might imagine, there are other bad and undesirable things that are happening too. For example, it is easy to imagine that all the lithium ions will travel together and happily make the journey from one electrode to another. In reality, these lithium ions will “collide” and they will bond together to form dangerous  deposits of lithium metal. These lithium ions are now out of the picture and can no longer participate in storing electrical energy. It is analogous to traffic jams on highways because cars do get into accidents; the notion that cars on a highway will travel merrily at 65 mph and stay in their own lanes is somewhat naive and left only to a utopian universe.

Now, it turns out that CCCV charging is greatly responsible in how lithium ions travel inside the battery. There is sufficient proof now that CCCV is one of the key factors that accelerate the damage inside the battery exhibited by the loss of lithium ions. Think of it as the ill-timed traffic lights or poorly marked signs on the road that can cause unnecessary accidents.

So you might ask, how come this issue was not observed in the past? Well, lithium-ion batteries of yesteryear are akin to a rural highway with very few cars on it. So even if the highway signage was defective, there were relatively few cars on the road. Batteries from past years have low energy densities, and consequently have fewer lithium ions to go around, so they were more forgiving. But modern batteries are now packing higher energy densities, in other words, they contain a lot more lithium ions than their older sisters ever did, and thus are extremely sensitive to how they are charged. This combination of high energy density batteries with CCCV charging is a recipe for excessive damage, less capacity, short cycle life, and consequently, a very poor consumer experience. 

Think about it next time you wonder why your battery seems to be losing its freshness.

23Sep 2014

I love my electric vehicle, or EV, as we affectionately call our electric cars here in California. I love that it is quiet. I love its fast pickup from a stop. I love that it requires practically zero service: no oil change; no transmission service; no timing belts. Of course, I love too that it is eco-friendly and driving in the carpool lane. I am bullish on the future of electric vehicles, but first, the technology has to evolve a little more to give the consumer less anxiety, the topic of today’s writing.

No, it is not a Tesla. It is not a Leaf. I am one of the early adopters of a Ford Focus Electric. It looks like a regular Ford Focus so it does not stand out in traffic. I nominally get about 80 miles of range which includes a lot of freeway driving…my normal daily commute. Slower driving in stop-and-go traffic increases my range to about 100 miles. Shave 10 or 15 miles during our mild California winters.

My vehicle is powered by a 24 kWh lithium-ion battery pack that is manufactured by LG Chemical, but in reality, only about 19 or 20 kWh are available to me. That’s because to provide a 100,000-mile warranty, the battery has to reach 100,000 divided by 80 miles = 1,250 cycles minimum. So battery manufacturers and car makers choose to reduce the capacity of the battery to gain cycle life. Remember the whack-a-mole strategy from earlier posts. Using the water analogy, if you don’t fill up the water bucket to the top, you can fill it more times over its life. Tesla Motors, Leaf and virtually every car maker employs this strategy. For the time being, it’s ok, but that has to be addressed over time in order to make electric cars more affordable for the broad population.

When I first bought my car, my range anxiety was high. The car dashboard displayed how many miles of driving I had available in the tank, ehem, battery. I charged my car overnight, and I started my morning with about 80 miles. By the time I got to work, the dashboard showed less than 60 miles.  I was nervous every time my dashboard dropped below 50 miles, so I charged as frequently as I could. That’s range anxiety. 

Now, nearly a year and half later, my behavior has changed drastically. I drive my car down to 10 or even 5 miles left in the battery. I plan my route. I know my destination and I know my return route. Keeping 50 or more miles for insurance does not make any more sense. I became comfortable with the given range of 80 miles and I use it effectively. I consistently get about 80 miles, and in the time since I bought it, my comfort level increased and my trust in my dashboard’s range estimate has increased. Of course, my maximum driving range was still limited to the greater Bay Area. I cannot drive my car to, say, Los Angeles, but I do use nearly every mile available to me in battery.

However, my range anxiety got replaced with something else: Charging anxiety. You see, if I am comfortable taking my battery down to nearly zero, I need to know that I am close to a charging outlet when I stop. Good news here! The San Francisco Bay Area has lots of charging outlets. But the problem is the speed of charging. If my battery is near zero, it takes a whopping 20 hours to charge it at 120-Volt, and a mere 4 to 5 hours using the 240-Volt chargers. Ouch! That is not acceptable. That is at the core of anxiety in battery-powered cars, phones, or anything else. We need to charge them fast, and I mean really fast….As fast as filling up your gas tank at the gas station. 

If you look at what Tesla Motors is doing and what Elon Musk keeps advertising, none of it is about extending the range of their cars. Their publicized priorities are about building cars for the masses (in other words, lower price point) and secondly about charging their cars fast, in half an hour or so.  

Fast charging…we need it. Remember that!