Fast or quick, just be better than slow
Words like fast and quick have very subjective meetings. For example, I showed up earlier than usual for a doctor’s appointment and the receptionist said that because I arrived early they would get me out of there very fast. Well, that set my expectations of being able to get back to work and make a meeting that I thought I would miss. A “Car and Driver” and a two-year-old “National Geographic” later, I was summoned to the examining room.
I even got a chance to read a couple of previous year’s “Road and Track” magazines before I saw my doctor. So much for making my meeting…. When I mentioned to the receptionist that this appointment seemed to take longer than expected, she said that they got me through as quickly as they could. Okay, so maybe I had expected fast and ended up with quick. Actually, from my perspective it was slow.
In the days when NiCd batteries were the rechargeable batteries for most portable electronics, we had terms for different charge rates that had somewhat clear definitions. There was the “standard” charge rate, C/10, which took about 12 hours to charge from empty to full. The batteries were designed not to require charge termination at this rate. Then there was the “quick” charge, C/3, which took close to 4 hours to get to full from empty and did require termination. Then there was the “Fast” charge, C/2 to C, which could get the batteries to full in little over 1 hour at the C rate. This charge rate required termination and usually included a top off charge before going to a maintenance charge.
As Lithium-Ion (Li-Ion) batteries have become the portable rechargeable energy source of choice, rapid recharging has become a good area for innovation. The transition of our devices to smartphones and tablets add to the challenges. One extreme example is the new tablet for Samsung, Note Pro 12.2, which I am considering. This tablet has a 9500 mHr battery and comes with a 2A charger. This means that it is definitely an overnight charge, if the user is operating off grid all day. So far from the reviews, I should get 10+ hours of battery operation, which is fine with me.
Getting back to the rapid recharging of these types of devices, consumer expectations need to be set. Terms like “quick” or “fast” do set expectations. Figure 1 shows the recharge time to full for various maximum charge currents when starting at different states of charge, SOC. From the graph, it shows that it really does not matter whether you use an adapter with 2A or 3A capability when starting from 75 percent SOC. It takes a little over 1.2 hours to get to full. This is due to charging in constant voltage mode. So if the consumer expects to be able to bring their battery to full faster because they have a fast or quick charger, then they will be disappointed.
However, if they are starting from empty, then there is about 20 minutes difference between a 2A and 3A charge rate. It is all about setting expectations. In a paper written by Botsford and Szczepanek1, they provide parameters for slow, quick, rapid, and fast charging. Although this paper is concerned with electric vehicles, it does point out a need for consistent definitions of these terms. In this paper, quick is slower than rapid, and rapid is slower than fast. This is consistent with other use of battery charging terms.
Figure 1. Recharge time to full when starting at various states of charge, SOC, versus maximum charge current.
An example from this paper is the mandate from the California Air Resources Board (ARB). It requires a fast charger be able to deliver 100 travel miles after 10 minutes of charge. Of course, this means that the vehicle has batteries capable of delivering more than 100 miles of travel.
We could use the same concept to convey what is meant by fast or quick in our portable devices. For example, we could state that our fast charger will provide battery capacity that is capable of at least 5 hours of operation after 30 minutes of charge. This is just an example and assumes that the fast charge current is the 1C rate and that the battery can support 10 hours of general operation.
I know this is more about communicating the benefits than solving the actual technical problems with providing faster recharging. If we cannot set the expectations so we can evaluate the consumer’s opinion, then fast charging may not be as valuable as we think it is. Providing a wall adapter that is not much bigger than today’s 10W adapter, while delivering twice the power at much less than twice the cost, will certainly keep us busy. In many cases the consumer will want to recharge the battery while using the device, so even more power will be needed. Solving the thermal issues of charging at twice the rate inside of these very slim smartphone and tablets will require significant improvements in conversion efficiencies.
Quick or fast, it is really about expectations. Whether waiting in the doctor’s office, waiting for a family member to get ready to go out, or recharging my new tablet, my slow is someone’s fast. I would like to know your opinion on this issue and what you think are the challenges for getting to either quick or fast. By the way, I did not even mention the possible capacity fade when charging faster.
For more information about this and other power topics, visit TI’s Power House blog: www.ti.com/powerhouse-ca.
Charles Botsford, Adam Szczepanek, “Fast Charging vs. Slow Charging: Pros and cons for the New Age of Electric Vehicles,” EVS24 Stavanger, Norway, May 13-16, 2009
About the author
Dave Freeman is a Texas Instruments Fellow and Chief Technologist for Power Supply Solutions in the Power Management business unit. Dave has expertise in the areas of battery management ranging from charging to capacity estimation. In the areas of power management, he covers low power DC/DC, high frequency power conversion and digitally controlled power. Other areas of focus for Dave are renewable energy systems and low power energy harvesting. Addition interest includes sensors and analytical methods used to evaluate physical properties of materials. Dave offices inside Kilby Labs where he manages the energy lab as well as power/energy related research projects.