Mobile devices have changed the way we work, rest and play — mostly for the better. But one downside is the nagging concern when rushing out of the house in the morning about whether our smartphone has enough juice for a day’s browsing, social networking, and video shooting.
The more diligent among us line up their mobile devices every night and plug each into the main supply before retiring and awaken to a full charge. But despite continuous advances in battery technology over the last several decades, large bright screens, powerful processors and always-on connectivity often mean even the most organized consumer can by hit by the dreaded red fuel gauge icon several hours before the end of a day’s electronic activity.
Little wonder then that spare power points in public places, work spaces and cars are rapidly occupied with chargers working hard to provide mobile device batteries with a quick refresh. Preliminary research of user habits suggests that 50 percent of “charging events” last for 30 minutes or less, and even then the user is often doing other things on the mobile device, thus reducing the power available to actually recharge the battery.
Almost unnoticed by the consumer, mobile telecom chipmaker Qualcomm has addressed this challenge by encouraging mobile device makers to adopt “Quick Charge,” a technology that accelerates charging. Now the company has released Quick Charge 3.0, which speeds things up even more while also improving efficiency.
Accelerating Charging
What effect does a brief connection to the mains outlet have on a mobile device battery? It will help, of course, but by how much depends on several things, including the output of the charger, whether the mobile device is idle during charging and, significantly, the amount of charge in the battery prior to plugging in for more power. But assuming a typical 3300-mAh mobile device battery is very low on capacity and is being replenished using a 5 V/1 A (5 W) USB wall adapter, the cell could receive around a 15 percent increase in capacity during a 30-minute charge (in practice, system losses would see this decrease to around 12 percent). While helpful when a consumer is down to the their last few hundred mAh, that’s hardly impressive and may well see them hunting around for another wall wart before the working day is out.
One way to accelerate charging is to provide more power from the wall adapter. Some manufacturers have adopted this approach and supply, for example, USB wall adapters that can push out 5V/2A (10 W), which would (theoretically) double the battery’s capacity gain during a 30-minute charge compared with the more modest units described above.
Unfortunately, Li-Ion cells, the (by far) dominant battery technology for mobile devices, are relatively fragile devices. Rapid charging generates heat, which subtly alters the battery’s internal structure, leading to reduced capacity, early failure, and in the very worst cases, spectacular but highly hazardous combustion. Faced with this challenge, manufacturers have understandably acted cautiously, preferring to slow down charging rather than multiply expensive warranty claims.
However, since its acquisition of charging know-how through its purchase of Summit Electronics in 2012, Qualcomm has promoted the merits of Quick Charge, a technology that claims to balance rapid charging against battery health. Quick Charge works by using an algorithm running on the company’s Snapdragon processor (the same processor at the heart of Qualcomm's DragonBoard) to continuously determine the maximum power that can be applied to a battery during the charging process without risking damage.
Quick Charge has been adopted by many mobile device makers who embed the Snapdragon processor and an associated power management IC (PMIC) in the smartphone and supply consumers with a USB wall adapter than can deliver more power than regular units. While consumers might not have noticed, many contemporary smartphones charge much more rapidly than previous models courtesy of Quick Charge. Qualcomm says, for example, that Quick Charge 2.0 (using a 9 V/2 A (18 W) USB wall adapter) can charge an empty 3,300-mAh smartphone battery to 63 percent capacity in 30 minutes.
This Fall, Qualcomm announced version 3.0 of Quick Charge, which will appear in mobile devices next year. The main changes this time round are a reworked algorithm, not-so-snappily dubbed “Intelligent Negotiation for Optimum Voltage” (INOV), which controls the PMIC such that it regulates the charging voltage in 200-mV increments over a 3.6 to 20-V range. That equates to a possible 82 voltage charging levels, compared to four accommodated by version 2.0.
The results of the improvements introduced with Quick Charge 3.0 are a modest speed increase (putting an extra 8 percent capacity into the battery in 30 minutes compared with version 2.0, see graphic) but a major gain in efficiency. The improvement in efficiency primarily comes from the fine-grained voltage scheme that allows the supply to be more closely matched to the battery’s demand at a particular time in the charging cycle, limiting system losses. Qualcomm claims that Quick Charge 3.0 dissipates 38 percent less power during a 30-minute charge of a 3,300-mAh cell compared with the previous generation of the technology. Lower power dissipation also means less heat, which in turn helps to reduce the stress on the battery.
Manufacturers adopting Quick Charge control the maximum output of the USB wall adapter. As there are no products on the market yet, it is difficult to predict what the typical outputs will be. Quick Charge 2.0 could theoretically support 60 W over a USB B connector, but the maximum power for USB wall adapters for smartphone use was capped by manufacturers at 18 W (typically from a 9 V/2 A output). Quick Charge 3.0 USB wall adapters will probably operate at a similar level.
Looking after Li-ion Batteries
While improvements in charging efficiency and speed during a 30-minute recharge are welcome, full charging of mobile device batteries using Quick Charge 3.0 can still take up to 90 minutes. This is because Li-ion batteries must be charged using a two-stage regime. The first part can be performed relatively rapidly using a constant-current source, but once the battery’s voltage reaches about 4.1 V (the threshold voltage), the charging system must progressively back off the current to maintain a constant voltage to “top-up” the cell. If too high a current is maintained after the battery reaches 4.1 V, damage can occur.
Quick Charge technology comes into its own in the constant-current stage of Li-ion battery charging where things can be accelerated by turning up the adapter’s wick. Once the threshold voltage has been reached however, charging proceeds at a pace dictated by the characteristics of the battery and is therefore no quicker than conventional systems.
There is a drawback in accelerating the constant-current stage of the charging cycle: the faster the charging, the lower the battery capacity when the threshold voltage is reached, effectively extending the period of the (much slower) constant-voltage cycle to complete the process. For example, charging the battery at a current of around 70 percent of that which the cell can provide for one hour (i.e. around 2.3 A for a 3,300-mAh unit) results in a capacity of 60 percent when the switch to the slower constant-voltage stage occurs. Slowing things down to a charging current of 20 percent of the cell’s one-hour current capacity results in virtually a full charge when the threshold voltage is reached. With the majority of consumers looking to boost capacity quickly and run, this trade-off is one that the designers of Quick Charge have presumably calculated is worth making.
The other concern that hangs over rapid-charging is the long-held belief that it stresses the battery (primarily through the generation of greater temperatures), resulting in compromised lifetime. In the case of Quick Charge, the smartphone maker ultimately decides the maximum current to which the battery is exposed and, no doubt armed with the history of warranty returns for failed batteries, manufacturers will be able to trade-off charging speed against ultimate battery life. In any case, with the continual improvements in battery technology resulting in more rugged devices, combined with the average smartphone life of two years, it would likely need to be a particularly aggressive charging regime to cause failure within a typical warranty period.
Besides which, there is some evidence that the systematic exhaust-and-then-charge-slowly-overnight schemes favored by conscientious smartphone owners may not be all they’re cracked up to be. Provided the charger is unplugged as soon as the battery is full, this is a good way to proceed, but who gets up at 3 a.m. to do that? In practice, the charger is usually left to periodically trickle-charge the battery as the voltage fluctuates during a long night, which—in the long term—adds to the cumulative stress on the cell.
So perhaps the undisciplined among us can, after all, rest a little easier as a result of serendipitously finding the best method of prolonging mobile device battery health. Frequent 30-minute fast boosts of partially discharged devices could well prove to be better for a Li-ion cell than the prolonged stress of a methodical overnight charge. That would certainly be music to Qualcomm engineers’ ears.