USB Type-C (or USB-C) had its coming-out party quite late—at this year’s CES in early January, even though the connection type made its broad debut with Apple’s 12-inch MacBook in April 2015. Since then, more devices have adopted the format, such as the Chrome Pixel C and Nexus 6P. (The new Apple TV has a USB-C port but only as a connection option for debugging and making screen captures with a Mac.) An ocean of USB-C devices is coming that will include more Macs as part of the Thunderbolt 3 update—which relies on that connector style—and possibly some iOS hardware.
I’ve been waiting to test USB portable batteries equipped with USB-C since theMacBook shipped. But as long as Apple relied on the MagSafe connector, you couldn’t get a licensed and certified adapter that would work with a Mac laptop. USB-C changes that altogether. It has bi-drectional power support, allowing energy to flow from a laptop or other controller’s USB-C to charge or power external devices and via USB-C to charge a MacBook or similar device’s internalbattery.
In general, USB battery packs used to have limited capacity, offer slow charging of devices and recharge slowly, and cost and weigh a lot relative to the benefit they offer. But they’ve matured very rapidly over the last few years. With the very large-scale manufacture of standard-sized rechargeable lithium-ion battery cells, electronics makers have created affordable, high-capacity USB packs that range from recharging your iPhone 6s by about 50 percent up to the equivalent of a week’s worth of multiple full recharges of a set of iPads and iPhones.
Laptops have typically been in a different category, because they not only have large batteries, but when in use, they draw power faster than previous USB packs typically provide it because of limitation in the previous generation of USB connector and cable standards. In such a case, a Mac laptop pulls juice from the USB-connected battery, but also gradually runs down its internal one.
The iPad Pro suffers from this problem. It ships with a 12-watt power adapter that can’t always keep up with power consumed while you’re using the iPad Pro. While its battery could safely be charged at a much higher wattage (at least twice as “fast” in terms of power flow), the Lightning standard appears to limit its maximum rate.
USB-C breaks through that limit by allowing higher-amperage charging even though USB has a set limit on voltage. (Wattage is the product of amps times volts, representing the total energy transferred.) This higher amperage can allow a USB battery pack to recharge a 12-inch MacBook relatively speedy when put to sleep, although the units I tested still can’t keep up with its power consumption.
OS X requires changes to better recognize the kind of external device providing charge, rather than treating them as a “power adapter” as the MacBook did for all the batteries tested.
USB-C’s higher rate of power flow lets some of the batteries I tested recharge rapidly, although you need to find a high-wattage USB adapter to make that work as well—none ship with such an adapter. (One pack can use Qualcomm’s Quick Charge 2.0 technology, which boosts voltage for faster charger when used with a USB power adapter with the same tech.)
Being able to bring a relatively lightweight battery (half a pound to a pound) that carries a partial or full additional MacBook charge or could partly recharge aMacBook and handle an iPhone and iPad (some charging three devices simultaneously) can make extended travel away from electricity very practical. This can especially include long-haul flights where onboard power isn’t available or that power isn’t enough to charge devices fully. You would no longer have to camp at an outlet or leave hardware in a vulnerable place to charge via AC.
In this roundup, I look at four USB packs that feature a USB-C port for charging; some can also recharge through the port. These models appeared on the market starting in fourth quarter 2015, and three are from companies with good track records on electronics, cables, or batteries. (The fourth is less known, but itsbattery tested very well.) I tasked them to discharge and recharge on their own, and replenish a USB-C MacBook.
Juice it up
Unlike the fancy design-to-purpose batteries you’ll find inside Apple products, every USB battery pack I’m aware of uses cells purchased from a battery-making firm. Apple and other companies mold or terrace lithium-ion (Li-ion) polymer batteries to fit every nook and cranny. Mass-produced cells, however, are typically round, like normal alkaline and rechargeable consumer batteries, although they are often much larger.
While some small packs use flat arrangement to stay compact—such as Amazon’s super-cheap $6 Micro-USB Portable Power Bank—larger USB packs like the ones I tested rely on standard cylindrical cells and package them with the circuitry, heat dissipation, and connectors needed to move power in and out.
If you’re not familiar with power basics, here are just a few. The simplest way to discuss electrical power is in units of volts (V), amperes or amps (A), and watts (W). These can be compared to water pipes and water flow. Voltage is pressure, or the amount of water in a given space; amperage is pipe diameter, which has an impact on pressure. Low-amperage (a small diameter pipe) requires high voltage (lots of pressure) to move the same amount of power as a high-amperage (big diameter pipe) with low pressure (low voltage). Wattage is the product of amps and volts, describing the power (the “work”) passing through the system.
Now, with batteries and battery packs, we want to describe how much capacitythey have—how much power they can store and then provide to other hardware. That’s measured in milliampere-hours, abbreviated mAh, which you’ve probably seen repeatedly and wondered precisely what it meant. That number can be confusing because it also requires a voltage, something you rarely see listed. The batteries used in power packs typically discharge at about 3.6V or 3.7V and charge at 4.2V. (Lithium-ion cells, used for all the packs I tested, charge best at about that rate.)
So when you see that a battery pack has 10,000 mAh, that’s 10,000 mAh available at 3.6V. USB, however, is 5V, while smartphone batteries used in iOS devices discharge at about 3.8V (and charge around 4.3V or 4.4V). This requires converting voltage to figure out the idealized capacity. Because these voltages are so similar, you can mostly ignore that; it mostly matters with higher-voltage device batteries. (A similar measure, watt-hours (Wh), avoids this conversion, but because it’s not consistently used, it’s harder to find it for comparison.)
As an example, the iPhone 6s battery has 1,715 mAh of capacity. That should mean that a 10,000 mAh USB battery can recharge it about 5.5 times. (For comparison, the iPad Air has a 7,340 mAh and the iPad Pro a 10,307 mAh one.) The single-port MacBook is a trickier case, because its internal battery is 5,263 mAh but at 7.55V. If you do the math, 3.6 divided by 7.55 gets you the factor to multiply against the battery pack’s pack—roughly 50 percent or about 5,000 mAh. So you should be able to charge a MacBook from 0 to 100 percent almost twice with such a pack, right?
But that omits three other factors! Bear with me, as these are easier to explain:
- Because power has to be converted among voltages to work over USB, both in the source battery and in the destination device, there’s always some loss. This is why you feel heat when batteries charge or discharge, as heat is wasted energy. (Some of the devices I tested seem to get noticeably hotter than others.)
- Lithium-ion batteries can’t be taken down entirely to zero percent. As a spokesperson at Anker, the maker of many batteries and one best one we tested, conveyed from its engineers, “If the battery power is discharged to zero it will adversely affect the durability of the battery cell.” So even when seemingly exhausting a USB battery pack, its circuitry prevents it from tapping out.
- Li-ion batteries also degrade over time and have a risk of expansion or even fires if they’re overcharged or charged too close to full too fast. (For reference, see all the Hoverboard fire videos from this last fall.) USB packs can charge rapidly at first, but as batteries approach full, they slow down, and stop short of 100 percent—sometimes far short in my testing.
To sum up? Batteries can’t give up their last ergs of juice, can’t be charged to 100 percent (and you never know quite how close), and lose power in converting over USB and back. This adds up.
In my testing, the best of the two highest-capacity batteries (both over 20,000 mAh) delivered 55 percent of its rated capacity to a MacBook. That was still enough to completely recharge a MacBook battery with some left over, which is magnificent both for performance and by price and weight. But it’s not as much as you’d reckon by using rated numbers alone. I’ll get into this more with individual reviews.
Another factor with power is “speed”—in this case, that’s directly related to amperage. Because USB’s voltage is fixed at around 5V, you have to up the amps to move more power, which equates to moving power “faster.” Devices with larger batteries, like tablets or these large USB battery packs, need high-amperage chargers to refill them in any reasonable amount of time. You also need high amperage to charge a device faster than it’s depleting power if it’s in use while charging.
Originally, most USB packs maxed out with ports that could each pass power at about 1A, fast enough to charge a smartphone at full speed. But an iPad Air 2and iPad Pro can charge at 2.4A (and the Pro even faster with a higher-amperage adapter), and iPhones for years charge fine at 1A, but can bump up to as fast as 2.1A with an iPad charger.
Modern packs typically have ports that can be rated at 2.0A, 2.1A, or 2.4A; all the packs tested have at least one Type-A 2.4A port, and one USB-C 3A port. Modern packs also use USB and other signaling to provide as much power as a device can accept but no more, while mobiles and computers won’t accept more power than they can safely use. (USB packs’ ports default to 1A or lower if they can’t sort this out with an attached device.)
Faster only works to a point: For keeping their lives long, batteries should only be charged between about a 0.50 and 1.00 ratio of amperage to capacity, which is called its C rating. An iPhone with a 1,715 mAh battery charging at 1A has a 0.58C rating, considered “gentle” and which maximizes cycles. Charge it at 2.1A, and you’re well above 1C, but Apple appears to have factored in, as it allows charging at that high a rate. The USB power packs I tested charge at about 0.15C to 0.30C; future packs might work with higher-wattage cables and adapters for faster recharge rates.
Only the Talentcell provided guidance as to the number of cycles it expects for the pack to perform as expected: 500. As with all lithium-ion charging cycles, that typically refers to complete cycles, so depleting to 50 percent and charging to full counts as a half cycle.
Finally (whew!) each battery pack has a maximum combined output across all ports. The internal electrical circuitry divvies up charge by port, but also can’t exceed that total when charging through multiple ports at once, like multiple iPads and iPhones. For example, the Anker PowerCore+ 20100 can output 2.4A on its two Type-A ports and 3A on its USB-C ports. With all three ports in use, however, it maxes out at 6A, with no more than 2.4A to any port.