USB-C battery packs: Understanding the basics of battery power

Editor’s note: This article is part of our roundup of USB-C battery packs.

The simplest way to discuss electrical power is in units of volts (V), current measured in amperes or amps (A), and watts (W). (Current is usually abbreviated as I, originally for intensité de courant; I use A throughout as an abbreviation for amps for consistency, as that’s the letter used in all reviewed batteries’ documentation and labeling.)

Volts, amps, and watts can be compared to water pipes and water flow. Voltage is pressure, or the amount of water passing through 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 high amperage (big diameter pipe) with low pressure (low voltage). Wattage is the product of amps and volts, describing the total amount of water coming out of the end of pipe—the power (or “work”) passing through the system.

Now, with batteries and battery packs, we want to describe how much capacity they 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 might have wondered precisely what it meant.

Milliampere-hours can be confusing because it’s not meaningful in isolation, even though it’s often used that way. It has to be paired with a voltage: mAh at a given V. The batteries used in power packs typically discharge (provide power output) at about 3.6V or 3.7V and recharge at 4.2V. (Lithium-ion cells, used for all the packs I tested, charge best at about that rate.)

When you see that a battery pack has 10,000 mAh, that’s 10,000 mAh available at 3.6V or 3.7V. Smartphones and tablets use batteries with roughly the same voltage—around a nominal 3.8V when discharging and about 4.3 or 4.4V when charging. It’s such a slight difference, it’s lost in rounding.

What about watt-hours?

It’s often easier to refer to the watt-hours (Wh) of a battery unit, which was printed on every battery pack I tested, because of regulations for carrying external batteries onto airplanes in the U.S. and elsewhere. It’s sometimes in type so tiny and sometimes silkscreened in black ink on a black background, that you may need a flashlight and magnifying glass to find it.

An iPhone 6s has about a 6.5Wh battery (1,715 mAh), thus you’d expect a 36Wh (10,000 mAh) battery pack could recharge it 5.5 times. An iPad Air sports a 32.5Wh battery (7,340 mAh), while the 12.9-inch iPad Pro has a whopping 38.5Wh one (10,307 mAh); the MacBook’s battery operates at 7.55V, so the 41.5Wh capacity pencils out to about 5,500 mAh. So you’d think maybe a full charge, more or less, for each of those.

But that omits four other factors that keep you from getting 100 percent efficiency! Bear with me:

• Some power loss happens during voltage conversion from the battery to USB and back again; there’s always some loss depending on the quality and efficiency of the circuitry. This is why you feel heat when batteries charge or discharge, as heat is wasted energy.

• Lithium-ion batteries can’t be taken down to zero percent. As a spokesperson at Anker 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. In some of my testing, batteries clearly don’t go below 5 to 10 percent, reserving a significant percentage for a margin of safety.

• Depleted 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. Lithium-ion batteries 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.)

• Li-ion batteries also degrade over time, and can no longer hold a full charge, though capacity should remain quite good for some time. My MacBook’s internal battery now charges only to about 85 to 90 percent of its capacity only 16 months into its life with 100 cycles of charging, and I’ve factored that into test results. I’m concerned about how it performs after another 100.

To sum up? Batteries lose power in conversion, can’t discharge entirely, can’t always be filled entirely, and lose capacity over time.

It’s also worth noting that the LEDs on battery packs round up to the next set of units while displaying available capacity: on a pack with four LEDs, all four are lit when the internal cells total between 75 and 100 percent, not just when it’s nearly full. With eight LEDs, as the Anker, that’s about 87.5 percent. When you’re charging, however, most packs’ documentation says the final LED remains blinking until it’s reached the highest charge it can take. (A few packs on the market that lack USB-C ports include a tiny display with the exact charged percentage, something that I expect we’ll see more of as a competitive feature.)

In my testing, I saw a fairly wide variance from about 55 percent to 80 percent of stated capacity winding up being passed to a MacBook. With higher-capacity batteries, that was enough to most or completely charge a MacBook, sometimes with power left over. The price differential can be huge: a heavier, larger, less-efficient battery might cost much less than a smaller and efficient one, but offer the same effective amount of charge.

Speed

Another factor with power is “speed.” In this case, that’s directly related to voltage and amperage. All the batteries I tested except the MOS Go used 5V over USB-C, and the best among them could use current up to 3A for an output of 15W. The higher the wattage, the “faster” power moves. Devices with larger batteries, like tablets or these large USB battery packs, need high-amperage or high-voltage chargers to refill them in any reasonable amount of time. You also need high wattage 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 out at about 1A, fast enough to charge a smartphone at full speed. But an iPad Air 2 and iPad Pro models can charge at 2.4A (and the Pro units even faster with a higher-voltage adapter), and iPhones charge fine at 1A, but models over the last few years can bump up to 2.1A with the appropriate charger.

You can’t overcharge a device with any well-made equipment, and I didn’t see any problems with any of the batteries I tested. Apple and other manufacturers design their hardware to only accept a combination of voltage and amperage up to a maximum usable level. USB packs’ ports seem to default 1A or lower if they can’t sort this out with an attached device.

Modern packs typically have ports that can be rated at 2.0A, 2.1A, or 2.4A; most of the packs tested have at least one Type-A 2.4A port, and one USB-C 3A or better port. The Type-A ports are sometimes labeled “smart,” “IQ,” or “iSmart.” There’s one outlier: while the MOS Go can deliver 5V at 3A or 14.5V at 2A over its USB-C port, but only 1A over its Type-A port.

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 without degrading a battery. Charge it at 2.1A, and you’re well above 1C, which may reduce your battery’s lifetime of holding its maximum charge.

The USB power packs I tested mostly charge at about 0.15C to 0.30C; in most cases, higher voltage not amperage pushes faster charging, as with both PD 2.0 and QC2.0/3.0, keeping the C rating lower. (The TouchJuice was a minuscule 0.03C, which I’ll discuss in its individual review.) Future packs might work with higher-wattage cables and adapters for faster recharge rates.

Batteries are also rated for a certain number of cycles of complete charges, usually in the hundreds, but they can be in the thousands. The lower the C rating, the more cycles you get. And a cycle typically counts as 100 percent: deplete to 50 percent and charge to 100 percent twice, and that’s a single cycle, not two.

Output

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. Not all battery makers provided the total maximum output; I’ll note in reviews as available.

Each pack I tested had a different set of ports. Packs around 10,000 mAh typically have just two ports; those at 15,000 mAh or above have three and sometimes four—even five! I’ll run down each port arrangement in individual reviews as well.

You might wonder about the maximum feasible amount of power to pack into a pack. With PD 2.0, battery makers may push this much higher than the MOS Go’s 12,000 mAh. Conceivably, a pack with nearly 100 Wh and over two full MacBook charges could be lightweight and not outrageously priced, but there may not be enough of an extended-power audience to go for the gusto.

The FAA imposes a practical upper limit on external battery packs: they must be in carry-on baggage, although there’s no limit up to 100Wh on how many you bring on. From 101 to 160Wh, there’s a limit of two per passenger.