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 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.
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