(For reference, see all the hoverboard fire videos from this last fall.) Lithium-ion batteries have a risk of expansion or even fires if they’re overcharged or charged too close to full too fast. In some of my testing, batteries clearly don’t go below 5 to 10 percent, reserving a significant percentage for a margin of safety.ĭepleted 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. 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. Lithium-ion batteries can’t be taken down to zero percent. This is why you feel heat when batteries charge or discharge, as heat is wasted energy. 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. So you’d think maybe a full charge, more or less, for each of those.īut that omits four other factors that keep you from getting 100 percent efficiency! Bear with me: 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. 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.Īn 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. 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. It’s such a slight difference, it’s lost in rounding. 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. When you see that a battery pack has 10,000 mAh, that’s 10,000 mAh available at 3.6V or 3.7V. (Lithium-ion cells, used for all the packs I tested, charge best at about that rate.) The batteries used in power packs typically discharge (provide power output) at about 3.6V or 3.7V and recharge at 4.2V. It has to be paired with a voltage: mAh at a given V. Milliampere-hours can be confusing because it’s not meaningful in isolation, even though it’s often used that way.
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