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A USB charger’s connector and advertised current do not, by themselves, tell you how quickly a particular device will charge. The device must recognize the source’s charging signal or supported mode, and its current draw can change as the battery fills. Those are the key ideas in Mitch Polonsky’s 2012 EE Times primer, which focuses on legacy USB charging and Battery Charging Specification 1.2 (BC1.2)—not modern USB-C Power Delivery.
Why can the same device charge at different speeds?
Polonsky frames the everyday puzzle this way: “I charge my phone, MP3 player …fill in blank , in x hours at home, but at work, with my laptop, with my monitor, with my new adaptor…fill in blank , it takes all day to charge!” It is an illustrative question from the article, not a survey result.
The reason is that USB charging is not determined by the plug alone. In the legacy USB context discussed in the 2012 article, a source can provide power while also signaling its charging capability over the D+ and D− data lines. A device may use those signals to decide whether it is allowed to draw more current, or whether it will charge from an unfamiliar host at all. Two sources with matching connectors may therefore be treated differently by the same device. Polonsky’s EE Times article describes this historical behavior.
That is why a charger’s nominal current rating is not a promise that every connected device will draw that amount. The rating describes a source capability; the device’s signaling rules and charging behavior determine what it actually uses.
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What changes as the battery fills?
A current reading is only a snapshot. Polonsky says a deeply discharged lithium-ion battery may begin with a low-rate trickle phase, and that a device can change its charging behavior as the battery moves through different states. He writes: “Many of us who have worked on cell-phone designs can tell you that a deeply discharged lithium-ion battery needs to be trickle charged before the real charging can start.” That is the author’s account of the devices and charging designs he discusses, not a universal rule for every battery or device.
For a useful comparison, check current at more than one comparable battery state: near empty, around the middle of the charge, and near full. A single measurement taken from one source at a nearly empty battery and another source at a nearly full battery cannot establish which source charges faster overall.
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Why can charging signaling affect USB data?
In the legacy USB arrangements covered by the article, a charger may place a voltage or signature on D+ and/or D− so the device can identify a suitable charging source. Those are also USB data lines. Signaling used to indicate charging capability can therefore conflict with ordinary enumeration or communication, creating a design trade-off when a device must charge and transfer data at the same time.
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Polonsky’s 2012 account describes the BC1.2 Charging Downstream Port mode as a way to permit both data transfer and charging. After the specified detection sequence, he says, a peripheral may draw up to 1.5 A while communicating. That figure is specific to the article’s description of this BC1.2 mode; it is not a general USB current limit or a statement about every USB charging standard in use today.
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The USB Implementers Forum lists the Battery Charging v1.2 Specification and Adopters Agreement. The listing confirms the document’s existence; the mechanism and 1.5 A figure above are attributed to Polonsky’s article.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare charging sources meaningfully
When comparing a laptop port, monitor, wall charger, or other legacy USB source, focus on the device’s actual behavior rather than the connector or advertised amps alone:
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- Source recognition: Does the device recognize the source’s charging signal or supported mode?
- Equivalent battery state: Are you comparing current at similar points in the charge cycle?
- Data needs: Does the setup also need to maintain USB data communication?
A device-compatible charger is a sensible baseline because its signaling is known to work with that device. It does not follow that an arbitrary source with a higher current rating will make the device charge faster.
What a charging measurement can—and cannot—show
For engineering characterization, the article describes measuring current on VBUS and exposing D+ and D− so their voltages can also be observed or applied. A USB interposer between host and peripheral can make those lines accessible. The device should then be examined across multiple battery states, rather than judged from one current reading.
An inline USB power meter or charging tester may help show current, but its capabilities vary. Unless it exposes and measures D+ and D−, it may not reveal the signaling that helps explain why a device accepted or rejected a source’s charging capability. Polonsky’s discussion is a measurement approach, not a test or endorsement of any consumer meter.
Where this 2012 explanation stops
This primer is useful for understanding legacy USB charging and BC1.2-era behavior. It does not explain current USB-C Power Delivery behavior, establish compatibility among present-day devices and chargers, or evaluate product safety certifications. For a USB-C charging question, the connector shape and this article’s legacy signaling examples are not enough to determine the applicable charging mode.
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