Charging from a USB Port

The Universal Serial Bus (USB) was introduced in 1996 and has since become one of the most widespread and convenient interfaces for electronic devices. The USB port is a bi-directional data port that provides a supply voltage to power memory sticks, keyboards, mice, wireless interfaces, cameras, MP3 players and chargers.

With 5V and 500mA of available current, the USB bus can charge a small single-cell Li-ion pack, but there is a danger of overloading the USB hub when attaching too many gadgets. Plugging in a charger that draws 500mA along with other devices will exceed the port’s current limit, leading to a voltage drop and a possible system failure. To prevent overload, some hosts include current-limiting circuits that shut down the supply when overdrawn. Another method is limiting the current of all attachments to 400mA to reserve 100mA for housekeeping.

The most common USB chargers are designed for single-cell Li-ion. The charge begins with a constant current charge to 4.20V/cell, at which point the voltage caps and the current begins to decrease. Due to a voltage drop in the cable, which is about 350mV, and losses in the charger circuit, it is possible that the 5V supply cannot supply the battery’s 4.2V charge threshold. This is no problem; the battery does not suffer but will deliver shorter than expected runtimes.

The rectangular Type A USB plug has four connector pins and a shield. The rightmost contact is number 1 and carries 5V; the leftmost contact is number 4 and forms the ground. The two shorter pins in the middle are reserved for data transfer and have no function in the USB charger. Figure 1 illustrates the rectangular Type A USB plug.

Adapters are available that convert the cigarette lighter to a USB port to charge a cell phone in a car. You can also charge a cell phone from a laptop USB port. Note that the USB port is unidirectional and cannot take power from an outside source. In other words, power only flows out.

Comments

On August 9, 2011 at 3:22pm

John B. wrote:

Thanks I allways wanted to know which side caried the positive charge !

chers
John

On August 10, 2011 at 6:54pm

DR ABBAS ABUDAYYAH wrote:

Charging from a USB

On December 28, 2011 at 10:35am

Bob Stuart wrote:

Have you done any research on AC powered USB chargers?  My family is accumulating a number of USB charged devices (i.e. mobile phones, game controllers, e-readers, etc.) and I’m wondering if any AC powered USB charger can be used with any of these devices or if there are limitations.  Thanks!!

On January 4, 2012 at 12:33pm

barak wrote:

Charging Li-Ion From USB trough Series 0.7V Diode will give 4.3V which is the charging voltage (~4.2V),
enother option is to use PNP transitor with the Base connected to the gnd or to 10K thermistor in order to protect from high temperature (50deg),

what do you think about this?

On February 10, 2012 at 12:27pm

David Olive wrote:

I need to find out charge time of lithium battereis versus depletion…ie…if it takes
2 hours to get a 100% depleted battery back to full (or close to) charge, how fast will it take a battery that has been depleted by only 80% back to full charge….I am trying to design a high powered dual bank battery based system and need to know how long it will take to charge one bank of batteries based on depletion %?

Also, do I need to “rest” the battery between charge & use cycles, or can I drop the battery into the circuit as soon as it is charged…ie…will I diminsh teh life charge cycle times by not giving the battery any “rest” between cycles?

Thanks.

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