Today's typical smartphone chargers are rated at 5 Volt & 1+ Ampere. The laptop USB can supply 500mA 100mA.

  1. Why is it safe to charge the phone with 1/2 the rated current?
  2. Is it like a slow charging mode where it's the phone's responsibility to recognize a computer USB port & lower down the current demand?
  3. Does this mean I can potentially use a 5V/0.5A wall charger with my phone without any fear of damaging it?

Edit : It seems that the answers & comments are focused on 100mA vs 500mA that the laptop USB port can supply. I'm amending that in my question & rephrasing. The real question is, even 500mA is lesser than what the wall charger supplies i.e. typically 1+ Ampere.

Edit 2 : To those who marked this as duplicate : You are confusing this question with the one that's asking whether a laptop can supply more than 500mA. It does not specifically answers the 3 points above. I'm wondering if a laptop can charge a phone, then why can't a lower current rating charger be considered safe?

  • \$\begingroup\$ I had seen the question before posting but it does not answer my query. How does the phone know when to draw how much current? Or potentially you can ignore the charger rating i.e. connect a 5V/0.5A charger to a 5V/1A requirement phone. \$\endgroup\$ – user Oct 3 '13 at 6:56
  • \$\begingroup\$ Most laptop USB ports can deliver full 500mA. \$\endgroup\$ – Turbo J Oct 3 '13 at 11:41
  • \$\begingroup\$ The question that it is marked as a duplicate of explains the spec and that you have to have the charger implemented to pass 500mA, it does answer it, read the question and the answers. We are not computer support, we are a design site, so it does explain it from a designing wit hit. \$\endgroup\$ – Kortuk Oct 3 '13 at 15:51
  • \$\begingroup\$ If the lower current rating being safe for a battery is your question that is different, batteries are not impacted by charging with lower currents, it is high currents that are dangerous. \$\endgroup\$ – Kortuk Oct 3 '13 at 15:55

Batteries can charge at less than the full rate. Just Slower. USB 2.0 supplies up to 500mA standard, after enumeration, not 100mA. Not all usb host devices conform to the 100mA limit without enumeration. Not all usb host devices limit to 500mA either.

Edit due to expanded question: Most new phones (4 years) will adhere somewhat to the USB Battery Charging Spec. For chargers, they will signal what kind of charger they are on the USB Data Line pair (mostly using Voltage Dividers to indicate a specific voltage). Samsung does or used to, have their own resistor pairs, but now mostly uses the standard signaling. Apple did as well, in fact, they are the first to be recognized as doing this. The standard pretty much adopted Apple's idea. Internally, the phones use some type of IC or Microcontroller to limit current.

The three common ratings are Computer (500mA), Wall (700mA to 1A), and High Current (2 A, mostly for iPads and tablets)

For computers, most new phones will enumerate, and draw 500mA. Almost no new phone has charge only usb ports, so that is not the issue. Some phones will treat dummy chargers as computer (500mA) chargers. Some phones will completely refuse to charge from them (iPhones, some Sansung)

So for the most part, no new phone was ever intended to have a non-standard current limiting. They expect to enumerate to a computer or see signaling on the usb pair. And with usb chargers being so common they can be had for single digit dollars in retail stores, there really isn't any excuse for using a 100mA limited supply. But batteries can charge from 100mA, some phones just won't try. Look at the iPad. It is intended to charge from a 2.1A high current charger or computer port with higher current (Which many new macs have). If you connect it to a low current 500mA port, it will SAY "Not Charging" but if you turn the screen off, it will very slowly charge.

  • \$\begingroup\$ If I connect 2 phones to a laptop (via USB), how much current would each get? \$\endgroup\$ – user Oct 3 '13 at 7:54
  • \$\begingroup\$ @buffer, depends on your laptop and on the phones. Some require drivers to charge. There is no single scenario. Frankly any laptop in the last 5 years that has usb ports that provide less than 500mA each, is garbage. Even most netbooks have full current ports. \$\endgroup\$ – Passerby Oct 3 '13 at 9:05
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    \$\begingroup\$ @Buffer it also depends on the organisation of the USB hardware in the computer, it may not be able to provide 500mA from all ports at once. \$\endgroup\$ – John U Oct 3 '13 at 9:57

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