Choosing USB Pin Voltages for iPhones and iPads – Update for iPhone 5By Phillip Stearns
We continually make minor tweaks to the USB output of our batteries to make sure we charge as many devices as possible. We pay particular attention to Apple products and now, with the introduction of the iPad, it has become slightly more complicated to have a one size fits all solution. This post tells you the Voltage that we use on each pin in current versions of our USB battery packs. We make these battery packs available standalone and as part of our portable solar chargers. Hopefully, this will be useful if you’re trying to make your own USB charger.
|Pin||Wire Color||Signal Name||Description|
In all cases, pin 1 is 5V (with a little wiggle room, spec allows from 4.75 to 5.25 V, and we aim for the upper end of that range to compensate for voltage drop due to high current draw) and pin 4 is 0V. But, depending on the configuration of the two data pins AND the current draw of your device, a USB output may charge all, some or no devices.
We ended up using the following Voltage configurations for current versions of our batteries:
|Battery||USB Port Current||D-||D+|
|V11 USB Battery Pack||900mA||2.75V||2.0V|
|V39 USB Battery Pack “low-power” port||900mA||2.75V||2.0V|
|V39 “high-power” port||2,000mA||2.0V||2.75V|
|V60 Universal Laptop Battery||900mA||2.75V||2.0V|
Here’s the logic and the tradeoffs associated with each of the decisions.
The V11 and the V60 both have a single output port so we need for those ports to charge every device (note: as of early 2012, all V11s have 900mA output instead of the previous 650mA). Only the 2.75/2.00 V (D-/D+) output charges both the iPhone and the iPad consistently across all models. If we went with 2.00/2.00 then it would charge iPhones but not all iPads. The risk with this configuration is that once or twice in hundreds of tests the iPhone 4 says “accessory not recognized” part way into the charge cycle. The solution is to disconnect and reconnect the charger. We don’t expect this to occur much in the field, but it is the tradeoff we needed so that you will be able to charge your iPad with its own charging cable from the V11. If you get have problems charging an iPad from our current V11 battery, you can use our iPod adapter which sets the right Voltages.
We kept the same 2.75/2.00 configuration for the V60 with its 900 mA output, but didn’t see any issues surface in testing with the iPhone at this higher current.
On the V39, included with our iPad solar charger, we have both a high and low power port so we chose the same 2.75/2.00 output for the low-power port and 2.00/2.75 (the same voltages supplied by an iPad charger) for the higher power port, which is specifically designed for tablet charging.
Update: The iPhone 5 works across all three pin configurations.
Note: A general observation with the Apple products is that they will attempt to draw the maximum current if and only if the voltage level on Pin 1 with respect to the current draw remains constant between 5 and 5.25 volts. Often, if a charger is unable to supply the proper amount of current the voltage output will drop. For instance, a 5.25V 1A power supply may only output to 4.5V when under a 2A load. For Apple products, when a device is presented with any one of the voltage configurations (500mA, 1A, or 2A) and then presented with a range of Pin 1 voltage levels from 4.5V to 5.25V, the actual current drawn by the device varies. In our tests, we found that when the voltage sent to the iPad 2 was 4.5V, the iPad 2 drew only about 1A, but steadily scaled up the current to 2A as the voltage was incrementally increased to 5V. Additionally, we noted Apple devices will not accept charge from power supplies with Pin 1 voltages of less than 4.5V or greater than 5.5V.