In a recent post, Cell Phone Charging Hack with Household Batteries, I described how you could use household batteries to charge your cell phone. It takes about 16 kJoules to charge an iPhone 6 cell phone. To get a full charge, you need to ensure you use enough battery energy to charge the iPhone. However, for iPhone and iPad, in addition to the energy source, there are secret impedances that need to be set up on the USB D+ (green wire) and D- (white wire) data pins to enable the fastest charging speed. Otherwise, your charging current will be just 80mA and will take days to complete.
The iPhone and iPad charging secrets described below are as follows:
- The maximum charging current occurs when the D- pin is at 1.8v and the D+ is at 2.6v with the correct pull-up resistors to 5v and pull-down resistors to ground. This is approximately 10% faster than the factory iPhone charger.
- The iPhone/iPad will charge at the 1amp rate when the D- pin is at 2.6v and the D+ is at 1.8v with the correct pull-up resistors to 5v and pull-down resistors to ground.
- The iPhone/iPad will also charge at the 1amp rate when the D- pins are shorted to the D+ pins
- The iPhone will drop charge current if the input voltage drops below 4.5volts and will latch at a lower rate at approximately 4.2v.
In order to analyze the current draw for the iPhone and iPad devices, I modified a USB cable so that I could monitor the current draw and the D+ and D- data lines. The smallest resistor in my lab was 0.68ohms, so I placed about 8 in parallel to achieve a 70mohm sense resistor for the current draw monitor network. Here is a picture of the setup.
Three different chargers were investigated. The factory iPhone and iPad chargers and a third party charger purchased from Amazon
Charging the iPhone 6 and Ipad at different rates
The schematics below show the correct resistor impedances to charge the iPhone and Ipad at the normal 1amp rate and at the fastest rates possible. The iPhone 6 will charge at approximately 10% faster than the factory charger rate when using the >1amps setting. You will also get this same 10% boost when charging from the 2.1amp iPad charger.
I measured currents of around 1.1-1.2amps when using this fast charge setting with the iPhone, while with the iPad, I measured currents up to 1.9amps.
The Third party charger had a simple impedance circuit where the D+ and D- pins were shorted (like the center schematic shown above), but they also added a 43k resistor from the D+/D- shorted net to ground. In my testing, this was not needed, but won’t hurt and may be safer for the phone.
From experimentations, there are several other resistor configurations that allowed for charging the iPhone 6 at the fastest current rate. However, this was not true when trying this with the iPhone 5. This table below was only verified with the iPhone 6, but failed on the iPhone 5. In this table, R7 was opened (not used) and R6 can be any value between 30k and 82k.
- R5 = 82k; R6 = 47k; R7 = Open ; R6 = 30k-82k
- R5 = 82k; R6 = 24k; R7 = Open ; R6 = 30k-82k
- R5 = 82k; R6 = 21k; R7 = Open ; R6 = 30k-82k
- R5 = 82k; R6 = 13k; R7 = Open ; R6 = 30k-82k
Less than 1 Amp charging
At startup, the iPhone will sense the Vcc voltage and adjust the charging current accordingly to keep the VCC voltage above a 4.2-4.5volt value. In this way, it can use chargers that limit the current to 500mA or below. To test this out, I first started the iPhone at 1amp charging rate (actually around 900mA) at Vcc equal to 5v. When the Vcc voltage to the iPhone was lowered below 4.5volts, the charging current starts to drop below the 1amp value. If this voltage drops below 4.2volts, the iPhone will drop into a 500mA rate even if the Vcc voltage returns to 5v. It will also drop to lower rates but will turn off somewhere below 4v.
How the iPhone and iPad determine the charging rate
The iPhone must provide handshaking through a reading of the D+ And D- signal line impedances with the charger device to determine what rate to charge at. It does this right after power up. While all the circuits above work as described, the exact handshaking is unknown. The image below shows the handshaking right after power up of the D+ and D- pins with R5=82k, R6= 49k, R7= open, and R6 = 38k.
Once power is applied, the D- line rises to 1.6volts because of the R5 and R6 resistors. The D+ line remains at zero voltage because R7 is open. After some time, the D- line is disturbed and the handshaking begins. My best guess is the following occurs:
- The D+ and D- lines are samples by the iPhone to determine the voltage. If they are within the normal iPhone and iPad charger values (2.8v and 1.8v ), then startup occurs. If they are not, then 2.
- The D+ and D- are placed into another state. the figure above shows that the D- line drops from 1.6v and the D+ line is pulled up. The iPhone then determines if the voltage and current draw are appropriate.
Here is handshaking that occurs when D+ and D- lines are shorted together
In this case, the D+ line is not pulled to 3v after the handshaking as was done in the previous case. Instead, the voltage returns to zero volts and the iPhone and iPad charge at the 1 Amp rate.
This post identified several iPhone and iPad charging configurations and how to set up the values to allow them to charge at their maximum rate. If the current is too much for a given charger and is current limited, the output voltage will drop, putting the iPhone into a lower charge rate.
Have fun and let me know if you have any comments and questions.