Charging worked fine on a 1st generation iPod Nano.
Charging not reliable with iPhone 3GS.
If phone was on when charger plugged in, 100% of time got the "Charging is not supported with this device" message. If phone was off when the charger was plugged in, the phone did not show the error msg and reported charging some of the time (I think it worked when the charger had been left disconnected for a while and the batteries could recover).
Multimeter measurements revealed the problem.
Two of these batteries are not able to supply sufficient voltage and current. As the table below shows, when the device was charging the 3GS, the battery voltage supplied to the charger was only 1.83 volt (dropped to 1.73 in 2 minutes), below the minimum 2.0 volt spec for the LT1302 chip.
According to the spec sheet, the efficiency of the LT1302 is 75% for 2.0 volt input and 500 mA output, and would be lower for a lower voltage. Assuming 70% and 500 mA 5.0 V output, 2.5 watts, then the supply current would be 1.95 Amp at 1.83 Volt.
It is quite amazing that the LT1302 circuit is able to supply 500 mA at 5.0 V with a supply of 1.83 V, but clearly this is not going to last long as the battery voltage will drop before long and the LT1302 will shut down.
I added a third battery, now three AA's in series. Now the charger worked as expected, the 3GS reported charging even if was on when the charger was connected.
The supply voltage when charging was now 3.67 V. The LT1302 spec sheet indicates 86% efficiency at 3.0 V input and 500 mA output current. Doing the math indicates that the batteries are now supplying 0.79 Amp. This is a more reasonable current draw for AA's, and a good input voltage for the LT1302.
Code: Select all
no load IPod iPhone charging for 2 minutes
2 X AA
Vout 4.98 4.89 4.72
pin 2,3 1.995 1.998 1.751
Bat V 2.8 2.53 1.728
3 X AA
Vout 4.92
pin 2,3 1.826
Bat V 3.67