iPhone 3GS - two AA alkalines - not enough power

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Lucidus
 
Posts: 6
Joined: Sat Mar 23, 2013 8:12 pm

iPhone 3GS - two AA alkalines - not enough power

Post by Lucidus »

MintyBoost v3.0 built, 2 X Energizer alkaline AA batteries installed (just purchased from a major retail store, but a pkg of 10 "on sale", so possibly not fresh).

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

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adafruit_support_bill
 
Posts: 88088
Joined: Sat Feb 07, 2009 10:11 am

Re: iPhone 3GS - two AA alkalines - not enough power

Post by adafruit_support_bill »

Yes. The 3Gs is one of the more demanding iPhones. Standard Alkaline chemistries do not handle high-drain situations well due to high internal resistance. There are high-drain alkalines available - at a price. you should also consider NMH rechargeables which generally perform better in high-drain situations - despite their lower voltage.

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