I specifically wanted 4xAAs to maximise the energy available for weekends away with the iPhone - and further, from the LT1302 data sheet the efficiency seems to improve with greater input voltage, though the sheet only shows Vin @ 2, 2.5 and 3 V. Given the device's Vin max of 8 V, I figured it shouldn't hurt to feed it 4 x AAs (5.4 V for fresh rechargeables, 6V for alkalines) and see what happens. Below are the results with the MB fed from a variable power supply, loaded with a 10 Ohm resistor (i.e. nominal 500 mA load):
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Vin Iin Pin Vout Iout Pout Efficiency
2.4 1.31 3.14 4.53 0.43 1.95 62%
3.0 1.05 3.15 4.79 0.45 2.16 68%
3.6 0.80 2.88 4.82 0.46 2.22 77%
4.2 0.66 2.77 4.91 0.47 2.31 83%
4.8 0.60 2.88 5.05 0.48 2.42 84%
5.4 0.53 2.86 5.16 0.49 2.53 88%
6.0 0.51 3.06 5.51 0.52 2.87 94%
* Vin/Iin were measured to only one/two decimal places (PSU meter), so all of the above should have nice fat error bars applied
* further errors would have been due to me using alligator clips to hook up my little test rig - not the best way to get a nice low-resistance connection (notice the output current is not quite the expected I = Vout / 10 Ohm)
* the MB stayed cool, warming slightly at the lower input voltages
* output voltage regulation was good, within 10% across the board, under 5% for the rechargeable input voltage range of 4.0 (flat) to 5.4V (charged)
* I measured significantly lower efficiency than the datasheet's claims, at ~500 mA: cf. 62% actual vs. ~82%
* one thing's quite clear: the circuit's efficiency improves with higher voltage, up to at least 6.0 V
Conclusion: 4xAAs are the way to go for peak efficiency. The current drawn from 4 AAs is less than half 2 AAs; indeed the cells should last more than twice as long as the effective capacity of most battery types is usually higher for lower discharge currents (e.g. Fig 5.2.1).
Rgds,
Ben
