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I tried using 3 AA NiMH batteries (total DC V=4.2, 1600mA) as a backup/charger for my HP Ipaq 612c battery. I inserted a resistor (4.7 ohms) in the circuit to reduce the current. The red light charging indicator of Ipaq blinks intermittently when I attached it using the usb cable. Is there really a problem/error in doing this? thanks.

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Hello,

You built one of these too, eh?

Yes, there can be a problem doing this. First off, 3 AA batteries does not equal 4.2 volts, they are 1.2volt nominal so 3 * 1.2 = 3.6volts. When they are just off the charger, terminal voltage will be closer to 1.5v, so 1.5*3 = 4.5 volts.

Using ohms law, E=I * R, we can find your current as I = E / R or I = 4.5v / 4r7 as .95Amps (almost a full amp) that you would see worst case. The resistor is doing nothing for you.

The reason that the charge light blinks intermittently is because the charge controller (lithium battery, right?) is pulsing due to low voltage.

Try this: Take *6*, yes, 6, AA batteries, your choice of chemistry, and put them in series. That will give you a nominal (alkaline) voltage of 9v. Connect a low dropout 5v regulator such as an LT1117-5. Then you will be able to use your battery backup much more effectively.

On a different note, I used 4xAA (NiMH) and powered my ipaq 3650 directly. Worked great. Your mileage may vary though.

Posted on Aug 27, 2010

Testimonial: "Thank you so much krbjmpr. BTW, am suing NiMH too and the Ipaq battery is rated 3.7VDC 1590mAh. Will surely try your suggestion. Thanks again."

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