# No exactly homework, but I need help!

Discussion in 'Homework Help' started by laduch, Jul 19, 2010.

Jul 19, 2010
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I have a 7.4V 2000mAh battery block, and one of my module works only with 3 AAA battery set, which is 4.5V 2550mAh.
So, I'm trying to convert 7.4V 2000mAh battery to 4.5V 2550mAh power source.

I used a voltage divider, but it doesn't seem to work... what should I try next?

Feb 17, 2009
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3. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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That would require a conversion with at least 78% efficiency. A linear regulator won't do that.

A switching regulator might do it. How much design work do you want to do to get to that point?

Finally, the mAH capacity of batteries changes with time and cycles. When you say 2550 mAH, I wonder whether 2500 or 2400 mAH might be enough. If not, then you probably need to reassess your requirements. Something that depends on the battery capacity to be within a 5% is just not going to be too practical.

John

Jul 19, 2010
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I want it to be as simple as possible. I was saying 2550 mAh, because each AAA battery has 850 mAh. 3 of them is used to power my IMote2 device using power pads. on the datasheet it says these on-board pads "can be used to connect any power source supplying a voltage range of 3.2  4.5V (after the diode drop)." There's no mention of current tolerance...
So, as long as it has a possibility of getting it to work, I would try.

5. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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Three AAA batteries in series would give "4.5 volts" (nominal, if NiMH, it is 3X1.2 = 3.6; if alkaline, it is 3X1.5 = 4.5V), but the combined current capacity would be only 850 mAH. In parallel, you would have 3X the mAH capacity but only 1X the voltage of a single cell. In series, it is the other way around, 3X voltage and 1X mAH.

So, back to your original question. A 7.4V/2000 mAH battery can easily be reduced to provide 4.5V/2000 mAH using just a linear regulator like mentioned in post #2. That will give you almost 3X the run time of 3, AAA batteries in series.

John

Jul 19, 2010
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Thanks for your help! I will try that out!

Jul 19, 2010
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Thank you! It worked.

8. ### cristine27 New Member

Sep 8, 2010
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Why don't you try to use a voltage converter or regulator also? Not only the voltage divider? I think it will also depend on the capacity of your battery. I was hoping that it will help on your homework.