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I want to use a 7.2V (4200mAh) battery with my raspberry pi but I need to convert the output to 5V (2000mAh), I tried the following formula:

Ω = V / A

but when I try to find the ohms the result is 0.78

|7.2/4.2 - 5/2| = 0.78

How can I find the correct ohm (to use a resistor) to get an output of 5V and 2A?

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This will NOT work.

You cannot just use a resistor. If you try you will destroy your Pi.

You will need a regulator. A switching regulator is recommended.

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Some ready modules do the task. Essentially, same circuit inside as UBEC switch mode DC/DC regulator but added USB socket.

Example, this one max 3A and ok as RPi max demand is less than 3A

6-24v-to-5v max 3A converter

Note other similar looking modules boost voltage (instead of dropping) from 3.7V Lipo cell to 5V

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As the load is RPi and it varies, I recommend using a voltage regulator. For 5V it is L7805. Connect as shown here.

L7805

If you want to use resistors only, try using a voltage divider network.

Vout = (Vin * R1) / (R1 + R2)

If you dont want theory use this calculator. As rule of thump use R2 = 2 * R1 so you get a 4.6V.

EDIT: The best option is using a UBEC switch mode DC/DC regulator

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    Stay right out of that Resistor network garbage. I also want to note that the Pi only requiers 750mA max. sourcing any more may damage your system. But, if you want a 2A power supply you can do it with a pair of 7805 regulators in parallel. Its a bit hackerish, but it keeps the cost low, and works with easily available components. – j0h Aug 5 '14 at 5:27
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Would this be a sufficient solution to this problem ? When the Ni-Mh battery is connected to the power port of this board:

These regulators also have internal diode protection and can handle transient surge currents from 50 A to 100 A.

The operating instructions state that the input voltage through the barrel socket must be between 6.5 V and 12 V. Hence, if you wish to use it to its maximum capability you will need to remain in that range. This is a non-adjustable fixed power supply model, which is good enough for most applications. The manual states maximum output current to be 700 mA. However, it is probably better to use much lower voltages and current to be on the safe side in case you make a mistake on your breadboard circuit.

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  • Please take the tour and visit the helpcenter to see how things work here. I am curios, is this an answer or a question? New questions should not posted as answers... either way the board is not well suited for the Pi due to its limit of 700 mA. – Ghanima♦ Mar 26 '17 at 16:17

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