user8220: I want to switch from using disposable 9V blocks to a proper setup of 3.7V LiPolys for all my portable Arduino (Uno/Mega) projects consisting of:

only 3.7V LiPo units
charger
upconverter (to 5V)

My questions:

Is this generally a good idea? Or is there a better solution for portable Arduino power?
How should I actually hook the LiPolys up to the Arduino? They usually ship with JST connectors. Should I use the Vin/GND Pins or should I build my own JST/DC-Connector adaptor? I want to create a robust, pluggable solution.
Am I actually losing power twice that way (once in the upconverter and then in the Arduino voltage regulator)?


user4143: Sparkfun do the ideal solution for you - their "LiPower" shield:

https://www.sparkfun.com/products/10711

It contains the Li-Ion / Li-Poly charger, 5V boost converter, and MAX17043G+U fuel gauge chip.

user6959: Consider using 2 x LiPo is series (2S). Vmax is 2 x 4.2V = 8.4V
This may be attractive compared to boosting from a single cell.
This is still < the 9V you used previously.
Vmin is say 2 x 3V = 6V - but higher is better for the LiPos.
You can use a buck converter or a linear regulator as desired, and could use one or other depending on project.
You could even use linear + buck in parallel for applications where sleep current is very small and buck converters may find efficiency at very low I challenging.
Using 5V rather than less than 5 is attractive in that you do not need to ensure the processor speed will accommodate the lower voltages.
Using a linear regulator and 5V:
Efficiency at 4V/cell = 5/8 = 62.5%.
Efficiency at 3V/cell = 5/6 = 83%.
Any sensible buck regulators will be more efficient at more than a few mA but at very low currents that may be acceptable. 
2 LiPo in 2S will notionally need balancing occasionally but this is easy to do informally occasionally or using any of the many available charging ICs.

user368: If you can get away with it, I recommend not using a boost converter and instead connecting the battery directly to the supply voltage input (the "5V" pin).
The AVR chips used in most Arduinos (but not the Due or Zero [and the Yún and Tre are a totally different kettle of fish entirely]) can operate with anywhere between 2.7 and 5.5 volts, with the ATmega328P found in the Uno able to run with as little as 1.8V. This is a much larger range than the 3.0-4.2V available from rechargeable Li-ion/Li-poly/LiFePO4 cells.
Naturally you will need to make sure that any external components can run with the reduced voltage as well, but quite a few are able to work with anything from 2.0 to 5.5 volts.

8220: <p>I want to switch from using disposable 9V blocks to a proper setup of 3.7V LiPolys for all my portable Arduino (Uno/Mega) projects consisting of:</p>

<ul>
<li>only 3.7V LiPo units</li>
<li>charger</li>
<li>upconverter (to 5V)</li>
</ul>

<p>My questions:</p>

<ul>
<li>Is this generally a good idea? Or is there a better solution for portable Arduino power?</li>
<li>How should I actually hook the LiPolys up to the Arduino? They usually ship with JST connectors. Should I use the Vin/GND Pins or should I build my own JST/DC-Connector adaptor? I want to create a robust, pluggable solution.</li>
<li>Am I actually losing power twice that way (once in the upconverter and then in the Arduino voltage regulator)?</li>
</ul>
 4143: <p>Sparkfun do the ideal solution for you - their "LiPower" shield:</p>

<ul>
<li><a href="https://www.sparkfun.com/products/10711" rel="nofollow">https://www.sparkfun.com/products/10711</a></li>
</ul>

<p>It contains the Li-Ion / Li-Poly charger, 5V boost converter, and MAX17043G+U fuel gauge chip.</p>
 6959: <p>Consider using 2 x LiPo is series (2S). Vmax is 2 x 4.2V = 8.4V<br>
This may be attractive compared to boosting from a single cell.<br>
This is still &lt; the 9V you used previously.<br>
Vmin is say 2 x 3V = 6V - but higher is better for the LiPos.<br>
You can use a buck converter or a linear regulator as desired, and could use one or other depending on project.<br>
You could even use linear + buck in parallel for applications where sleep current is very small and buck converters may find efficiency at very low I challenging.</p>

<p>Using 5V rather than less than 5 is attractive in that you do not need to ensure the processor speed will accommodate the lower voltages.</p>

<p>Using a linear regulator and 5V:<br>
Efficiency at 4V/cell = 5/8 = 62.5%.<br>
Efficiency at 3V/cell = 5/6 = 83%.<br>
Any sensible buck regulators will be more efficient at more than a few mA but at very low currents that may be acceptable. </p>

<p>2 LiPo in 2S will notionally need balancing occasionally but this is easy to do informally occasionally or using any of the many available charging ICs.</p>
 368: <p>If you can get away with it, I recommend not using a boost converter and instead connecting the battery directly to the supply voltage input (the "5V" pin).</p>

<p>The AVR chips used in most Arduinos (but not the Due or Zero [and the Yún and Tre are a totally different kettle of fish entirely]) can operate with anywhere between 2.7 and 5.5 volts, with the ATmega328P found in the Uno able to run with as little as 1.8V. This is a <em>much</em> larger range than the 3.0-4.2V available from rechargeable Li-ion/Li-poly/LiFePO4 cells.</p>

<p>Naturally you will need to make sure that any external components can run with the reduced voltage as well, but quite a few are able to work with anything from 2.0 to 5.5 volts.</p>
