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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)?
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    If you are using the boosting regulator to get 5V you wouldn't need to put it into the Vin pin, so you won't loose power there. The boosting regulators are also efficient anyway. – RSM Apr 17 '15 at 17:00
  • Keep safety in mind when moving to LiPo batteries. Many people don't know that they are more volatile than your average battery. Research/youtube LiPo fire, they're not common but actually do occur. Lots of people out there charge in a fire retardant lipo bag. Your choice, but always good to be informed. – Kenneth Ito Apr 20 '15 at 21:00
  • Thanks to all the replies! However, I still feel my questions are not actually addressed yet. – Hauke Holtkamp Apr 22 '15 at 7:47
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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.

  • When the lipo is near the 3V range and the uno board is at 16MHz one might need to disable the resonator surely? – RSM Apr 17 '15 at 18:57
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    Or just set the prescaler to /2. For added safety, program the CKDIV8 fuse and set the prescaler in setup(). – Ignacio Vazquez-Abrams Apr 17 '15 at 18:58
  • good point ;) should have thought. +1 – RSM Apr 17 '15 at 19:27
  • You don't mention the DC plug. That should not be used in this case? How can I create a solution with plug? – Hauke Holtkamp Apr 20 '15 at 9:10
  • Isn't the "5V" pin output? Or do you mean "VIN"? – Paul Apr 20 '15 at 10:18
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Sparkfun do the ideal solution for you - their "LiPower" shield:

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

  • Note their warning re a hardware bug that disables low voltage cutoff and the need to implement a software work-around. – Russell McMahon Apr 18 '15 at 0:19
  • @RussellMcMahon Yea, that is a bit of a pain - but with the fuel gauge chip you can guestimate how much power is left, and with a simple P-channel MOSFET circuit you can shut off the power completely when it gets too low. – Majenko Apr 18 '15 at 10:02
  • I just ordered one. What is that bug exactly? – Hauke Holtkamp Apr 20 '15 at 9:14
  • There is a facility in the charge chip to monitor the voltage and if it's too low to trigger a shut-down circuit. They just didn't use it. You can connect the + of the battery direct to an ADC input and monitor the voltage yourself if you want to protect the battery against over-discharge. – Majenko Apr 20 '15 at 9:36
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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.

  • How would I plug the two LiPos together reliably (and portably)? And should I use the Arduino's DC plug? – Hauke Holtkamp Apr 20 '15 at 9:13

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