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I will power a Raspberry PI with 4 NIMH batteries, but they quickly lose charge, and end up at 3V instead of the 5V needed. I was wondering about a DC-DC step up converter.

I want a converter that will have a fixed output and will not be affected by the input voltage. E.g. This may transform 3V to 5V, but when the batteries are fully charged at 5V, I want the converter to output 5V too rather than 7V for example which will damage the PI.

I was thinking of using this converter, would this work? XL6009 DC-DC Step Up Boost Voltage Converter Power

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  • \$\begingroup\$ can you use more than 4 cells? \$\endgroup\$
    – hassan789
    Jan 20, 2014 at 2:45
  • \$\begingroup\$ I'm sure you have considered it, but battery life on 4 NiMH batteries is going to be relatively short. \$\endgroup\$ Jan 20, 2014 at 9:27
  • \$\begingroup\$ Note: there's a linear regulator on the board that takes it down to 3.3v again. Some people mod the board to bypass that and inject 3.3V from a buck/boost converter directly. \$\endgroup\$
    – pjc50
    Jan 20, 2014 at 11:52

3 Answers 3

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seems like you answered your own question... a buck-boost would be optimal, but a boost would also do the trick. Here is a simple boost circuit you can build yourself:

enter image description here

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  • \$\begingroup\$ This particular part is in a 3x3mm 10-lead QFN package with thermal pad, not exactly hobbyist-friendly. Also cutting it pretty close on output current capacity, since the PI could require 1A@5VDC, plus whatever gets plugged into the USB ports. Unfortunately, there are few new parts that are easy to work with for a hobbyist (packaging-wise). \$\endgroup\$ Jan 20, 2014 at 1:55
  • \$\begingroup\$ The package definitely presents a problem for a hobbyiest... current wise, you can sync two in parallel for more current \$\endgroup\$
    – hassan789
    Jan 20, 2014 at 2:44
  • \$\begingroup\$ The small inductor is like the Swiss flag-- a big plus. \$\endgroup\$ Jan 20, 2014 at 2:48
  • \$\begingroup\$ Yes, thanks for the diagram it would be a nice challenge to make it. \$\endgroup\$
    – Hamoudy
    Jan 20, 2014 at 14:56
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That part they used is only guaranteed to operate down to 5.0V, which is actually worse than the Raspberry PI. Maybe some of them will work okay, maybe they won't.

XL6009 input voltage rating table

Something based on the LM2577 should work. It actually cuts off at a bit too low a voltage to protect the batteries, so I'd add something external to shut it down at about 4.0V pack voltage. You don't gain much run time from running the batteries down to 0.725V/cell, and series-connected batteries can be damaged.

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  • \$\begingroup\$ Interesting, do you know any converters that will perform this task? \$\endgroup\$
    – Hamoudy
    Jan 20, 2014 at 1:23
  • \$\begingroup\$ Something based on the LM2577 should work. It actually cuts off at a bit too low a voltage to protect the batteries, so I'd add something external to shut it down at about 4.0V pack voltage. You don't gain much run time from running the batteries down to 0.725V/cell, and series-connected batteries can be damaged. \$\endgroup\$ Jan 20, 2014 at 1:51
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You don't need a transformer per se, since you don't need isolation. What you need is a step-up / step-down converter.

Texas Instruments makes the TPS63002 which works from three NiMH batteries and generates a 5V rail at input as low as 1.8V. There's also the TPS63060 family which can accept up to six cells on the input. Other manufacturers make similar parts.

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  • \$\begingroup\$ like a buck boost? \$\endgroup\$
    – Hamoudy
    Jan 22, 2014 at 21:10
  • \$\begingroup\$ Both parts are buck-boost converters. \$\endgroup\$ Jan 22, 2014 at 21:32

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