Lithium Ion Battery Power

All of my headless projects use Lithium Ion batteries.  These are made up of 1 or more battery cells which each produce 3.7V and potentially many amps in a small, light package.  Their only downside is you need 5v to run a Raspberry Pi, and so need to add some form of regulator.  My three projects have 3 different power requirements, and use 3 different regulator circuits which I thought I’d share:

  • Turtle needs a 5V supply able to supply up to 3A for the stepper motors.  This means starting with a 2 cell combination delivering 7.4V.  However delivering 7.4V at 3A means 2.4V (7.4 – 5) @ 3A is wasted as heat – given it’s a recharchable battery, I don’t really want that level of waste (nor heat melting the Lego Turtle!).  So the turtle uses a low drop out, switched mode regulator LM2596T-5.0 with the circuitry as suggested by the data sheet (a schottky zener diode, a couple of electrolytic capacitors and a 33uH 2.5A inductor).  This has proved perfect in all but one respect – the LM2596T has 5 pins packed into a TO-220 package – that means the pins are closer that the standard 0.1″, meaning they need to be bent carefully to fit.  I broke one doing so, so caveat emptor!
  • SkySpy has an 11.1V LiPo (Lithium Polymer) battery to supply up to 30A to the blade motors.  So the low-drop out is not needed in this case, but since the drop-out is now 6.1V at up to 1A for the Raspberry Pi, I still don’t want to waste the power; once again another switching regulator steps in: 78SR105HC. this one is not cheap, but it’s efficient, has only 3 pin (like a TO-220 plus heatsink) and lies flat on the breadboard which is critical when you have helicopter blades whizzing round.
  • Finally for the SkySpy controller, this is using the same battery as the Turtle, but doesn’t need anything like the high current, so the wasted power will be minimal, so I’ve gone for a non-switched regulator (LM2940T-5.0) which has feedback circuitry to minimize the loss in high drop out situations

All of these came from Farnell as always!

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