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Background Information

My college has entered an annual racing competition called Greenpower, and it's been agreed to use a Raspberry Pi as the car motor controller. Since I am the only knowledgeable one in the group where Electronics and Programming are concerned, I'm managing most of this myself.

The regulations have limited the amount of power we are allowed to use to 2x 12v lead-acid batteries, as well as 6x 1.5v AA batteries. We are additionally allowed a 3v coin cell battery. I was planning to power the pi using 3x AA's(totalling 4.5v), and have the other 3 AA's hooked up in parallel to double the capacity. As well as the Pi, there will be 2 other electronics using the battery, including a 3.6v sound system, and a 1-watt amplifier.

On the dashboard there will be 3 status indicators, a Red LED (11mA), a Green LED (1mA), and a Yellow LED (9mA). There is an additional buzzer on board (1mA) which will sound if an error occurs. All these status indicators share the same ground, and have independent inputs with different value resistors (for brightness) and a 10v zener diode. These have all been tested using a voltmeter and a 3.3v power supply. These will be hooked up to the Pi's GPIO ports. These have already been soldered to a board, but another can be made

There will be 2 relays controlling the motor output, only one of which will be active at a time. The relays I've tested so far are 6v relays (3mA), but only activate at 3.45v(Pi may not power them).

There will be 3 inputs: Acceleration, Gear 1(16v to motor) and Gear 2(24v to motor). I am completely unfamiliar with sending an input to the Pi.

The code which will be controlling these has been written already, but has empty spaces where it will check and set inputs/outputs.

The race is in October so there is plenty of time for changes to be made. It is 90 minutes long, and batteries cannot be changed for the duration of the race. If the Pi dies, I'm planning to setup an emergency switch to power the Motor at full 24v.

I am running Raspbian with Mono installed.

What I would like to know...

Regarding power, is there any way I can optimise the current setup?

Additional suggestions and improvements are welcome.

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  • @goldilocks removed the additional questions.
    – TheJamaicanGuy
    Jan 10, 2015 at 15:56
  • Fair enough -- you might be better off putting that particular one on E.E., but someone here may have some thoughts. BTW, I think 4.5 V on the B+ might be a no-go; it has an undervoltage/brown out detector and may just shut off. But I don't know for sure.
    – goldilocks ♦
    Jan 10, 2015 at 16:04

2 Answers 2

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It's hard to see any use for the 6 1.5V cells. They are unlikely to reliably power the Pi for more than a few minutes.

The 3V cell might be useful if you want to attach a RTC to the Pi so that it can know the time when it is powered up.

I'd power the Pi and everything else from the lead acid cells.

A UBEC is an obvious candidate to power the Pi. It takes in a varying voltage (something like 6-27V) and converts it to a stable 5V at several amps.

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  • Yea, completely agree. Use the lead acid batteries only. The AA batteries wont help much. Although find and use an SBEC instead since its more efficient for 5V to the Pi and any other relays and things. If the Lead Acid runs out of power then that's it, no power to motors so no power to Pi will help. The only way to optimise the power is to build your own power efficient power supply and that is NOT easy.
    – Piotr Kula
    Jan 10, 2015 at 19:09
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As cool as this project sounds... What exactly would the Pi be doing in this setup? It would appear that a few smart switches would accomplish the same thing? The reason I say this is not to just be negative on the project so far because I think it is awesome to work on something like that!!!

But the Pi will draw quite some power and it would seem preserving power is your first and major concern..??

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  • The pi is there to allow for better expandability, e.g. adding sensors, or even an LCD. A simpler example would be a timer on the Pi running when the car is at full speed. When that timer runs out, the Pi will automatically slow down the car to relieve the strain on the lead acid batteries.
    – TheJamaicanGuy
    Jan 13, 2015 at 19:13
  • I see the possibilities you could have as soon as you have "intelligence" inside... I'd use an Arduino for such tasks, much more lightweight... but harder to program and debug I'm afraid... Still I'm afraid the Pi will consume too much power to run from AA's for such a long time (easy to try though)... Maybe underclock it?? You probably don't want to run the Pi from the acid batteries..??
    – Erik Z
    Jan 13, 2015 at 20:22
  • I've tested it on 4.8v NiMH batteries and it topped out at 2 hours 20 mins, with CPU usage at maximum. That is long enough for the race, but taking into account other things that may be using the AA's (safety lights & an onboard sound system) the battery may not last as long. I'm considering getting a dc buck converter to provide a steady 5v for the pi, since I've read somewhere that a 5v regulator isn't power efficient.
    – TheJamaicanGuy
    Jan 13, 2015 at 21:13
  • If you can feed the Pi a direct 4.8V you may be fine (if the battery actually STAYS at 4.8V). you cannot get more efficient than a direct connect. A buck converter is great when you for example need to use 12V and drop it to 5V... But 2:20 is pretty ok, how many mAh was this 4.8V NiMH cell?
    – Erik Z
    Jan 13, 2015 at 21:18
  • About 2000mAH,,
    – TheJamaicanGuy
    Jan 13, 2015 at 21:47

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