DC to DC Converter

Andrew and Morgaine,

It is kind of easy, but it all depends on how much electronics you're into. The trickiest part is if you want to be able to charge the batteries while using the R-Pi.

So, let's drill into it! Powering the R-Pi:

(1) 12 volt sealed cell battery.

This is the most easy one: a linear regulator (LM7805) with a heat dissipator and 2 bypass capacitors should do the trick.

(2) 2 to 4 cell lithium packs

It depends... with the packs in series  (2S or 2P2S) you have between 5.6V and 7.3V. In that case, the voltage is too low for a 7805.

The circuit is almost the same, but using a LP2954-5V or a LM2940CT-5.0, the last one being more common. A 2S2P pack would drain in around 1-2 hours.

(3) perhaps even a set of AA batteries.

I would go for 4 AA batteries and a LM2940CT-5.0. The problem is that they will discharge very quickly and the system will shutdown.

The most beautiful solution I can think of is a 2AA pack running thru a LT1302 boost converter (see the Minty Boost http://www.adafruit.com/products/14). The main problem is that they would drain in around 1 hour...

Thank you Rafael.  It looks like the AA solution is generally "out", unless it's "just for fun".  But I will check out the MintyBoost.  It sounds like fun.

I do have four packs of 2150mAh, 20C, 3 cell, 11.1v lipo packs.  Would one (or more in parallel) of these work with the LM7805ACT.LM7805ACT.?  Is there a cut off voltage?  I don't really want to kill the lipos on the first discharge.

For the 12volt sealed, it is a small one with 1.4 Ah.  However, I may graduate to something bigger.

I would like to charge the batteries while running the RPi.  I have a small solar unit that I was hoping to use to get me started.  (1.8 watts/ 125mAmps Max, current 125mAmps at 15v, I see 16 to 25 volts on a meter in full sunlight.)

I was going to ask about the solar charger in a separate post, but it seems the overall project should be considered.  For my first project, I want to run an RPi outdoors, with a web cam and motion detection software.  I would use IR at night.  I'd like to run it off battery at night and solar/battery during the day.  Later, I'd like to network it (WiFi, Bluetooth, or modem) with another RPi unit for image storage on a common drive. 

I don't have a clue how to calculate the overall drain.  While I used to be able to read a schematic, calculate loads, voltage drops over cabling, etc. that was MANY years ago.  Trust me, I've forgotten most everything except how to follow instructions :-)

If you will lend some guidance, it will be greatly appreciated.

Thanks in advance,

Andrew

Morgaine,

  the education market may have taken a bit of a hit after Eben's quote today:

"There's nothing specifically educational about this device. The only reason it's applicable is it's cheap."

http://www.pcpro.co.uk/news/education/375106/founder-no-raspberry-pi-for-every-student

Haha, good ol' Eben.

See, you can tell he's an engineer, he's being accurate rather than strategic.  PR is basically about lying, no matter how generously you try to slant it, and engineers are not good at lying.  In an important sense, it conflicts with the engineer's "religion" or core principles.  This is why engineers should never do PR.  The lying is best left to those who lack the engineer's respect for honesty.

That said, he could have been accurate while still promoting Pi for education, simply by following his "There's nothing specifically educational about this device," with "except in the sense that it is mostly open source and hence inherently educational."  That's still accurate.

And, if his audience showed signs of wanting more, he could have added "However, what does have a strong educational element is the motivation that brought this device into existence, because Raspberry Pi was developed for the specific purpose of providing a widely accessible platform for education in computing."

That might have been a bit more encouraging, without descending into manipulating reality for PR.

Morgaine.

When I wrote "This is why engineers should never do PR", I did of course mean "if you want to preserve the current social order".  In contrast, if what you want is a better world, then only engineers should ever do PR.

- "These shoes were made by slave labour with the cheapest materials we could find.  They have a year's guarantee by law, but we really don't expect them to last that long.  And you really shouldn't be wearing shoes with these ridiculous heals, they're bad for your health and my employers should be ashamed."

Andrew,

Check Roger's solution! This is a better solution, as switched regulators are much more efficient. I didn't point it out, because designing one is not easy, but since we have them ready at ebay...

This converter is a step-down converter, so you must have an input voltage well above the 5V. Somewhere above 6V should work, as Roger pointed out.

As for the uninterruptable charging, the solar minty boost might shed some light. It uses only one cell and a step-up converter, so you might have to change it.

As for the endurance, let me explain how you calculate it:

1) The R-Pi uses 5V at 700mA, leading to a 3.5W power consumption

2) All your batteries offer 11.1V, with 6450mAh total charge (in parallel), which goes to 11.1*0.645=71.595Wh available energy

3) Dividing the available energy by the power consumption: 71.595/3.5=20.45h. But this implies in 100% efficience.

4) The converter has a minimum efficience of 85%, so the final endurance must be: 20.45*85% = 17.39h or 17h 23m 15s.

I think it might work for your application...

Your calculations miss that the 'pi uses only 440mA on average. So there is only 2.2W of power used by the 'pi. (on the other hand, a "worst case" calculation is also worth doing).

The 700mA minimum is including the 2x 100mA for the downstream USB ports. If you're not using anything there, the maximum drops to about 500mA. You'll be a bit closer to that than my 440mA measurement if you are running full-HD video decoding on the GPU..... which you probably won't as I don't see you powering an HDMI screen from that battery......

On the charging front, the currently suggested solar panels are too small. They would provide almost enough to power the 'pi in ideal conditions. But no margin to charge the batteries during daytime so that you can survive the night. You need about 3x more solar power to survive in summer and 5-10x (wild guess) more in winter.

I usually do the worst scenario, since I don't want to estimate 27h (using 440mA) or 24h (using 500mA) and be surprised by a low battery after 17h of usage, because a wifi adapter and a webcam went nuts!

Anything beyond 17h is profit! ;-)

BTW, I think that, if Andrew is using it with a webcam and a wireless link, he will need a powered hub! Which lead us to even more consumption...

Morgaine,

   Sometimes engineers can be very effective at marketting because

they are believed even when they should be challenged, just like a discount

store can be very profitable because shoppers tend to mistakenly assume that

all their prices are equally discounted.

   Eben is asked what the greatest challenge currently is, and says its ramping

up volume.  This may be true, but if unchallenged, tends to lead one to believe

that he thinks the design is solid and all that's necessary is more production. 

But I think this theory ought to be challenged.

Wow.  Thank you guys.

Okay, I was just going at this project laissez-faire, but with all this help and to be fair to you then I really should spec out the requirements and at least the high-level use cases.  That is not something I have done.  I've got a big project now at work and it may be several days before I can even find some free time at home to do this.  But let's keep the dialog going, as knowing what is reasonably possible allows me to write requirements that don't have to be revised for workarounds.  Anyway, let me know if I'm on track here...

• Solar Panel (will most likely need to increase the capacity).
• Minty Boost to charge LiPo batteries.
• LiPo batteries
• Battery Eliminator Circuit to prevent over discharging the LiPo batteries.
• L2596 module to control the voltage to the Circuits 1 & 2.
• Circuit 1 - PiPower to power the RPi (later wireless something).
• Circuit 2 - Other peripherals (at least the web cam  and probably also some IR LEDs for night).

Perhaps we need to consider a method that will allow the entire system to restart in the case where the batteries have discharged to the point of shutdown but then after time the solar has recharged the batteries enough to support operation again.  (I'll put this in the requirements.)

Roger, yes, I'll take you up on the beta PiPower.  But I'm thinking I should wait until we have the full strategy defined and parts identified before I start ordering.  What do you think?

Thanks again guys.  Aside from me needing to properly define the project and write the requirements, what are the next steps?

Regards,

Andrew

High level goals of the project -> strategy (what components). -> box diagram.

What are you planning for "power strategy"? Are you going to be able to conserve power by shutting down the 'pi for some lenghts of time?

Farnell's Q1 financial results are out (covering Feb-Apr)

http://www.premierfarnell.com/content/results-first-quarter-financial-year-ending-03-february-2013

RPi is mentioned under "Strategic Highlights" as bringing in new customers, but no mention of

bringing in profits.

• "Continued progress as a digital enterprise, with 2.7m visits in the quarter to our element14 community as it played an important role in the global launch of Raspberry Pi which will add in excess of 60,000 new customers for the second quarter."

Farnell's Q1 financial results are out (covering Feb-Apr)

http://www.premierfarnell.com/content/results-first-quarter-financial-year-ending-03-february-2013

RPi is mentioned under "Strategic Highlights" as bringing in new customers, but no mention of

bringing in profits.

• "Continued progress as a digital enterprise, with 2.7m visits in the quarter to our element14 community as it played an important role in the global launch of Raspberry Pi which will add in excess of 60,000 new customers for the second quarter."