solar powered BS2

billy1180

Here's the situation. I am using a rev. C BOE dev. board and I'd like to have it connected to a gel cell battery that is recharged during the day by a solar panel. I know that the BOE board can be powered with a 12 VDC 1 Amp power supply and, since we bought a lot of our sensor equipment from Dr. Allen of EME systems, we decided to go with a 12 VDC 1.2 Amp gel cell rechargeable battery, found here: http://www.emesystems.com/batcharg.htm#gelcells, and an AE5 solar panel from alternative energy engineering, which is similar to the SM5 solar panel (5 W 12 VDC) found here: http://www.emesystems.com/solar.htm.

So, I have the following questions.

Can I hook the 12 VDC 1.2 Amp rechargeable battery directly to the BOE development board without any problems? I can't see why the extra .2 Amps over what Parallax says to use would hurt anything, after all how much current is drawn is dependent on the load (in this case the development board) and not the battery. However, I'd rather be on the safe side and ask around first.

How would the solar panel figure into this equation? Can I simply connect it to the rechargeable battery? I saw on another forum here that I would need a diode to block the solar panel from drawing current from the battery, and I could do that without difficulty, but is there anything else I might need?

In any event, thanks for all of your help. This forum has been an invaluable resource along the way for our senior design along with Dr. Tracy Allen's page: http://www.emesystems.com/ as this page enabled us to get working soil moisture sensors and rain sensors without difficulty. It's a good site if one is going to use the BS2 for data logging and environment purposes. However, I come here for anything and everything Basic Stamp.

Louis Blessing

metron9

Some random thoughts I had on the subject.

With a solar powered battery you may want to also incorporate a way to read the voltage of the battery so you can shut down the unit if the battery falls below 80% charge.

I would use another chip to monitor the battery and supply the power via a mosfet to the system, that way you can draw microamps while solar charging and when the unit is not in use.

Since the solar panel can generate 17V you should also use a regulator in case the battery is not installed and the solar panel produces 17V (unless the BOE has regulators that can handel that voltage, i am not familiar with the BOE bots)

Looks like pretty high prices there as well. Sparkfun has panels at 9.15 volts 280 ma for 35 bucks, two of those would supply 18V at 280 ma for 70.00 the one you are looking at is twice the price. I have both sizes and they work very well.

I would look at a higher storage capacity battery as well. At 1.2AMPS storage and a 270MA charge rate (actually hiher as this is at 17V rated and you are charging only 12V) you may have more solar energy capacity to store that will get wasted.

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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Phil Pilgrim (PhiPi)

Louis,

In your situation I would certainly consider a DC-DC converter to convert your 12V to 6V before it enters the BOE. This would almost double the power efficiency of your system and, perhaps, let you use a smaller and less expensive solar panel. Here's an example of one that Mouser carries: www.eta-usa.com/pdffiles/dcdc/oc1-sc1224.pdf

-Phil

Tracy Allen

Hi Louis,

Thanks for your kind words!

There is no problem in using a battery with larger amp-hour capacity. The issue with any solar powered system is to add up the total integrated current that the system will draw 24/7 and balance that against total current that can (best guess) be provided by the solar panel. It can be a sobering exercise in worst case thinking. Does it have to operate on short winter days? How much mission non-critical stuff can be shut down at intervals to conserve power?

One thing about the BOE is that its voltage regulator and its LED draw quiessent current even when they are not doing anything. I forget the exact figure, but suppose 10 milliamps. What is the power budget of your system, for Stamp operation, sensors, other devices, and ratio of "wake" to "sleep"? From your description, it sounds like the real power requirements are quite low, for data logging and monitoring. In that case, it might save a significant fraction of the power budget to disable the BOE LED, and run the project off of the Stamp's onboard LT1121 regulator, which is very efficient at low currents.

The battery should have a charge regulator, to clamp the battery voltage at the float level of around 13.8 volts, or certainly no more than the fast charge value of 14.2 volts. The danger is overcharging the battery, drying it up and generating (explosive) gas. There are a number of ways to regulate the charging and it could even involve the stamp in a feedback loop to monitor the battery voltage.

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Tracy Allen
www.emesystems.com

billy1180

metron9,

"I would use another chip to monitor the battery and supply the power via a mosfet to the system, that way you can draw microamps while solar charging and when the unit is not in use."

That sounds like a great solution! How would I go about doing this? As far as using another chip, could I just use a basic stamp II i/o pin to measure RC time (which appears to be another way to do this from this site: http://72.14.209.104/search?q=cache:t0kjcrxEAtoJ:ece-classweb.ucsd.edu/eureka/abstracts/corax_abstract.doc+"solar+panel"+to+basic+stamp+mosfet&hl=en&ct=clnk&cd=2&gl=us&client=firefox-a). They are using a mosfet as well, but I didn't see any circuit diagrams and am a little lost as to how to do this. Any advice would be great. Thanks.

Louis

Tracy Allen

Hi again,

There is further info about using RCtime for battery monitoring at,
www.emesys.com/BS2rct.htm#B_voltage

I'll see if I can come up with a schematic for a simple Stamp controlled charging circuit. My first thought using a ground side n-mosfet won't work.

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Tracy Allen
www.emesystems.com

Post Edited (Tracy Allen) : 5/8/2007 8:36:46 PM GMT

metron9

Hey Tracy, Since you are only dealing with 270mA Why not just test the battery voltage with the rc circuit. If it is full charge turn on a resistor with a mosfet to bleed the current. I think you can use the solar panel side to test as the voltage drops close to the battery charge level. That way you wont be bleading any current when the sun is not generating power.

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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Tracy Allen

Right, that's an easy solution. I think I'd want a shottky diode to isolate the panel from the battery, and a power n-mosfet like an IRL530 with drain to source directly across the solar panel. It would not need a resistor, as the the mosfet could easily absorb the 270 ma at near zero volts, zero power. The Stamp brings the gate high to turn off charging when the voltage gets above 14 volts, and brings the gate low to enable charging when the battery voltage drops below 13.6 volts. The charge regulation depends 100% on the Stamp, and if the Stamp loses its mind, the battery might be overcharged. I usually use a low dropout, micropower regulator as the basis of the charging circuit.

Solar panel +   --o---->|-------- battery positive and Vin for Stamp
                       |
                       |
                       `-|| IRL530
                           |           1K
                       ;-||---o--/\/\------------gate control
                       |        |
                       |        `--/\/\---;
                       |             1M    |
                       |                     |
Solar panel - ----o-----------------o----------common, Vss

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Tracy Allen
www.emesystems.com

billy1180

Dr. Allen and metron9,

Thank you for the insight into this problem! If I might ask a rather strange question; how did you guys become so good with electronics? It seems to be one thing to be able to get all of the problems right in any circuit analysis or electronics course, but an entirely different thing to have this intuition that you two have with electronics. We used Microelectronic circuits by Sedra and Smith for our electronics courses at the University of Cincinnat, but I feel like knowing that book is completely useless when it comes to practical electronics. Any advice on how to gain this "electronic intuition"? I imagine the answer is just practice, practice, practice, but it would be wonderful to have this skill and I am just wondering how to go about attaining it regardless of the time and effort it may take.

Louis

metron9

I just kept buying chips and components I did not know how to use and puttered around with them, asked lots of questions here and at avrfreaks when I had problems. Now I have 6 cabinets with many drawers full of components. Components are like software commands or lines of code, once you know what they can do you can weave them into circuits and logic.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Tracy Allen

I started with electronics in early high school with ham radio and experimention (which, miraculously, I survived). EE was my major in collage, where like you I learned all about 1 farad capacitors in parallel with 1 henry inductors and 1 ohm resistors and ideal op amps. It may sound terribly impractical, but quite the opposite when you start to think in terms of networks. Questions of which direction the electrons flow and which end is the source and which is the drain become far less significant. But you still have to become familiar with real components by building things and learning to troubleshoot, troubleshoot troubleshoot. In grad school, I got deeply into what happens when different electronic or biological oscillators are coupled together. And I was doing instrumentation for my own projects and started building things for others, and that is really what I've been doing ever since. Reading is a big help. Get ahold of the trade magazines like Electronic Design or EDN. Learn how to read data sheets. A lot of the books out there are good. Come at things from lots of different angles. I grew up on the "Cookbooks", TTL, CMOS, Op-Amp, IC-array. If you don't have a copy already, get ahold of Horowitz and Hill, The Art of Electronics.

I don't want to get too far off topic. There is an "old school hackers" thread in the sandbox forum you might find interesting, how people got hooked into this field.

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Tracy Allen
www.emesystems.com