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I am looking to make a dog feeding mechanism that will operate regardless of power outages. The main point of this query initially is to figure out how to work with the materials I have (or can get) to store enough power to run a small Arduino or two for a non-predetermined amount of time.

I have a small collapsible solar pack that is 5v 8 watts, made for slow charging phones and stuff camping. I have started experimenting with Super Capacitors and currently have 6, 2.7v 500F Samwah (MH47765) caps. I also have a rather large supply of 18650 Li-Ion batteries.

I am having a problem understanding how to balance charge my supercaps. I am also unsure how to approach power distribution between each segment. I have a small converter that takes 0.9v-5v and outputs 5v at approx 96% efficiency(claimed). I have the necessary parts for a lm2596 5v switching voltage regulator circuit. I may have pre-assembled ones out of china as well as enough to make 7 or 8 more.

Would it be prudent to regulate the voltage or the solar cells to the caps with the small 5v buck converter or better to do that after the caps? if I use the lm2596 it seems like it would be just as good to use the arduino's onboard voltage regulator? The Diodes have have from the lm2596 are 5amp Schottky diodes; are they appropriate for balancing purposes?

The idea of charging two 2.7v caps in series to 5v and then when full, perhaps use a relay to switch to another pair maybe another pair after that. This would maximize power harvesting from the sun. Then once two pairs had charged I could use a relay to put two pairs into a series at 10v making the use of the onboard or lm2596 voltage regulators work, possibly utilizing extra energy to charge some of the li-ion batteries as well?

The system would need to be able to run a real time clock, or could be simplified to work with the sunrise and set. It would need to trigger the feeding mechanism twice a day. So I should have lots of "extra" stored power to pour back into Li-Ion batteries.

The problems:

I live on an island (Guam) and we get very bad storms and typhoons. The power system on the island is antiquated (WWII Diesel)and we suffer from rolling blackouts frequently and prolonged outages during stormy periods. I goal is to ensure food is provided to two 95lbs (44 Kilograms?) German Shepherds. I have to travel off island for medical and if a bad storm hits the pet sitters may not be able to get to them.

This doesn't talk about the feeding mechanism side of this but I figured one step at a time. I am happy to add this for those who are curious or if anyone thinks it may affect the approach so far, but I am trying (and failing) to keep this shortish.


Any ideas, creative alternatives, or any help understanding my options would be very much appreciated!

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    \$\begingroup\$ I would use a big car battery. What current are we talking about? A 12V 72 Ah car battery switched down effectively to 5V would be able to supply 1A for about a week. And charging it is already solved. \$\endgroup\$ – Szidor Jan 30 '16 at 18:26
  • \$\begingroup\$ @Szidor his PV panel is a 5V model, not one of the 12V car batt. trickle chatge ones. -- Preator, charging all series pairs of 2.7V supercaps in parallel (with small shunt/balancing resistors) @ once will more efficiently harvest solar powe than switching from pair to the next, then the next. \$\endgroup\$ – Robherc KV5ROB Jan 30 '16 at 18:41
  • \$\begingroup\$ Yes, I know, but if hes not away for weeks, it may just solve his original problem without the complexities of the solar panel, the caps, ... I was not sure whether the project is for the problem or the problem for the project :) \$\endgroup\$ – Szidor Jan 31 '16 at 13:49
  • \$\begingroup\$ You have a point about the problem and the project :) I have been planning a dog feeder for a while now and when I picked up the caps and solar panels I thought I would work them into the project. But I see the complexity is probably not the right project to try it on. Thanks! \$\endgroup\$ – Preator Feb 2 '16 at 17:07
  • \$\begingroup\$ Tell me about it! :D (The RFID door lock at my workplace is controlled by a dsp with 16bit audio dac only so it can say "access granted" in a sensual female voice :))) ) \$\endgroup\$ – Szidor Feb 2 '16 at 19:59
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You are going at this backwards. Always, always, always start by defining your requirements. Only then can you move on to considering solutions.

In this case, you need to first build a prototype dog feeding station. Only then can you get a feel for how much power/energy you'll need. With a nod to your idea of using supercaps, you can use motors which run at 5 volts. With a mechanism in hand, you can measure how much current the station draws (this establishes power requirements), and how long the motors run . From this you can determine how much energy you need. Remember to specify a worst-case power outage duration - if you size the system for 2 days but the grid is down for a week, your pups will not be happy. You'll want to run the feeder while you are home, anyways, in order to monitor it for reliability. After all, you don't want to find out that there's a weakness in the system by coming home to hungry dogs.

Only now can you start looking at candidates for energy storage. I suspect (very strongly indeed) that student is correct, and supercaps are not the way to go - they simply don't store enough energy to run motors for any length of time.

I'd also cast a jaundiced eye on solar power for your backup. If you are absent when a storm blows through, and the sitters can't reach your place, how will you depend on the solar cells not taking damage during the blow?

Frankly, I'd be inclined to start out assuming a biggish battery backup charged from the power grid would be the default position for your needs.

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Simplicity is your answer. Do not use supercaps, my experience is that they are evil, but with that aside, there is no need for them. Use the solar cells to charge the baterries directly. Buck-boost converter can be used to create the right charge voltage.

Simple charging will suffice. Use constant current at 0.1C for 12h.

For the consumption, RTC is not gonna be a problem when compared to the motor that you will have to use.

Anyway, even small batteries should support weeks of operation.

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    \$\begingroup\$ Supercaps sound like a good idea, but they are far from a perfect energy storage solution. Some have significant internal resistance, then there is the balancing issue, and most have significant leakage, meaning they discharge completely in about a month. In other words, more hassle than they are worth. The lithium batteries offer far more charge storage, and are 99% efficient at storing/releasing it. \$\endgroup\$ – rdtsc Jan 30 '16 at 18:47
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The initial intention of mine was to leave a comment, but I'm rookie so... be gentle. I agree with @whatroughbeast. I just want to add a note. What ever power storage you'll use. If you're going to use PV be aware of a problem.

The problem:

The sunlight energy varies during the day and between days. So the energy we sink from the cell can not be calculated. If for example we design a switching power supply to charge the battery with a constant of some mA for example, we may reach a point (and we will), that the available energy from the PV will not be enough to produce this amount of mA. Now consider that all the power supplies have feedback to regulate voltage. And what the feedback does? When the output is less than the requested, the produced error is bigger and so the signal is maximized. It's like when you need more speed, you press the gas pedal more. This approach may be good for a lot of situations, but is not for solar panels. If you try to sink more current (more than the available based on the sunlight) from the panel the voltage will drop and the resulting sinking power will be far less.

The solution:

A system called MPPT (Maximum power point tracking) system. The whole idea behind this system is very simple. Sink as much power as you can from the panel :-)

How it works?

Consider the next circuitenter image description here (This circuit is just for reference, is not a working system)

The controller performs the following algo: enter image description here

This way the amount of energy flow to the battery/Capacitor will be at the maximum all the time ;) Of course you can use a solution like this from ST

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