Solar Coop – Update II


_DSC6362 - 2015-01-25 at 16-14-33 - Version 2Since writing the first update on the solar panels on the chicken coop, we had quite the cold snap.  Along with the persistent cold, there was persistent cloud cover and strong winds.  The combination of cold weather meant the battery for the coop had diminished capacity and the cloud cover meant that there would not be enough solar irradiation to fill that reduced capacity.

Each night, I would disconnect the battery from the system and lug into the garage; connecting it to a battery charger for the night.  We also brought the chicken’s waterer into the house to prevent the need in the morning of having to thaw the water. In addition to the routine changes, I removed two things from the electrical-mix: the electric timer and the ammeter. With the battery’s stored energy being consumed so quickly and the water and battery being brought in at night, it did not seem like the timer was needed at all. The ammeter was removed because it stopped working in the sub-zero cold.  Subsequent tests, while it has been warmer outside, show that the meter still works; it just does not like the cold.

We have since made it through that cold spell, and have been in a pleasant middle ground of nice amounts of sun, above freezing day-time temperatures – several days in a row, and light amounts of wind.

With the warmer stint of weather, the battery has been under lighter duty. Even when the solar charge controller has been indicating that the battery is not strong enough to power the inverter, the next day’s sun will be more than enough to give the battery a good charge.

Being a curious, amateur scientist, I wanted to know a bit more about why a lead-acid battery appears to be quite poor at being able to provide energy when the ambient temperature is very low.  This inability to provide energy is quite noticeable and prevalent in colder regions during the winter.  For those who are familiar with starting a car while in the depths of a cold winter – think of how the car’s starter seems to struggle to turn the engine over.  It’s a battle between cold lubricants with a higher than normal viscosity and a lead acid starter battery with diminished capacity.  But, why does cold cause this diminished capacity.

With my day-job being at a university, and I am surrounded by academics and researchers, my first thought was to look into published research on batteries or modeling batteries.

The first paper I found was A mathematical model for lead-acid batteries co-authored by Dr. Ziyad Salameh – Dept. of Electrical Engineering, UMass Lowell, Margaret A. Casacca (student), and William A. Lynch (student).  The paper was published in the IEEE Transactions on Energy Conversion, Vol. 7, No.I, March 1992.

Aside from equations and a mention of a BASIC program that was developed (but this program is nowhere to be found in the paper), the main take aways from the paper are list of five factors that effect a battery’s ability to store energy; for the most part, the list of things is obvious.  (1) State of charge, (2) battery storage capacity, (3) rate of discharge, (4) environment temperature, and (5) shelf-life or age.

(This list is originally from the Complete Battery Book.)

We can say that our battery is fully charged (state of charge is 100% at the on set), has a capacity of 110 amp-hours, the waterer has a draw of at most 10 amps (this is likely not constant as the waterer will turn on when the water is below 35 degrees, and turn off when it reaches a temperature above this), the shelf-life or age is basically “brand new”.  Temperature is likely the deciding factor.

Looking at how temperature effects capacity, you can see that as the temperature drops, the capacity drops, as well.

Capacity vs Temperature

Assuming this graph is true (there is no documentation or available analysis), at our coldest, the battery is likely to be running at a bit over 60% capacity; meaning, we’ll only get about 66ah from it.  The inverter will shut off when the voltage drops below about 12.10V; according to another battery university article, this likely means the battery has about 50% capacity remaining.  This could roughly be translated into a capacity of 33ah at our coldest, and about 55ah at optimal temperature (which we won’t have until sometime in late May).

Now that I have some numbers that better explain the observation of why didn’t the battery last more than 8 hours on the coldest day, I still want to know why does this happen.  Why does ambient cold have such an effect on lead-acid batteries?

The short answer is internal resistance.  To understand this, you first need to know a bit about how lead-acid batteries work.

A course offered at the University of Colorado Boulder‘s Electrical, Computer and Energy Engineering department has a great set of lecture slides (or here) explaining how lead-acid batteries work.

The gist of how lead-acid batteries work are electrons drifting or flowing from the negative terminal – most often made of lead (\(\ce{Pb}\)) – to the positive terminal – most often made of lead dioxide (\(\ce{PbO2}\)).  The two terminals are submerged in an electrolyte solution.  This is usually in the form of sulfuric acid (\(\ce{H2SO4}\), where, in solution, it takes the form of aqueous ions \(\ce{{H^{+}}+{SO4^{-2}}}\)).  As you draw electricity from the battery via the positive terminal, electrons flow from the negative terminal to the positive terminal.  How easily or difficult the electrons can move from negative to positive terminals in the internal resistance.

I think of what happens with the electrolyte solution and effect of cold temperatures on it is sort of like what happens to honey when it is chilled.  It is not quite analogous but I think it gets the point across.  If little droplets of honey are running down a piece of glass and you suddenly cool the glass, the honey will begin to run much slower.

Similarly, as the electrolyte solution cools, the ability for electrons to drift efficiently to the positive terminal decreases.  The decrease in electron flow results in less power to be consumed – in our case, by the inverter and waterer.

So, what can be done be to keep batteries from losing their capacity in the cold?  Some of the documentation that I read suggested gently warming the battery to keep it from getting too cold.  This might be possible if we added another electricity generator – like a wind turbine.  We would have to build a better storage place for the battery, too.  We can’t really put the battery into the coop (where it is noticeably warmer because of sheltering from the wind as well as the use of deep-litter) because lead-acid batteries generally need venting (there is a small amount of oxygen and hydrogen released from batteries).  In addition to the idea of keeping the batteries slightly warmer (which, I will admit, I’m likely not going to do), adding capacity seems like a more viable option.  This is done by simply adding additional batteries in parallel.  With the idea of adding in a wind turbine, I have already been thinking of adding additional batteries.

To wrap things up, I just hope that the recent temperature bounce-near-and-around-freezing continues.  The battery likes not being choked by the cold; nice amount of sun – the adequate sun light we have been receiving has had two benefits: the battery is able to be recharged successfully, and, equally important, the chickens have begun laying eggs once again.

 

For a better look at the specifics of the chemical reactions that occur within a lead-acid battery, check out this page.

Buckets of Chicken

Blood in the SnowThe sun was out and there was a fresh spring-like smell in the air.  Water dripped from the barn roof into a puddle near the door.  The cows, near the barn, were slurping from their trough – the ice in it was melting.

I rotated and rolled the large cook’s knife I held in my hand; the rivets through the handle and tang were still cold.

I lifted the knife up; a quick and heavy downward swing of the knife, and I felt the heel push into the chopping block below.  A slight forward movement with the knife on the block, and the rest of the blade was in contact with the surface.

One down, ten more to go.
List of Butchering TasksI flipped the now headless bird into the snow; the carcass flipped and jumped; I knocked the head into a bucket – wiped the blood from the knife, and walked to the coop to get another bird.

Walking back from the coop, the headless bird now lay motionless; the snow in front of the chopping block starts to take on the appearance some twisted form of a snowcone.

Two down, nine more to go.

We ended up dividing the lot into two batches.  Six birds and five birds.   With each batch, the process was the same.  Two to three headless birds in a 5 gallon pale – I’d walk up to the farm house from the coops – buckets in hand.  Scald, pluck, scorch the remaining fine feathers with a torch, remove feet, remove oil gland, remove trachea and neck skin, gut, since and chill. Repeat.

This was the first time that I witnessed the scald method.  Usually, when cleaning fowl, the carcass is too small, as with ruffed grouse, to really warrant going beyond the breast meat, or, the skin just seems far too greasy, as was with the ducks we had the last summer – in this case, I skinned the ducks.   The pigs we helped butcher a few years ago, we opted for skinning instead of scalding.  Rabbits, squirrels and deer are all skinned, too.

The killing turns out to be almost the easiest part (assuming you are not bothered overly much by the removal of life); the plucking is messy – feathers stick to your hands, and the removal of the entrails is slippery – there is quite a bit of fat on chickens.  There is water involved at both ends of the uncomfortable for hands spectrum – hot for scalding, and icy-cold for chilling.

IMG_0003 - Version 2 - 2015-01-18 at 12-20-32By late afternoon, the carcasses were chilling in tubs of ice water; we had cleaned up the work area in the farmhouse’s basement; the entrails that some consider edible were in the farmhouse freezer and the buckets of soggy feathers were out of the house.

On the ride home home, I found myself mulling over the sensorial aspects of the day’s task.  The red snow, noises made by headless carcasses, and smells.  I had given my mom a call – knowing that she, during her quixotic-commune days, had butchered chickens, as well.  She asked me, “What’d you think of the smell?”  I thought for a second or two, and replied, “It smelled like yellow.”  I knew she wasn’t referring to the smell of burnt feathers from the scorching, she was thinking of the bird itself – freshly plucked and at the point where you have started to clean out the cavity.   It has a yellow smell to me.   My mom knew exactly what I meant.  That’s the best way I can describe it.  I’m curious what others describe the smell as – leave a comment below.