Just When You Thought It Was Safe ...

As the manufacturer says “It’s one mean battery”!

Below is Dennis's second post in the continuation of the Solar Installation. Cue music here.

The Solar Installation Part Two – Batteries and Battery Bank

In the first installment, we discussed how to site the solar panels on the roof of a new cabana built so that the roof was oriented optimally.   In this installment, we are going to go over the steps to estimate the electrical usage; how many storage batteries are needed and other concerns about the batteries.  We will save consideration for how many solar panels are needed and how much sunlight reaches your panels (insolation) for a subsequent blog. J

How Much Electricity Do You Use?  The next step in setting up your solar system is determining what you use electricity to power, how long you use it, and ultimately how many watts are used.  This is the dreaded (i.e., pain in the butt) “electricity inventory”. You can find forms for this on-line that vary in complexity – search for  “battery calculator solar”. I opted for simple.  We itemize all the items we are likely to use on a regular basis. There are tables of values for electric appliance wattage on-line, but I felt more comfortable using a Kill-a-Watt meter to quantify the electricity usage where I could.  Plug the Kill-a-Watt meter into an electrical outlet and plug the device/appliance into the meter.  Set for Watt.hrs and collect the data.  It will sum the amount of wattage used over the length of the collection period.  Some would argue this is another aspect of my compulsiveness – ahem.  (Note from Wilma:  some would be right!)  For those items we could not quantify with the meter, we did use average values found online.

Push the "Amp" button to make it easy!

We got some surprises however.  We were using more electricity than we thought, so would have underpowered the solar system by solely using the values published online. For example, the very efficient (aka expensive) refrigerator we bought was actually using 50% more electricity than claimed.  Hrumph! (Note from Wilma:  Maybe because our ambient temps are around 85F instead of the 70F used in the test setting?)

Here are the results from the dreaded “electricity inventory”. 

We excluded power tools used in construction since these would be used on an intermittent basis and would be powered by our gas generator if used for more than a few minutes.  I decided to round up the daily usage estimate to 6000 watts to allow for devices and appliances we do not have on site currently:  a television and a microwave.  (Note from Wilma:  also a sewing machine, washing machine, maybe a small toaster oven.)  These will ultimately be here and used, but not for some time.  We have a good collection of DVDs too.  We will not be connecting to satellite TV service however.

One item to note is how you get Amps, in the first column, from watts AC in the second column.  It is calculated from a rearrangement of Ohm’s Law. Basically, Watt.hours = Amp.hours X Voltage, or Amp.hours = Watt.hours/Voltage.  Luckily there is an easier way to get the Amp.hours however - just push the Amp button on the Kill-a-Watt meter!

The Battery Bank.  You now have an estimate of how many Watt.hours of solar electricity you need to generate and capture for use.   Now you need to determine how much battery capacity you need to store sufficient electricity for daily use, keeping in mind a few other items, however.  You want your usage of electricity to be a small amount of the total capacity of the battery bank, referred to as the Depth of Discharge.  If you have a smaller amount of usage, the batteries last longer.  If we use only 5 to 10% of the battery capacity, our specific batteries may last around 20 years.  In other words, the battery capacity is not a measure of the total that you can use because you want to avoid discharging the batteries more than 10% of their charge carrying capacity.  Unless, of course you plan to replace your batteries frequently – not a good financial decision!

Redrawn from:  http://support.rollsbattery.com/support/solutions/articles/8136-cycle-life-vs-depth-of-discharge-5000-series

You also want to make certain that the solar system will be able to fully charge the batteries, otherwise the batteries will be consistently undercharged which may lead to premature failure due to sulfation of the plates in lead-acid batteries.

So what type of battery do you want to use?  We had some bad experiences with deep cycle cheapo lead acid batteries and more expensive AGM (Absorbed Glass Mat) batteries when we were relying upon our diesel generator for charging, and were not encouraged.  But we had seen a battery installation in Western Belize at DuPlooy’s Jungle Lodge and Botanical Garden in 2010.  They had installed twenty-four 2V batteries wired in series to produce a 48volt DC system.  These batteries are manufactured by Surrette Batteries in Canada, a Rolls company and are the top of the line batteries for renewable energy applications.  I looked at space, weight, price, and decided we could do something similar with Surrette 4V batteries, requiring twelve 4V batteries wired in series to get the 48 V system. 

Here is how batteries are wired in series using 12V batteries as an example:

This is what we did for our original battery bank and is similar to how we would wire the 4V batteries together. The negative pole of the first battery is connected to the positive pole of the second battery … and so on.  While the voltage is additive, “The Amps Remain The Same”.

According to the manufacturer, the capacity of the 4V Surrette battery is ~1900 Amp.hr at a slow discharge rate of 100 hours.  The Watt.hours for the bank of 12 batteries would be 1900 Amp.hours X 48V = 91,200 Watt.hours or 91 kilowatts.  With numbers like this it seems we are becoming our own electric company!  (Note from Wilma:  Well, that is actually the idea, isn't it?!)

We now have enough information to perform a battery bank calculation to determine how many batteries are needed.  Here is the actual analysis, but this calculator does not appear to be online any longer. 

You should use no more than 50% of the battery bank capacity, to improve battery longevity, which gets us to having 45 kilowatts of storage and at least 5 days of using electricity without reducing usage and not having to run the generator (by the morning of day 3 of overcast skies, we would turn the generator back on).  If we are careful and reduce our usage during overcast periods, then we could go much longer.  By keeping the depth of discharge to between 5 and 10%, the batteries should last around 7000 charging cycles or approximately 20 years before (anticipated) failure.  Poor maintenance can decrease the time to failure – ahem.   We certainly don’t aim to be guilty of that!

To summarize, we have planned to have the capacity to use 7500 Watt.hours of solar power for at least 5 days of autonomy, which will not draw the battery bank down to less than 50% of capacity, while normally keeping the daily depth of discharge to between 5 and 10%.  We are going to wire twelve 4volt batteries together to produce a 48V system.

Now a practical issue:  each battery weighs 315 pounds when filled with battery acid (34 liters of acid, BTW).  There are 12 in series plus a spare battery.  How do you support this weight 10 feet off the ground since the combined weight of the batteries is 4095 pounds?  You make floor joists composed of two 2”X8”X8 ‘pieces of treated pine lumber with plywood in between, glued with Liquid Nail, a construction adhesive in the USA, and nailed from both sides, spaced 16 inches on center, supported on reinforced concrete beams.  Then you lay a plywood subfloor, glued and screwed down on the joists, with the edges epoxied to prevent water intrusion and delamination.  And finally nail tongue and groove flooring on top of that.  The box enclosing the battery bank weighs about 1060 pounds (more on this in a subsequent post on Insurance).  The total weight of batteries and box is 5055 pounds, distributed over a 27.5 square foot area (2.75X10 feet).  This reduces to 188 pounds per square foot.  Whooh!

This is overbuilt support for the batteries, done intentionally since you not only have to support the weight of the batteries and box, but you have to consider there is an earthquake slip fault line about 130 miles away off the coast of Honduras that moves Guatemala eastward every now and then.  A million years from now Guatemala may have some nice seashore and a decent East Coast!  See the very bottom of the Magnetic Declination Map in the first installment for where Guatemala is moving eastward. The last significant earthquake was a hefty 7.1 on the Richter Scale in late May, 2009 (see here for images of the aftermath, originally linked to Trip Advisor by our neighbor Sue Harris).  It shows the destruction in Monkey River Village, with cabanas sinking to ground level from the effects of liquefaction of sediment, concrete buildings cracked asunder, and mud geysers erupting from formerly solid ground.

We would like to ensure that the batteries and box remain where they are and not fall through to the ground below during the next “Big One”.   Our original cabana and this addition include features to mitigate damage from liquefaction as shown in a previous blog post.

This is getting to be pretty thick reading, so I will stop here for now.

The 3rd installment will deal with how many solar panels you need, how much sunlight you have, how many watts produced, and does this meet your needs.

It takes a fair amount of planning and effort to “go green” or off-grid.  Cash too, as will be explained in a future installment on costs and safety.