Hi, welcome back.
I'm Amy Beaudet from the altE Store.
Thank you for watching the second of our video series on designing an off-grid solar system.
Step 2 is to size the battery bank.
We'll discuss the different considerations that go into sizing your battery bank.
We've already done a loads list in our previous video, so we know how much power we will use
each day.
Now let's see what size battery bank we need to store it.
The answer may surprise you.
Just a quick reminder of the components that make up an off-grid system.
We will start at the battery bank.
You may recall the loads list we did in our previous video.
We came up with a total usage of 2191.5 watt hours a day.
Note however that 44Wh are DC, not AC, and therefore not going through the inverter,
so we'll need to use that information later.
So let's get started.
The first thing we need to decide is what voltage we will make the battery bank.
Most off-grid battery banks are either 12, 24, or 48V.
So how do you decide which to use?
First is what voltage are your loads?
Are you just powering a small video surveillance camera or light that runs off 12V DC?
Or is it an AC system that will be using an inverter to convert from DC to AC?
If it is using an inverter, what size does the inverter need to be?
Generally, the higher the output wattage, the higher the DC input.
For example, if it is a 2000W inverter, it may be available in 24VDC, but a 6000W inverter
will certainly require 48V.
Another possible consideration is the distance between the solar panels and the battery bank.
Depending on what type of charge controller you use, you may need to match the voltage
of the solar array with the voltage of the battery bank.
I'll get more into that in a future video.
But if you do need to match voltages, keep in mind that if the panels are far away from
the batteries, you can reduce the gauge of the expensive copper wire needed by using
a higher voltage.
Since using a higher voltage array results in lower current, you can potentially save
money by running the system at 48V instead of 12V.
Let's get into this a little deeper.
We'll use an 8A4D battery as an example.
It is 12V and 200Ah.
12V x 200Ah = 2400Wh for 1 battery.
Remember that wiring in parallel increases the amphours, but keeps the voltage the same,
and wiring in series increases the voltage, but keeps the amp hours the same.
If I needed 4800Wh capacity, I can wire 2 of these batteries in parallel . However,
if I needed 9600Wh, I would need 4 of these batteries, 4 x 12V x 200Ah.
Since I want to limit the number of parallel strings I use, I can't wire them all in parallel
, but I could wire them in 2 parallel strings of 2 in series.
In doing so, I made a 24V 400Ah battery bank, 24V x 400Ah = 9600Wh.
Or, I can make a 48V system by wiring them into 1 string of 4 in series.
To decide which voltage you use, you can then refer back to the other considerations of
if you have any specific voltage DC loads, or if the inverter you picked requires a certain
voltage.
If you did need to have a 9600Wh battery bank, as in the previous example, but you need a
12V bank for your loads, you can still accomplish this.
You would want to pick a lower voltage but higher amp hour battery.
9600Wh / 12V = 800Ah.
So how can we build that?
We can pick a lower voltage, higher amphour battery, like the Concorde PVX-405 at 6V and
405Ah, and wire them 2 parallel strings of 2 in series.
The 2 6V batteries in series makes 12V, and the 2 405Ah batteries in parallel equals 810Ah,
more than enough.
When wired together, you get a 12V, 810Ah, 9720Wh battery bank.
Now that we've figured out how much power we use a day, we need to know how many days
we plan on running our equipment off the battery bank if there is no sun to recharge it, or
days of autonomy.
This is a delicate balance, because the more days we select, the bigger, and more expensive
the battery bank gets.
But we don't want to go too small either, because the less we drain the batteries, the
longer the bank will live.
This is where that generator I mentioned can come in handy.
For example, you could pick 3 days of autonomy, and plan on using the genny to charge up the
battery bank if needed on day 4.
Depth of discharge, or DoD, is how far down you can drain the battery.
A lead acid deep cycle battery that is made for renewable energy systems can be drained
down pretty low, but the less you drain it, the longer it will live.
You'll often hear people say you can drain a deep cycle battery down to 50%.
That's true, but if you do, it will last half as long as if you drained it to 20%.
Each battery will have a depth of discharge chart.
you can see here that if you drain this battery down to 50%, using half it's power, you can
get about 1500 cycles, or 1500 days if you do that every day.
That's just over 4 years.
But, if you only drain it 20%, you can get 3400 cycles, over 9 years.
That sounds great, except you have to remember that requires a bigger bank to use a smaller
percentage.
So you have to balance the upfront cost of the system with how often you have to replace
the batteries.
You may also hear the term State of Charge (or SoC).
That is the percentage of how full the batteries are.
It is the inverse of DoD.
So a battery that is at 30% depth of discharge is at 70% state of charge.
Batteries are rated at 77 degrees Ferenheight, or 25 degrees Celcius.
When the temperature gets colder than 77 degrees, the amp hour capacity decreases, but the lifespan
increases.
When a battery is hotter than 77, the capacity increases, but the lifespan decreases.
To compensate for lower temperatures, we will need to increase capacity.
This chart shows the change in capacity based on temperature.
You see here at 77 degrees, the capacity is at 100%, what it is rated for.
For example, 100Ah.
But you see here at 50 degrees, you are at 81% of the rated capacity.
So if you still need 100Ah, you would need to multiply that by 1.19 to get a battery
rated at 119Ah, and at 50 degrees, it will be able to store 100Ah.
The colder the battery is, the larger the rated battery needs to be to store your power.
OK, now that we know the variables, let's do some math to figure out what size battery
bank we need.
From our loads list (you remember the loads list, don't you?), we are using 2192Wh a day,
but only 2148Wh was AC.
We will divide it by the efficiency of the inverter we use, let's say 92%, to make up
for lost power used by the inverter.
Then we enter our DC loads, 44Wh.
this gives us 2379Wh.
Next step , Then we multiply the 2379Wh by days of autonomy and temperature compensation
. I'm going to be storing it in a 50 degree room, so I use 1.19.
We'll divide that , by .5 for 50% depth of discharge.
Now notice that we are using 50% depth of discharge, but that's after our 3 days of
autonomy, so I can run my loads for 3 days with no solar recharging the batteries, and
after 3 days, I'll have used half my rated capacity.
That should give me plenty of stored power, and a long battery life.
This gives me 16986Wh battery bank needed.
Then , I divide this number by the voltage of the battery bank we picked.
I'm going to use a 48V battery bank, so I divide by 48V to get 354Ah.
OK, we are almost there.
We take our 354 amp hours, and divide it by the maximum number of strings we want to use.
I'll go with 2.
That says I need 2 strings of at least 177Ah batteries.
Let's pick some batteries that will fit this bill.
An MK 8AGC2 battery is rated at 6V 190Ah, we can use that one.
Next , we take the system voltage of 48V, divided by 6V battery, which tells us we need
8 6V batteries in series.
Let's add that all up.
2 parallel strings of 8 in series = 16 batteries needed.
That's it for the second video for designing an off-grid PV system.
Watch the next videos in the series for how to size the solar array, and charge controller
and inverter, using the numbers you came up with from your loads list.
Also watch more of our Video Series on our web site, and peruse our selection of deep
cycle batteries.
We've got a team of highly trained Technical Sales Reps available to help you plan your
system, give us a call.
Thanks!