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How to calculate battery bank capacity, this is the step 2 of Solar power system design. What factor need to consider when you design the system?

How to calculate battery bank capacity

What factors need to be consider

Thank you for reading the second of our video series on designing an operating system.
Step two 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 blog, so we know how much power 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'll be starting at the battery bank today. You may be called the loads list we did in our previous blog. We came up with a total usage of 21, 91, . 51 hours a day. Note, however, that 44 watt hours are DC, not ac, and therefore not going through the inverter.

So we'll need to use that Information later.


Let's get started. The first thing we need to decide is what voltage will make the battery bank.

Most of great battery banks are the 12, 24 or 48 volts. How do you decide which to use? First is what voltage are your loads? Are you just powering a small video surveillance camera or lights that run off 12 volts DC?

Or is it an ac system that will be using 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 voltage, the higher the DC input. For example, if it is a low frequency inverter, it may be available in 24 volt DC but 6,000 more inverter will certainly require 48 volts.
how to calculate battery bank capacity-InkPV
how to calculate battery bank capacity-InkPV

Distance between the solar panels and the battery bank

Another possible consideration is the distance between the solar panels and the battery bank.

Depending on what type of charge control 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 the 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 results in lower current, you can potentially save money by running the system at 48 volts instead of 12 volts.
how to calculate battery bank capacity-InkPV

How much power you need to store in the batteries

A big consideration is how much power you need to store in the batteries.

This is combined with the restriction of how many parallel strings you can use without negatively affecting your battery bank.

Most experts recommend no more than two parallel strings, some even say you should only do one, or some will be scratchingly, say that three is all right in the pinch. The reason is that the more parallel strings you wire, the more likely you are to have an uneven charging and discharging. Even a slight difference in voltage between the strings can result in a shorter lifespan for the batteries.

Now that we figured out how much power use a day from step 1.
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, called 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 we don't want to run out of power unless we drain the batteries, the longer the battery bank will live. This is where the generator I mentioned can come in handy.
how to calculate battery bank capacity-InkPV
For example, you can pick 3 days of autonomy and plan on using the journey to charge up the battery bank. If you need a day four depth of discharge or dod is how far you drain the battery down.

Let us a deep cycle battery that is made for renewable energy systems can be drained down pretty low. But the less you drain it, the longer will live.

You 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 only 20 %, each battery will have a depth of discharge chart.
how to calculate battery bank capacity-InkPV
You can see here that if you drain this battery down to 50 % using half its power, you can get about 1,500 cycles. What 1,500 days if you do that every day?

That's just over 4 years. But if you only drain it down to 20 %, you can get 3,400 cycles. That's over 9 years. That sounds great, except you have to remember that requires a bigger bank to use a smaller percentage.

You have to balance the upfront cost of the system with how often you need to replace the batteries. You may also hear the term state of charge or SOC. That is percentage of how full the batteries are.

It's the inverse of dod so battery that is 30 % depth of discharge is at 70 % state of charge. Batteries are rated at 77 ℃ or 25 ℃.

When the temperature gets colder than 77 °, the amp, our capacity decreases, but the lifespan increases. When a battery is hotter than 77, the capacity increases, but the lifespan decreases.
how to calculate battery bank capacity-InkPV
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? 

Don't you? We're using 21, 9021 hours a day, but only 21, 40 81 hours was ac. We'll 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 of 4041 hours. This gives us 23, 70 91 hours.
how to calculate battery bank capacity-InkPV
Next step, we multiply that twenty three seventy nine one hours by the 3 days of autonomy, the temperature compensation, I'm gonna be storing it in a 50 ° room. So I used 1 . 19. We'll divide that by . 5 for 50 % depth of discharge. 

Now notice we're using 50 % depth of discharge, but that's after a 3 days of autonomy. So I can run my loads for 3 days with no solar recharging the batteries. 

And after those 3 days, i'll have used half of my rated capacity of my battery bank that should give me plenty of stored power and a long battery life. This gives me 16,900 hour battery bank needed. 

Then I divide this number by the vote to the battery bank, we picked 48 volts. I get a result of 354 empire battery bank needed. 

We're almost there. We take our 354 and hours, and divided by the maximum number of strings we want to use. I'll go with two. 

This says that I need two strings of at least 170 78 hour batteries. Let's pick some batteries that will fit the bill. And mkaagc two battery is rated at six folds, 198 hours. We can use that one.

Last word

we take the system voltage of 48 volts, and divided by the 6 volt battery, which tells us we need 86 volt batteries and series to get 48 volts. Let's add that all up.

Two parallel strings of eight in series equals 16 of those batteries needed for our system. That's it for the second blogs of designing an off grid pv system. Watch the next videos in the series for how to size the solar Ray and the charge controller and inverter. 

Using numbers you came up with from your loads list. Also watch more of our blogs series on our website and feel Free to 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.
Wang has over 10 years of experience in off-grid projects. And know how to wire the system well. Making every installation step easier.

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