Renewable sources of energy, such as wind and solar, are fantastic but without the means to store this energy, they become redundant. It comes as no surprise then that many more people are wondering how to size a battery bank.
The deployment of renewable energy sources has sparked a deep interest in the development of technologies that can store more energy, for longer periods of time.
If you’re reading this article, you may already know about this, but do you actually know how to size a battery bank? If you are thinking of a DIY project or simply do not want to hire an expert to do the sizing for your application then you’ve come to the right place.
Sizing a battery bank requires that you address the following:
- Determine the number of days you need your system to run for.
- Calculate the required capacity.
- Select the most ideal battery model.
- Lay out the required connection along with the battery.
This is easier said than done, so let’s break it down step-by-step so that you know how to size your battery bank.
Sizing a Battery Bank – What Should I Know?
Before going any further it is important to define a couple of factors that you’ll need to understand about batteries.
One of the most important elements of a battery is its capacity. The battery capacity refers to the amount of energy that can be stored inside the battery for later usage. These units can be provided in watt-hours (Wh) but are generally provided in amp-hours (Ah).
Conversion between these two units is very simple. If you have Ah units, multiply that value by the nominal voltage of the battery to get Wh. Conversely, if you have Wh, divide that value by the nominal voltage.
Depth Of Discharge
Another factor to keep in mind is the depth of discharge (DOD). This refers to the percentage of the battery that can be discharged without causing any damage to the internal structure. This is very important to keep your battery in good health.
Having a large DOD means that you will be able to extract larger amounts of energy from a single charge. It also means that the battery will not last as long. Each battery technology has a maximum DOD which should not be exceeded.
Number Of Cycles
Lastly, it is important to consider the number of cycles in a battery. The number of cycles estimates how many charge and discharge rounds the battery can withstand before losing capacity below nominal values. This is directly related to the DOD since the higher the DOD, the fewer cycles the battery will last.
The number of cycles demonstrates how much time will need to pass before you need to replace the battery with a new model.
There are two main deep cycle battery types in the market currently – lead-acid and lithium-based batteries.
Lead-acid battery categories:
Flooded batteries are the oldest and cheapest option available on the market. They are an old form of technology but their efficiency values are low and they require a lot of maintenance.
AGM and Gel batteries have comparatively higher efficiency values, longer lifespans, do not need much maintenance, and are suitable for higher energy demands.
Moving on to lithium batteries – a much more cutting-edge technology. They have higher efficiencies, higher DOD, more energy density, and are suitable for very high-energy demands. The downside of these batteries is their price.
How To Size A Battery Bank?
Step 1 – Estimate You Energy Demand
One of the most important steps is energy demand estimation. What do you want to back up? Is it some critical loads? Your entire house? An RV or boat?
Defining specifically what you want to back up is essential to have accurate battery bank sizing that suits your needs and that doesn’t exceed your budget.
So how do you estimate your demand? Well, there are several options depending on the application.
If you’re talking about your entire house and you want to live off-grid, then the best option would be to check your utility bill.
Having an annual consumption estimate from the meter is the best way to represent your consumption patterns across the seasons.
Depending on your area, you may consume more power during winter or summer, therefore it is important to have a complete demand scenario for a whole year to accurately represent your consumption.
But, what happens if you don’t have access to that data or if you actually want to back up the loads for a mobile RV?
In that case, you would need to take the long route. Each electrical load, whether it is a laptop, a TV, or a microwave, has a nominal amount of watts that they consume in order to function. That represents the power demand of the device.
If you add the number of hours that we are consuming said device, then you get the amount of energy consumed within that time frame. If you add this step to every device then we will get the whole energy demand of your system.
In the table below, you will find examples of devices that would typically be in an RV as well as their average usage, in hours. This will result in Wh units.
Table For Demand Estimation
|Appliance||Estimated Power Consumption (W)||Estimated Hours of Daily Use (h)||Energy Consumption (Wh)|
|Small LCD TV||25||4||100|
Step 2 – Define Your Autonomy Days
The days of autonomy represent the number of days that the battery bank is able to sustain the load with a single charge. This must be balanced with the source of energy that you will use to charge the battery bank.
If you are installing an off-grid solar power system, increasing the days of autonomy will also increase the size of your PV system, which will impact the cost as well.
If you are using the battery bank for an RV or boat, you will have a reference point of how many days you will be able to run in the outdoors relative to your load demand.
For the purpose of this exercise let’s assume that you want 2 and a half days of autonomy.
Step 3 – Calculate The Battery Capacity Needed
Now, the most important step is to accurately calculate the capacity needed to power the selected loads for the desired autonomy days. In order to do this, we must make use of the equation below:
As can be seen in the above expression, you need to define the DOD and the DC Voltage. As previously mentioned, DOD is the depth of discharge. Typical battery banks with good performance would be sized to 50% DOD.
The DC voltage is associated with the battery voltage which can be 12V, 24V, or 48V.
This will be directly related to the charge controller that you choose to work with. A higher battery bank voltage will require a lower energy storage capacity, while a low voltage will demand higher energy storage capacity.
For the purpose of this example, let’s assume that there is a 48V nominal voltage and a 50% DOD. Using the above equation, the result would be:
Step 4 – Choose Your Battery
The next step is to select the battery. Keep in mind the following criteria when doing so – battery technology, weight, dimensions, nominal voltage, nominal capacity, brand, price, number of cycles, and more.
Based on the above criteria, we will focus on the technical specifications and select a battery that has a 12V nominal voltage and a 100Ah battery capacity.
Step 5 – Arrange the Battery Setup
Now to arrange the layout of the battery bank using the capacity needed and the selected battery bank specs.
Batteries must be connected in series and parallel connections. Series connections mean joining the positive lead of one battery with the negative lead of another battery.
A Parallel connection means joining the positive lead of one battery to the positive lead of another battery.
Making series connections maintains the electrical current and increases the voltage.
Parallel connections maintain voltage and increase electrical current. The following diagram illustrates how these connections are made.
Use the expression below to determine the required amount of parallel connections.
We must take the next whole value from the result above since it must be a whole number. The number of parallel connections would be 3.
Next, calculate the number of connections in series using the expression below.
In other words, you must connect 4 of the selected batteries in series and add 3 strings of batteries of the same size for a total number of 12 batteries.
Now that you’ve learned how to size a battery bank, you can apply the above steps to any off-grid application of your choosing. No matter the battery technology, the steps to sizing you are always essentially the same.
Make sure to do some in-depth research into the battery technologies as well as their inherent advantages and disadvantages – this will have an important impact on your final decision. We wrote an in-depth article about the different battery types here.
Finally, always remember to differentiate between what you need to back up versus what you want to back up – this will end up saving you hundreds of dollars in the long run.