How to Size the Battery Bank
Battery Bank for Off-grid Solar System
After you know what the electrical lifestyle is on an average day, you need to translate that into the amount of energy stored in the battery bank, also known as the battery bank’s capacity. All of the following dictates the battery bank capacity you’re looking for:
- The efficiency of the inverter
- The number of days you expect the battery bank to last without recharging
- The batteries’ operating temperature and voltage
- How much of the battery bank you are willing to use
- The voltage at which you want the battery to operate
Now let’s dig into great details on these variables and explain how to put them all together so you can accurately determine the battery capacity needed and create the battery bank for your solar system.
When you buy solar batteries to make up the entire battery bank, you have a few options. The most common battery type for off-grid PV systems is a 12V nominal solar battery. You then take these batteries and wire them in a series-parallel arrangement to achieve the voltage and capacity characteristics you’re after.
There’ll always be some losses associated with turning DC into AC, which is why no inverter can deliver 100 percent of the energy from a battery bank to the loads. However, if the inverter can be more efficient at inverting, the battery bank can be smaller. Consider the AC loads attached to the proposed inverter and the inverter’s size (in terms of power output) in order to maximize efficiency levels. What I mean by this is don’t put in a 5kW inverter if all the client will ever draw is 1kW. Instead, try to match the loads and the inverter.
Inverter manufacturers list the efficiencies of their units on all of their spec sheets. What you need to remember is that the number listed by the manufacturers is the peak efficiency value. As such, it’ll almost always be an impressive value that’s somewhere near 97 percent. Although 97-percent efficiency may be possible, it’s not achievable on a frequent basis. Most battery-based systems are regularly closer to 90-percent efficiency. Like all variables, this percentage will vary, but 90 percent is a fair value that represents a typical operating efficiency for an inverter.
The days of autonomy
The number of days you want the battery bank to sustain your electrical lifestyle is known as the days of autonomy. In other words, it’s the number of days you expect the battery bank to provide you with your average daily energy requirements without needing to be recharged by the Solar PV array and the charge controller, generator, or utility.
The local climate usually plays a major role in this decision, as does the available budget for the project. As you can imagine, the more days of autonomy, the more batteries you need, and the higher the system cost climbs.
Many off-grid residential applications use two or three days of autonomy as the starting point, whereas most utility-interactive systems use just a single day. For commercial applications, the grid is typically present, so one day of autonomy should suffice. You can consider adding more days of autonomy, but then you have to play a balancing game with the size of the battery bank and the size of the PV array.
The temperature used for battery operation
The temperature that batteries operate at affects their capacity. The colder a battery is, the less capacity it can deliver. Why? Because the efficiency of the chemical reaction occurring inside the battery increases and decreases at different temperatures. Battery manufacturers publish the exact effects that temperature has on their batteries, so you should be able to find that data for the battery you’re considering in order to apply the correct temperature derate factor (the percentage of the capacity you can expect from a battery based on the temperature).
Because most systems use lead-acid batteries and the technology is pretty consistent among the different manufacturers, we’d like to recommend you use a single temperature derate factor: 90 percent. This percentage corresponds to a battery temperature of approximately 60 degrees Fahrenheit (15.5-celsius degree) and indicates that at that temperature, the battery will only be able to deliver 90 percent of its rated value (the battery’s capacity at 77 degrees Fahrenheit or 25-celsius degree).
Battery bank’s depth of discharge (DoD)
Depth of discharge (DOD) is the amount of energy drawn from the battery bank; it’s generally given in terms of a percentage. The higher the DOD value, the more energy has left the battery bank. As with days of autonomy, DOD can (and should) be dictated in the system-design process because it affects the overall size of the battery bank. When you look at a typical chart provided by battery manufacturers that shows the number of cycles versus DOD, it becomes apparent that the smaller the DOD is, the greater the number of cycles (a cycle is a period from when the batteries’ capacity is drawn down to when it’s recharged). Although this fact probably isn’t surprising, it doesn’t mean you should try to baby the batteries and design a system around a small DOD.
What you really need to do is evaluate where on the curve the maximum amount of energy will be delivered over the battery bank’s life. To determine the ideal DOD to use with a battery bank, look at the whole picture in graph form; a graph shows a battery bank’s number of cycles against the percentage of its discharge. The figure below shows an example.
If the battery bank in the above chart is rated at 400 amp-hours (Ah), you can use that information to estimate the energy delivered over the course of the battery bank’s life. From the chart, you can see that this battery bank will last for approximately 2000 cycles if the DOD is only 30 percent. The number of cycles is reduced to approximately 600 when the DOD is 70 percent. So which DOD delivers more energy over the life of the battery bank? Run the numbers to figure it out:
- 400 Ah × 30% DOD per cycle × 2000 cycles = 240,000 Ah
- 400 Ah × 50% DOD per cycle × 1000 cycles = 200,000 Ah
- 400 Ah × 70% DOD per cycle × 600 cycles = 168,000 Ah
Even though the idea of reducing the DOD looks good at first glance because it increases the overall life of the battery bank, it results in fewer amp-hours delivered. Because the battery bank’s job is to store and deliver energy, you may want to consider discharging the batteries more often to maximize your investment (and reduce the system’s initial cost). When evaluating the DOD, most battery bank designs use a value that’s somewhere between 50 percent and 80 percent, but there’s really no exact “right” answer.
You have to evaluate the options and make a suggestion based on the information in the charts from the manufacturer of the batteries you use in the bank.
Be careful to never exceed an 80-percent DOD (we are talking about lead-acid batteries) in your design. Repeatedly reducing a battery bank’s capacity more than 80 percent harms the batteries and causes premature failure of the bank.
For any battery-based system you install, you need to look at battery bank nominal voltages of 12, 24, or 48VDC. These voltages correspond to the inverter input requirements for the majority of commercially available inverters.
Systems using inverters that produce relatively small AC power levels (less than 2,000 W) may be able to justify using a 24V battery bank, but with the advancements made in inverter and charge controller technologies, 48V battery banks have become very popular. (Note that the wattage levels listed here are by no means absolute values. Rather, they’re common guidelines you can follow. They represent the goal of keeping conductor [wire] sizes down by increasing voltages and
reducing current values.)
Figuring out the battery capacity you need
Step 1. Determine the average daily AC watt-hours (or kilowatt-hours) consumption level.
For the purposes of providing an example, let’s use 5kWh as the average daily energy consumption.
Step 2. Divide the watt-hours value from Step 1 by the estimated inverter efficiency.
This step increases the required capacity due to the fact that an inverter loses some of its stored capacity during the process of turning DC into AC (10 percent loss is common). Continuing with the example, you find that 5kWh ÷ 0.9 = 5.56kWh (90 percent is a fair inverter efficiency to estimate).
Step 3. Add any energy consumption from DC loads to the watt-hours value in Step 2.
This value represents the total daily energy consumption for all the loads connected to the battery bank. If you have three 20W DC LED lights that run for 5 hours each day, the total DC energy consumption is 3lights × 20W × 5hours = 300Wh, or 0.3kWh. The total energy consumption is therefore 5.56kWh + 0.3kWh = 5.86kWh.
Step 4. Multiply the energy value from Step 3 by the desired days of autonomy.
Doing so tells you the amount of energy the battery bank needs to store (two or three days is a pretty typical value). My example here wants three days of autonomy, so that makes the new energy value 5.86kWh × 3days = 17.58kWh.
Step 5. Divide the value calculated in Step 4 by the temperature compensation value provided by the battery manufacturer.
Ninety percent of manufacturers estimate the adjusted capacity at 60 degrees Fahrenheit. Apply the manufacturer’s value here for the estimated temperature of the battery bank you’re considering. So if the example battery bank will be stored at 60 degrees Fahrenheit, perform
this calculation: 17.58kWh ÷ 0.9 = 19.54kWh.
Step 6. Divide the value from Step 5 by the allowable depth of discharge.
The greater the DOD, the smaller the battery bank can be because you’ll be using more of the capacity (approximately 50 to 80 percent). This example we’ll use DOD of 50 percent, so the math looks like this: 19.54kWh ÷ 0.5 = 39.08kWh.
Step 7. Divide the value from Step 6 by your desired nominal voltage for the battery bank.
Batteries are rated in amp-hours, not watt-hours. By using the nominal battery bank voltage, you can determine the required amp-hours for the battery bank (use a 12V, 24V, or 48V value here). The system in this example will be installed at 48V to keep the current values at a minimum and reduce the conductor sizes. Here’s the math: 39.08kWh ÷ 48V = 0.815kAh, or 815Ah.
Wiring the battery bank
As soon as you know what the capacity of the battery bank should be and the nominal voltage, you’re ready to evaluate the different battery options and decide which one is best for the battery bank you’re constructing.
To determine the number of batteries required in a string, divide the nominal battery bank voltage by the individual batteries’ nominal voltages.
From the example, we calculated that the battery bank would need to have a capacity of 815Ah at 48V. And let’s say we want to have two strings of batteries in our bank, we need to look for a battery that has 408Ah (which is difficult to find, so we may need to settle on a battery with 400Ah. Batteries with this level of capacity are commonly found in 6V nominal options. So if you’re going to wire a bank for 48V and each battery is 6V, you know the battery strings should be eight batteries long. Here’s the math: 48 V ÷ 6 V = 8 batteries per string.
AGM Vs GEL Batteries – What’s the difference?
AGM vs Gel batteries, it’s a tough battle! AGM batteries and Gel batteries are easily confused among a lot of people, but we can’t blame them. AGM batteries and Gel batteries have a lot in common. For example, they both use valve regulated lead acid technology and they are both maintenance-free and non-spillable. However, there are a number of differences between the two types of batteries. Read this blog entirely and you’ll be able to highlight some major differences.
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