How to size your battery bank to extend your solar batteries’ lifespan

Properly sizing your battery bank is crucial for an efficient and reliable solar power system. This guide will walk you through the process of determining the right battery bank size for your energy needs.

What is a battery bank?

A battery bank is a collection of batteries connected to store energy generated by solar panels. It's essential for providing power when the sun isn't shining and ensuring a stable energy supply. The two main types used in solar systems are lead-acid (including AGM batteries and gel batteries) and lithium-ion batteries.

Here are the steps to guide you how to size your battery bank.

Calculate the battery capacity

How to know the watt-hour capacity you need? Firstly, determine your daily energy consumption. Make a list of all appliances and devices you plan to power, noting their wattage and hours of use per day. Multiply wattage by hours used to get watt-hours (Wh) per day for each item, then sum these up for your total daily energy needs.

Then calculate the battery capacity, typically measured in amp-hours (Ah). You need to convert your daily energy needs from watt-hours to amp-hours, dividing the watt-hours by the system voltage (usually 12V, 24V, or 48V).

For example, if your daily usage is 5000Wh and you have a 24V system, the battery capacity is 5000Wh / 24V = 208.33Ah of capacity. You can use Renogy battery calculator to help you size your battery bank.

Considering Depth of Discharge

Depth of Discharge (DoD) is a critical factor in battery bank sizing and longevity. It refers to the percentage of the battery's capacity that has been used before recharging. Shallower discharges (lower DoD) lead to more charge cycles and longer battery life. For lead-acid batteries, the standard DoD is 50%. The depth of discharge for AGM batteries can reach 80%, but 50% is recommended to extend the service life. The DoD for lithium-ion batteries is 80%-100%.

If the battery capacity you need is 200Ah per day, and the battery is a lithium-ion battery, then the actual capacity required is: 200Ah/80%=250Ah.

Lower DoD means you'll need a larger battery bank, Higher DoD allows for a smaller bank but may reduce overall lifespan.

Factoring in Days of Autonomy

Days of autonomy is the number of days your battery bank can power your needs without recharging. This depends on your location and typical weather patterns. In sunny areas, 2-3 days might suffice, while cloudier regions might need 5-7 days. Multiply your daily Ah requirement by your days of autonomy.

Adjusting for Temperature

Batteries perform best at room temperature (around 77°F or 25°C). In colder climates, battery capacity decreases. As a rule of thumb, for every 15°F (8°C) below 77°F, increase your battery capacity by 10%.

For example, if your location averages 47°F in winter:
The temperature difference: 77°F - 47°F = 30°F
Adjustment needed: 20% increase (2 * 10% for each 15°F drop)
Adjusted capacity = The real capacity * 1.2

Choosing the Right Battery Type

Lead-acid batteries are cheaper upfront but have a shorter lifespan and lower DoD. Lithium-ion batteries are more expensive initially but last longer, have higher DoD, and perform better in most conditions. Consider your budget, space constraints, and long-term plans when choosing.

Calculating the Number of Batteries

After determining the total capacity needed for your battery bank, the next step is to calculate how many individual batteries you'll need. This process involves understanding series and parallel connections, as well as considering your system voltage and the specifications of individual batteries.

In series

Connecting batteries in series can increase the voltage of the battery pack while keeping the capacity the same. Connecting 4 12V 100Ah batteries in series will give you a 48V 100Ah battery pack.

In parallel

Connecting batteries in parallel will keep the battery voltage the same and increase the battery capacity. Connecting 4 12V 100Ah batteries in parallel will give you a 12V 400Ah battery pack. Connecting batteries in series and parallel can increase both voltage and capacity.

Let's say you need a 48V system with 600Ah capacity, and you're using 12V 200Ah batteries.

Step 1: Batteries in series for voltage 48V system / 12V battery = 4 batteries in series.

Step 2: Parallel strings for capacity 600Ah needed / 200Ah per battery = 3 parallel strings.

Step 3: Total batteries needed 4 batteries per string * 3 strings = 12 batteries total.

Please remember several key considerations :

1. Always use identical batteries (same make, model, capacity, and age) in a bank.
2. Ensure your charge controller and inverter are compatible with your battery configuration.
3. Consider future expansion when designing your system.
4. More parallel strings can reduce overall system reliability, so balance capacity needs with simplicity.
5. Some battery types (especially lithium-ion) may have built-in management systems that affect how they can be connected.

How to increase the lifespan of your batteries

• Limit the number of batteries in your bank: The more batteries you have in your bank, the higher your chances are of having unequal charging and increased resistance. Therefore, instead of having many batteries of smaller sizes, consider using fewer batteries of larger sizes.
• Keep your batteries charged: Don’t let your partially discharged batteries sit without being recharged for a long period of time. Keep them fully charged and you’ll extend the life of your batteries.
• Adhere to your batteries’ depth of discharge specifications: All batteries specify the recommended depth of discharge. A higher DoD means you can use more of the energy being stored in your battery.The more frequently a battery is charged and discharged, the shorter its lifespan will be.
• Select the right batteries for your system: Not every deep cycle battery is the right battery for you. For example, absorbed glass matt batteries offer a better temperature range than gel batteries, but gel batteries have a greater lifespan than AGM batteries and can be mounted in any orientation. Design a system around your specific needs.
• Use batteries of the same type and amp hour rating: If you go with lithium batteries, all the batteries in your battery bank should be lithium batteries. This will limit any efficiency loss due to having different batteries.

How to maintain your battery bank for best performance

Depending on what batteries you end up using for your system, you’ll want to make sure you keep up with the maintenance of your battery bank. If you’re using flooded lead acid batteries, you need to check water levels with a hydrometer and add water to keep them topped off each month. You’ll also want to make sure your battery terminals are clean and free of corrosion on all battery types.

Charge your batteries fully once a month, in order to equalize the batteries in your bank. If sunshine is not readily available, Renogy has a range of battery chargers available to help you maintain your batteries without overcharging them. You can use an amp hour meter to check to see if the batteries are fully charged.

We also recommend checking to see what your solar system is outputting at least twice a year to make sure you don’t have any underlying issues in your system.

Conclusion

Properly sizing and maintaining your battery bank is key to ensuring you have an efficient and long-lasting system. By taking into consideration battery types, adhering to charging and discharging best practices, and properly sizing your system, you’ll have a well-running system for years to come. If you are still at the stage of choosing an appropriate battery for your solar power system, you can visit Renogy, where offers a wide range type of batteries such as the 12 volt deep cycle battery, 48v battery, and marine battery.