# RV/Boat PV system: How to size your battery bank?

**Disclosure:** Some of the links in this post are affiliate links.This means if you click on the link and purchase the item or sign up for services I trust and recommend, then I will receive a commission at no extra cost to you. Thanks.

The next step after deciding your solar power need is **to size your PV system battery bank**. It is a very important component of PV systems. **Batteries are energy storage units that help us deal with the irregular nature of power generating of PV systems**. They provide energy at night and during periods of limited sunlight.

First, in this article, we will take a look at some basic characteristic parameters of **solar batteries** (**rechargeable batteries used in PV systems**) that we need to know for our purpose. Next, I will show you how to calculate your **battery pack** (**set of identical battery modules**) size (A.h) and its proper configuration.

## Nominal Voltage (V)

This is the voltage reference at which the battery is supposed to operate. In an RV/Boat PV system, the rated battery voltage is usually 12 V or 24 V. Practically, **the actual voltage of PV system could be different from the nominal voltage**, depending on the **instant load conditions**, **state of charge (SoC)** and **temperature** of the batteries.

## Battery Capacity

**It is the total amount of charge (coulomb, C) available from the battery at the rated voltage**. The battery capacity is measured in **ampere-hours (A.h)**, which is also used as an alternative unit of measurement for electric charge.

For example, if a battery’s capacity is 100 A.h, it indicates that the battery could be fully charged/discharged in 100 hours with a constant 1 A current.

## Depth of Discharge (DoD)

**It refers to the percentage of the battery’s capacity that has been discharged over the rated capacity**. The **maximum allowable DoD** is used **for sizing** the battery bank. This limitation is necessary **to increase the life of the battery** and depends on battery type.

For example, if we discharge a 100 A.h battery for 80 hours with a constant 1 A current, the DoD will be 80%.

## Days of Autonomy

In an **off-grid PV system**, designers consider the **reserved energy capacity** stored in batteries for safe operation during periods of limited sunlight. This reserved energy will provide some ** days of autonomy** from PV panels energy generation. For off-grid home PV systems, it could be up to 5 days, depending on the geo-location.

It is not the case for an RV/Boat PV system. Because **there are other energy resources available on your vehicle that could charge the batteries** and will make the additional battery storage pointless and economically inconvenient. However, it is your system and you have to design it based on your needs. Here I just wanted to show you how to consider it in the calculations if you wish to. We could** put this factor equal to 1 to prevent oversizing the battery pack**.

## Battery bank Capacity and Configuration

In the previous article, through steps 2 to 5, we decided our **system nominal voltage** and learned how to find our **PV system total energy requirement in amp-hours**. Now we can proceed with our calculation and determine our **PV system battery bank capacity**.

Like PV panels, batteries can be connected in **series** (forming a **string**) or **parallel** (forming a **block**). **In a series connection, the overall voltage will increase** as the sum of the voltage of each battery in the string increases, but the capacity will remain the same as a single battery. **While in a parallel connection, the total capacity** will be the sum of the capacity of each battery and the voltage will not change.

**The total number of batteries** you will need depends on your selected **battery characteristics**. To find **the required battery capacity of our PV system**, we need to know the **recommended depth of discharge (DoD) of our battery** and use the formula below:

To give you a numerical example, we continue with 127.37 A.h energy requirement (step 5) and 12 V nominal system voltage. Also, I consider a 12 V, 80 A.h battery with 80% DoD and a day of autonomy for my system. (In your calculations, round up to the highest whole number)

*(127.37 × 1) ÷ 0.8 = 159.21 A.h round up to 160 A.h*

The number of required **batteries in parallel**:

*N _{bat, p} = 160 ÷ 80 = 2*

The number of required **batteries in series**:

*N _{bat, s} = 12 ÷ 12 = 1*

And the total number of batteries will be:

*N _{bat} = 2 × 1 = 2*

As you can see in this example, I will need two of this type of battery connected in parallel. You should be aware that **having multiple batteries connected in series/parallel could damage battery pack performance**. The **mismatching effects in series connection** could lead to **system failure** and if you connect an old battery with new ones **in parallel**, **it will accelerate** their **ageing**. The good choice is to **select a higher capacity battery with proper voltage** to reduce the number of series and parallel connections.

## Conclusion

**Batteries are the part of the system that will most likely experience problems**, so you have to select carefully. A good battery will cost more but it will pay off over the years. Find the **cost per cycle-life** of different batteries and compare their **characteristics**, then choose the one that best meets your PV system requirements.

Lastly, I want to point out **the temperature effects on batteries**. All batteries have a specific range of operating temperatures, depending on their types (AGM, lithium-ion, etc.), but their overall behavior is the same.

Representative chart of temperature effects on battery capacity and life [src]

**A high temperature will increase the battery capacity (A.h)** but on the other hand, **will decrease the battery life-time**. The **optimal operating condition** is at **room temperature** around 25 C (77 F). Knowing this, you should **place your battery pack inside your RV/Boat cabin**, where it will perform best and will last longer.

##### References

[1] Xiao, Weidong. Photovoltaic Power System : Modelling, Design and Control, John Wiley & Sons, Incorporated, 2017.

[2] Smets, Arno & Jäger, Klaus & Isabella, Olindo & Van Swaaij, R.A.C.M.M. & Zeman, Miro. (2016). Solar Energy – The physics and engineering of photovoltaic conversion, technologies and systems.

[3] Mohanty, Parimita & Karnamadakala, Rahul Sharma & Gujar, Mukesh & Kolhe, Mohan & Azmi, Aimie. (2015). PV System Design for Off-Grid Applications. 10.1007/978-3-319-14663-8_3.

[4] Shuai Ma, Modi Jiang, Peng Tao, Chengyi Song, Jianbo Wu, Jun Wang, Tao Deng, WenShang. (2018) Temperature effect and thermal impact in lithium-ion batteries: A review.

[5] Lead Acid Batteries – Battery Lifetime.

[6] Series and Parallel Battery Configurations.