There are different elements involved in the designing of a solar street light system. It is highly important to make an accurate measurement of the required values of all the elements for the smooth operation of the system. The two most important elements involved in the designing of battery powered street lights are the calculations of the battery and the solar panel. The ratings of these panels depend upon the solar street light that they have to operate. For the fundamentals, our guide on the solar street light working principle is a helpful starting point.
Therefore, people should first purchase a solar street light that is capable of meeting their lighting requirements. We recommend our readers carry out extensive research before purchasing the solar light, since it will affect the parameters of all the other elements of the system. People can check the luminosity of the solar lights and their power consumption ratings before making the investment. Our list of the 10 things about the best solar street lights can guide this decision.
Once they have purchased the street light, they can move ahead with designing an appropriate battery and solar panel to suit their requirements. In this article, we will explain how users can calculate the battery ratings for battery powered street lights.
Important Concepts to Understand
Battery capacity and power
The capacity of a battery is a measurement of the maximum amount of energy that it can store. Batteries having large capacity can store more energy compared to those with lower capacity. The capacity of a battery is determined in terms of kilowatt hours or kWh. In simple terms, it is the amount of energy that a battery can provide in an hour.
Similarly, the power of the battery is given in terms of watts and is known as the power rating of the battery. The power rating is the measure of the multiplication of voltage (V) and current (I) supplied by the battery to the load. Understanding these values is essential when sizing battery powered street lights.
Ampere hour (Ah)
This is also a measure of the capacity of the battery. However, under this criterion, the capacity of the battery is represented in terms of the multiplication of the total current that it supplies over a number of hours. For example, if a 12 volt battery supplies a current of 5 amperes for a period of 5 hours, then the capacity of the battery will be 25 ampere hours.
The capacity of the battery = 5A x 5h = 25 Ah.
This representation of the capacity is necessary because usually the specifications of batteries are given in terms of ampere hours.
Depth of discharge or DoD
This gives an account of the energy in a battery that can actually be used by the load. It is possible that a battery may not be able to make use of all the energy that it has stored. Therefore, the amount of energy in a battery that can be used for productive purposes is represented in terms of the depth of discharge of the battery. In common terms, it is the energy that a battery supplies when it discharges from full capacity to the lowest safe capacity.
For example, if a battery has a depth of discharge (DoD) of 80%, then out of one hundred percent, only 80% of the energy will become available for the load while 20% of the energy will not become available for the load. In quantitative terms, if a battery has a capacity of 50 kilowatt hours and a depth of discharge of 60%, then only 30 kilowatt hours of energy will be productive.
The working temperature of a battery
This is also one of the most important factors that affect the performance and parameters of a battery. The battery should have an optimum temperature of around 27 degrees Celsius, as it might not perform well above or below this temperature. Maintaining the right conditions is key to the longevity of battery powered street lights, as covered in our 10 tips to maintain your solar street light battery.
Calculations of the Battery Requirements for Battery Powered Street Lights
In this section of the article, we will explain a step by step process with which people will be able to calculate the requirements of a battery for their battery powered street lights.
Step 1
The first step in the calculation of requirements for the battery is the collection of relevant data necessary for the process. The data necessary includes the power rating of the street light that has to be operated using the battery, the working voltage of the street light, the brightness requirements, the supported rainy days for the operation of the street light, and the depth of discharge (DoD) of the battery.
The rated power and voltage of a street light are available in its specification sheet.
The brightness requirements of the street light have to be determined by the user. It is probable that they may decide to operate the light at 60%, 70%, or 80% of its brightness instead of operating it at 100%.
Users will also decide the requirement for the number of rainy days of support.
The depth of discharge of a battery will also be available on its specification sheet. We recommend users consider 80% depth of discharge (DoD).
Step 2
In the second step, users have to calculate the daily consumption of the battery powered street light. Daily consumption of the street light will also be in terms of ampere hours (Ah). The formula for calculating power consumption is as below:
PC = (Power Rating / Voltage) x Time
Step 3
In the next step, users should account for the requirements of rainy days for which they need the battery to supply power to the load. For example, if users require support for rainy days for up to three days, then they will have to multiply the power consumption of the load by this factor. Therefore, the formula for accounting for rainy days support in battery calculations is as follows:
Power consumption (PC) x number of rainy days
This is the rating of the battery that users will require in order to support the load. However, there is still a need to account for the depth of discharge (DoD), which means that a battery of higher rating will be required depending upon the DoD percentage.
Step 4
The next step is to account for the depth of discharge (DoD) of a battery, which can reduce its overall usable capacity. In order to account for DoD, users need to divide the value obtained from Step 3 by the DoD percentage. For example, if the power consumption of the load with three days of support is 100 Ah, and users want to consider 80% DoD, then the requirement of the battery will be 100 / 80% = 125 Ah.
Therefore, in order to support the load, users will require a battery of 125 Ah rating for their battery powered street lights.
Battery Powered Street Lights: Conclusion
We have discussed that the ratings of a battery for battery powered street lights depend upon a number of different factors, and people should consider all these factors for making accurate calculations. Some of the factors on which the ratings of the battery depend include the required capacity/rating of the load, the depth of discharge, and the working temperature of the battery. People can follow the steps above in order to calculate requirements for battery powered street lights. For those who enjoy the technical side, our guide on how to make a smart solar street light circuit explores the electronics in depth.
Frequently Asked Questions (FAQs)
1. How do I calculate the battery size for battery powered street lights?
To size the battery for battery powered street lights, first gather the light’s power rating, voltage, brightness level, required rainy day backup, and battery DoD. Calculate daily consumption (power ÷ voltage × hours), multiply by the number of backup days, then divide by the DoD percentage to get the required ampere hour rating.
2. What depth of discharge (DoD) should I use for battery powered street lights?
For battery powered street lights, a depth of discharge of 80% is generally recommended. This balances usable energy against battery longevity discharging too deeply shortens battery life, so reserving around 20% capacity helps the battery last significantly longer.
3. Why do rainy days matter when sizing batteries for battery powered street lights?
Rainy or cloudy days reduce solar charging, so battery powered street lights need extra stored energy to keep running without sun. Sizing the battery to support 2 to 3 backup days ensures the lights stay illuminated through overcast periods without interruption.
4. What is the ideal operating temperature for batteries in battery powered street lights?
The ideal operating temperature for batteries in battery powered street lights is around 27°C (about 80°F). Performance drops when temperatures rise too high or fall too low, so choosing a battery rated for your local climate is important for reliable year round operation.
5. What type of battery is best for battery powered street lights?
Lithium ion batteries are generally the best choice for battery powered street lights because of their high energy density, longer cycle life, higher charge discharge efficiency, and lower self discharge rate compared to traditional lead acid batteries, making them more compact and reliable.