What size solar panel to charge 120Ah battery
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I have seen a variety of answers when I googled how many solar panels you need for a 120Ah battery.
I would say 90% of the answers on this matter were confusing. So I thought I'd guide you to a very simple way to calculate your answer.
To charge a deep-cycle battery with solar panels, you need a solar charge controller. Without the controller, you will have to replace your solar batteries much sooner because they will die faster.
Therefore, all you need to know before calculating the size of the solar panel is just the maximum charging rate for your battery type (lithium or lead-acid).
The short answer is; about 280 watts. But you need to know more.
2 Steps to calculate the size of solar panel needed to charge 120Ah battery
Between the battery and the solar panels, there is a charge controller, an important tool that has inputs and outputs for current and voltage.
The outputs provide the charging current, and it is important to choose a unit that will provide the appropriate charging current for your battery capacity. If you choose a large unit, it should not be fully loaded with solar panels.
The inputs provide the Nominal PV power or maximum PV power, which should not be exceeded to avoid damaging the charge controller.1. Determining the healthy max charging current for a 120Ah battery (charge controller size)
The healthy maximum charging current of a deep cycle battery depends on its capacity and type. You can refer to the datasheet of your battery to find the maximum charging current.
Let's divide this section into two parts. One for lead-acid battery and one for lithium (LiFePO4) battery.
Max charging current for lead-acid battery
The literature says that the healthy max charge rate for lead-acid battery is 0.2C (A) (below 20% of their actual capacity).
- Max charging current = 120Ah × 0.2 = 24 Amps
Meaning max 24A for a 120Ah lead-acid battery
keep in mind that you can't really use more panels to pump (for example) a full 120Ah of charge into the battery in one hour just because that's all the sun you have.
Max charging current for lithium battery
With lithium batteries, you can charge them up to five times faster than lead-acid batteries, although it is only recommended to charge them up to 50% of their capacity to extend battery life.
Keep in mind that you can charge up to 100% of the battery capacity, but this will shorten the battery's cycle life, and the Battery Management System (BMS) can handle a 100% charging rate.
- Max charging current = 120Ah × 0.5 = 60 Amps
Thus, the healthy max charge rate for lithium battery is 60A for a 120Ah lithium battery.
2. Determine what size solar panel to charge 120Ah battery
The appropriate size of the solar panel for the charge controller that will provide the maximum healthy charge current for the battery depends on the type of the charge controller (MPPT or PWM), and the nominal voltage of the battery (12V or 24V).
MPPT charge controller
To calculate the size of a solar panel to charge 120Ah battery, you need to multiply the battery voltage (V) by the healthy max charging current of the battery (A). The formula is as follows:
Max solar panel array wattage = Battery voltage (V) × Max charging current (A)
For 12V battery:
- For 12v 120Ah lithium battery: Max solar panel array wattage = 12V × 60A = 720W
Therefore, if you only get 2 hours of direct full sun per day, you would need a solar panel with an output of 720 watts to charge the 12V 120Ah lithium battery.
keep in mind that you can use a smaller solar panel size when you have more than two hours of full sunlight available, you only use for example 60% of the battery capacity daily, or if you plan to use a charge controller smaller than 60 amps.
- For 12v 120Ah lead-acid battery: Max solar panel array wattage = 12V × 24A = 280W
Therefore, you would need a solar panel with an output of 280 watts to healthy charge the 12V 120Ah lead-acid battery.
If you oversized your charge controller, it is not recommend to exceed this value, as during full sunlight hours, the battery will be charged with a strong current.
The only way to charge the lead-acid battery quickly with a larger solar panel is by adding more batteries in parallel. In this case, the charging current will be divided among the batteries.
You can use only a portion of the total battery capacity daily and be able to safely recharge them in a few hours. Additionally, the discharge rate will be lower, which will not harm their lifespan.
For 24 battery:
- For 24v 120Ah lithium battery: Max solar panel array wattage = 24V × 60A = 1440W
- For 24v 120Ah lead-acid battery: Max solar panel array wattage = 24V × 24A = 576W
You can also refer to the datasheet of the charge controller to find the maximum solar panel array.
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| 15 Amps MPPT charge controller victron energy |
PWM charge controller
For 12V battery:
You will need a solar panel that provides a slightly higher voltage than 12V to charge the 12V battery. Most 180W 12V panels have 30 or 32 cells, each generating about 0.6 volts. They are connected in series to produce an open circuit voltage of about 20 volts. It will drop to around 16 volts when loaded, which is good for charging a 12V battery with a PWM charge controller without losing much power.
You will need 2 of them, connected in parallel to charge your battery at less than 20 amps.
For 24V battery:
For a 24V battery, you will need a solar panel that provides a slightly higher voltage than 24V. Most 200W panels have a voltage of 36 volts when loaded (60 cells), which is acceptable for charging a 24V battery with a PWM charge controller, but you will have some energy lose.However, you will need 4 of them, connected in parallel to charge your battery at less than 24 amps. You can use 100W panels, but you will need 8 wired in a 2S4P configuration.
Mohamed Bellamine
I'm the blogger behind Goo SolarPower. As a solar engineer, and a strong supporter of renewable energy. I've always been interested in Solar devices. I loves to write articles, guides, and tutorials about solar PV technology. Through goosolarpower.com I want to help people build and understand how solar power systems work.