What Size Solar Panel To Charge 100Ah Battery?

Charge 100Ah Battery with solar panels

What size of solar panel will I require for charging a 100AH battery?

In the beginning, I’d like to know: What percentage of battery capacity has been utilized? If I can determine how many amp-hours have been used up and discharged in the process. I’ll know how much energy I will need to replenish and determine the size of your solar panels accordingly.

I’ve never discharged the lead-acid battery. Ultimately it would not be normal in the event of an emergency.

Certain lithium batteries can be nearly empty lead-acid batteries should be discharged at 20% and 20% depending on whether they’re normal car batteries or deep cycle designs.

I’ll be discussing lead-acid batteries in the context of this article since they are the most popular type of battery used. In the category of lead-acid, you can find two types available in lead-acid cells style.

It is generally accepted that a 100Ah deep-cycle lead-acid battery will require around 180W of solar power to fully charge from 50 percent Depth of Discharge (DOD) in the event of 4.2 maximum sun-hours every day. It could take up to 8 hours to fully recharge under sky clear.

What is the difference between a normal car battery and a deep-cycle battery?

Batteries referred to as leisure, or marine batteries may be discharged at a more extensive level than a regular automobile battery. This is why they are higher priced. They are also known as deep-cycle batteries.

The two kinds of cells are designed to supply power in different ways. Deep-cycle batteries are intended for applications that require moderate continuous currents for an extended period. One time. Auto batteries must deliver many amps quickly to crank heavy engines.

Their discharge capacities differ also. It is possible to discharge a deep cycle battery as low as 50 percent of its capacity, and even down to 80 percent in the event of need, without any risk of damage.

However, it’s recommended not to charge the car battery by over 15 percent or 20 percent at the maximum.

The difference between auto and deep-cycle batteries?

Then, there’s first, the 100Ah automatic battery, let’s say it’s discharged regularly to 20 percent:

100Ah x 20% = 20Ah

A 10Ah deep cycle leisure battery can be charged regularly until 50 percent:

100Ah x 50% = 50Ah

At 50 percent discharge, the deep-cycle battery will require more than double the amount of solar power to recharge it completely.

I’ll take that the discharge is between 20 and 50 percent for the various types in my calculations.

What exactly is Battery SOC and DOD, and why is it important?

SOC means State of Charge The SOCmeans State of Charge. DOD is the initials that represent Depth Of Discharge. They’re identical.

The State of Charge indicates amp-hours of capacity remaining in the battery. Depth of discharge indicates the amount of power taken out or discharged. DOD is more suited to us because it’s the energy we must replace with solar energy.

How do you determine the remaining capacity of the battery?

An easy and quick method to test your remaining capacity in amp hours of the lead-acid battery is to check the terminal voltage using a tester, like a multimeter.

The battery must be kept in a state of dormancy for approximately six hours before taking the voltage measurement to ensure that there isn’t any chemical reaction occurring.

Chart of discharge depth for a lead-acid battery

Use the table below to match your terminal voltage with the percent column to determine the battery’s remaining capacity. This chart works for any lead-acid battery.

State Of Charge % (12 V Pb)Battery terminal voltage
10012.73
9012.62
8012.50
7012.37
6012.24
5012.10
4011.96
3011.81
2011.66
1011.51

Amp-hours to Watt-hours – Calculating battery capacity

Amp-hours (Ah) is an efficient method of measuring the battery’s capacity. It can result in an oversimplified understanding of how batteries produce current.

For instance, the connection of amps and duration isn’t as simple as it appears. A 100Ah battery could give 10 amps for a short period. However, it’s not going to last over 10 hours!

A deep-cycle model may only provide 10 amps for five hours, with the battery in a car being much lower. Temperature is also a factor, and so does the speed of the current draw.

Energy in Watt-hours (Wh) is a useful method to quantify battery capacity since the solar panel’s output is measured expressed in watt-hours. This is logical to measure like against like.

Let’s take a look at the solar energy required to charge each kind of battery:

Auto battery (100Ah 20 x 100Ah) 12 voltages = 20 x12 = 244 watt-hours
Deep-cycle (leisure) battery (100Ah 50 percent) = 50 12 Volts equals 600 W-hours

Another observation of mine is written in an article on this topic: “What Size Solar Panel to Charge a 12v Battery?

How to find daily solar irradiance data by location

While many other factors impact solar panel effectiveness, The most significant influence on the power output of solar panels is the irradiance which is the quantity of sunlight’s energy hitting the panels at your location.

Two methods can do the measurement of irradiance:

  • Kilowatt-hours per square meter the day ( kWh/m2/day)
  • high-sun hours (basically the identical units)

Radiance varies according to the area of residence and by the season.

It is a lot higher in Texas than London, UK – around three times more. That implies that solar panels of similar dimensions will produce three times the energy within Houston, Texas, London.

The most efficient way to determine the hours of maximum sun for your city is to go to the website Global Solar Atlas and look up the historical irradiances for your area. The picture below illustrates how to do this:

Austin, Texas direct normal irradiation
Direct normal irradiation for Austin, Texas, USA.

If you know the number of peak hours for your specific location, multiply that by 100, and you’ll get the amount of energy in watt-hours the 100-watt solar panel generates in your area.

A good estimate to use as a guideline is that a 100-watt solar array produces around 400 watts-hours per day, which will vary according to where you live.

How to find daily solar irradiance data by location

I’ll take 400Wh/day as an example to demonstrate how it functions. The equation shows the amount of 100-watt solar panels I’d need to recharge the batteries:

Car battery (100Ah 20 percent) 12 Volts = 20 x 12 = 240 Watt-hours.

A 100-watt solar panel can recharge this battery in less than an entire day, about five hours.

Battery with deep-cycle: (100Ah x 50 percent) = 50 12 volts = 600 Watts-hours.

I’d require 2 solar panels rated at 100 watts to charge the battery within a matter of a day fully. Two 100 watts will produce around 800Wh/day.

Solar circuits are always subject to losses. Therefore, I suggest you consider adding 30-50 percent to the size of the solar panel you calculate. Along with the energy lost due to the inherent system losses, each passing cloud reduces the generated energy.

This chart offers a glimpse of how the sun’s energy can vary across physical locations and how sunlight hours during peak times can affect the size of solar panels:

Location Glasgow, UK Chicago, Il Lancaster, Ca
Peak-sun-hours/day 1.86 4.03 8.07
Solar panel rating in watts needed to fully charge each battery type in one day – full capacity 100Ah Car Battery – 20Ah discharged (20%) – 240 Wh required 130 watts solar panel rating required 60 watts solar panel rating required 30 watts solar panel rating required
Deep-cycle – 50Ah discharged (50%) – 600Wh required 322 watts solar panel rating required 149 watts solar panel rating required 75 watts solar panel rating required

The conversion of Ah into one kWh is a great method to consider because you’re comparing comparable types of energy. For example, the energy required to recharge the battery as kWh is equivalent to what kWh is produced by the solar panels.

Instantaneous measurements of current don’t have any connection to the solar panel’s power output, as it changes during the day by the sun’s angle and is very dependent on any shade caused by cloud coverage.

Peak-sun-hours is an ideal average of the energy produced by the sun throughout the day. It is the most accurate method to measure solar panels for charging the battery.

Can I connect a solar panel directly to the battery?

Do not connect a solar panel directly to batteries if its power is so small that it barely produces any generated. It is possible to have two options in solar-powered controllers, and their operation differs.

The process of charging the battery from the solar panel
The process of charging the battery from the solar panel.

MPPT or PWM – which is the better choice?

Its Pulse Width Modulation ( PWM) solar charger is less effective than the two options. It can measure the battery’s volts and reduce the panel’s output voltage to just over the battery voltage.

The PWM principle of operation does not take into consideration what is the MPP for the panel. When the MPP is set on solar panels, the panel’s voltage and the currents are highest, and the most charge power will be produced.

The MPPT ( Maximum Power Point Tracking) solar charge controller functions differently.

Every circuit has an internal resistance. In the case of solar panels, it’s known as”characteristic resistance.” Characteristic resistances.

A solar panel with MPPT alters its internal resistance to be in line with the panel’s, which means that the maximum power is generated.

Solar charge controllers with MPPT can offer up to 40 percent more power than PWM.

Can you connect two or more solar panels?

MPPT controllers utilize higher voltage inputs than PWM controllers. Even a 400-watt model can be able to handle up to 60 voltages. In reality, MPPT-powered chargers can be better than other chargers when they have greater energy inputs from solar.

The open-circuit voltages for a 100-watt solar panel with 36 cells are around 21 Volts. In a pinch, you can connect three of them in the series arrangement, like the one shown below:

Parallel and Series connection of solar panels
Parallel and Series connection of solar panels.