# Solar Battery Charge Time Calculator (12v, 24v, 48v)

Use our solar battery charge time calculator to find out how long will it take to charge a battery with solar panels.

## How To Use Our Solar Battery Charge Time Calculator?

To use the calculator, follow these steps:

1. Enter the total solar system size in watts: If you have multiple solar panels connected together, add their rated wattage and enter the total value in watts into the calculator.

2. Enter the battery capacity in amp-hours (Ah): If the battery capacity is given in watt-hours, divide the watt-hours by the battery voltage to find out the amp-hours. For example, enter 50 for a 50Ah battery.

3. Enter the battery voltage (V): Is this a 12, 24, or 48-volt battery? Enter 12 for a 12V battery.

4. Select your battery type from the options provided.

5. Enter the battery depth of discharge (DoD): Battery DoD indicates how much of the battery capacity is discharged relative to its total capacity. For example, enter 50 for a battery that is half discharged, and enter 100 for a battery that is fully discharged (which is achievable only with lithium batteries).

6. Select the charge controller type: Which type of charge controller are you using? PWM or MPPT?

7. find out the estimated battery charge time.

```Note: The calculator assumes the following efficiencies:

1. Battery charge efficiency - Lead-acid --- 85%, lithium --- 95%
2. Charge controller efficiency - PWM --- 80%, MPPT --- 95%
3. Solar panel output efficiency - 80%```
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## How To Calculate Solar Panel Charge Time?

Dividing the battery amp-hours (Ah) by the solar panel’s output amps (Ah ÷ charging amps) is the most inaccurate way to calculate the battery charge time. Instead, use this formula:

### Formula

Solar battery charge time = (Battery Ah × Battery volts × Battery DoD) ÷ (Solar panel size (W) × charge controller efficiency × battery charge efficiency × 0.8)

This method takes into account most of the real-world factors that affect the battery’s charge time.

### Steps

Let’s say you have a 12v 100ah lead acid battery with 50% Depth of discharge, a 100-watt solar panel, and an MPPT charge controller.

1. Multiply 12 by 100 to convert the battery capacity into watt-hours.12 × 100 = 1200 watt-hours

2. Multiple the battery capacity in watt-hours by its depth of discharge.1200 × 50% = 600 watt-hours

3. Multiply the solar panel-rated watts by the charge controller efficiency. PWM — 80%, MPPT — 95%.100 × 95% = 95 watts

4. Take into account for battery charge efficiency rate by multiplying the battery charge efficiency by the solar panel’s output (W) after the charge controller.

• Lead-acid batteries have a charge efficiency ≈ of 80 − 85%
• Lithium-ion batteries have a charge efficiency ≈ of 90 − 95%

95 × 85% = 80 watts

5. Take into account the solar panel’s output efficiency. Solar panels are designed to produce their rated wattage under ideal conditions, but in real-world conditions, on average, you’d receive about 80% rated output from your solar panel.80 × 80% = 65 watts

6. Now divide the battery capacity after DoD by the solar panel output (after taking into account the losses).600 ÷ 65 = 9.2 hours

Turns out, a 100-watt solar panel will take about 9 peak sun hours to fully charge a 12v 100ah lead acid battery from 50% depth of discharge.

## How Fast Should You Charge Your Battery?

Deep cycle or solar batteries are designed to charge and discharge at a specific rate, which is referred to as the c-rating. It’s important to note the recommended charge time for different types of batteries:

• Lithium (LiFePO4) ≈ 2 hours
• Lead acid ≈ 20 hours
• AGM ≈ 5 hours

Charging your battery at a higher rate than what’s recommended can increase the battery’s internal cell temperature, which will ultimately decrease the battery’s overall health and charging efficiency.

Note: Refer to your battery’s data sheet to find its c-rating or maximum charge rate.

## Why none of The Above Methods Guarantee 100% Accuracy?

Calculating the accurate charge time for a battery is a challenging task because there are numerous real-world factors that can impact it.

Some of these factors include the state of charge of the battery, the absorption stage for lead acid and lithium batteries, solar panel output efficiency, and PWM charge controller efficiency.

1. Lead acid battery charge efficiency will depend on its state of charge (SoC).

The below chart illustrates the lead acid battery charge efficiency based on its state of charge.

lead acid battery charge efficiency will decrease drastically after 70-80% charge, that’s because of the absorption stage.

2. Absorption stage for lead acid and lithium battery: During the absorption stage, which is a phase in the battery charging process, a fixed amount of time is used to charge the battery, regardless of the amount of input power

For lead acid batteries, this stage typically lasts between 2-3 hours and helps the battery reach its total capacity from 80% charge. The absorption stage is important for the health of the battery, as it helps to balance the battery cells and prevent damage.

However, lithium batteries do not require an absorption stage, although charge controllers may perform a brief 20-30 minute absorption charge to balance the battery cells.

3. Solar panel output efficiency will depend on many factors, such as the tilt angle of the panel, weather conditions (e.g., sunny or cloudy), and ambient temperature.

For example, if the panel is not tilted towards the sun, it may not receive optimal sunlight, resulting in lower efficiency. Similarly, if it’s cloudy or there is low light, the panel’s output will decrease.

Moreover, solar panels become less efficient at higher temperatures, so if the ambient temperature is high, the panel’s output may be reduced.

4. The efficiency of a PWM charge controller can be affected by different factors, such as temperature and the voltage difference between the solar panels and the battery.

A PWM charge controller can only lower the high voltage to match the battery voltage for safe charging but cannot increase the amps. As we know power (watts) = Amps × Volts.

In contrast, an MPPT charge controller can reduce the voltage and boost the amps to compensate for any losses.

High temperatures can cause the controller’s efficiency to decrease, while a greater voltage difference between the solar panels and the battery can lead to lower charging efficiency. This is because the solar panel generates a higher voltage at higher temperatures.

It’s important to consider these factors when estimating the battery charge time using a PWM charge controller.

## How Long To Charge 12v Battery With Solar Panel?

Here’s a chart showing how long will it take to charge a 12v battery with different capacities of lead acid and lithium batteries using a 100-watt solar panel with an MPPT charge controller.

#### Summary

• 12v lead acid battery from 50% depth of discharge will take anywhere between 2 to 20 peak sun hours to get fully charged with a 100-watt solar panel.

### 12v Lithium (LiFePO4) Battery

#### Summary

• 12v lithium battery from 100% depth of discharge will take anywhere between 3 to 30 peak sun hours to get fully charged with a 100-watt solar panel.

## How Long To Charge 100ah Battery

Here’s a chart showing how long to charge 100ah lead acid and lithium batteries with different size solar panels.

### Summary

• 12v 100ah lead acid battery from 50% depth of discharge will take between 2 to 40 peak sun hours to get fully charged with solar panel.
• 12v 100ah lithium battery from 100% depth of discharge will take between 4 to 80 peak sun hours to get fully charged with a solar panel.
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