How Long It Will Take To Charge 12v Battery With Solar Panel? (Explained)

Have you ever wondered how long it will take to charge 12v battery with solar panel? Look no more. We´ve got you covered.

Most RVs nowadays are powered by 12V batteries. These batteries are then combined with an inverter to step up the power to ensure that all of those appliances in your RV can be properly be powered.

So, how long it will take to charge a 12V battery with solar panel? On average you can expect 5 hours of sunlight to charge a 12v battery with a 300-watts solar panel. However, how long your battery takes to charge will also be dependent on the amperage of the battery and the amount of sunlight you get.

We are going to answer that question in detail for you. However, do bear in mind that the information here is only general information.

This is because there are a lot of factors that can influence the charge speed. Although, we are going to talk about those later on.

Assumptions

On this page, we are not going to be making any assumptions about the type of battery technology that you are using in your RV. Our only assumption is that you are using a deep cycle battery.

The charge time may differ ever so slightly if you are using certain battery technology. It may also differ if the battery is older, or if it has been fully depleted.

For most battery technology, we don’t think that the information here is going to be wildly off. Although, you may want to monitor your battery charge time if you are really concerned about how fast the battery is charging.

Let’s jump into the actual information now, shall we?

How Long to Charge 12V Battery with Solar Panel?

How long your battery takes to charge will be dependent on four things:

• The amperage of the battery
• The amount of sunlight you get
• The number of watts on the solar panel
• The amount of power still left in the battery.

The amperage is the important number here. It is the amount of power that the battery is able to store. The higher the amperage, the more stored power in that battery and, therefore, the longer that battery is going to take to fully charge.

Basically, you can expect a battery that is 100AH to take half the time to charge as a battery that is 200AH.

While it will be different depending on the time of year, the average amount of sunlight per day where the solar panel will be operating at full capacity will be around the 5-hour mark.

We will talk about the number of watts left in the solar panel shortly. We are going to work through this equation using a little bit of math!

Determine how much power needs to be charged

We are going to assume that you are using a 12-Volt, 100ah battery. This is the most common type of battery.

If you are using something that is a greater amperage, or even lower amperage, it doesn’t really matter. You can just substitute the numbers as we work through this math.

You then need to account for how much charge is left in the battery. Since you are only supposed to discharge deep cycle batteries a maximum of 80% before recharging them, there should be around 80ah of power to charge.

Although, of course, this can differ if you have not fully depleted the battery.

So, now we know that we need to charge 80ah of battery on a depleted 100ah battery. This leads us to the next calculation.

The Size of the Solar Panel

The solar panel is going to be the biggest determining factor in how long your battery takes to charge. We are going to assume that you already own a solar panel.

If you don’t, then you can buy one to speed up the rate at which your battery is charging. We will talk about that here.

As a rough guideline, for each 100-watts of power that a solar panel has, you will be able to produce 5-amps of power per hour.

As we know that there will be an average of 5-hours of direct sunlight each day, we know that 100-watts of solar power will be able to charge a minimum of 25-amps per day.

This leads us to the final part of the calculation and the information that you have been waiting for.

How long for the battery to charge?

Let’s go back to the amount of charge that we need to put into the battery. As we said before, we are working with the 80ah figure here, but yours may be slightly lower or higher than this.

It is all dependent on the capacity of the battery that you have. But, as we said, we are going to roll with that 80ah, and you can substitute the information as you see fit.

Since we know that 100-watts of solar power can charge 5amps an hour, we do this calculation:

80 / 5 = it would take 16-hours to charge the battery fully using a 100-watt solar panel.

Since there are just 5-hours of usable sunlight a day, it would take about 3-days total to fully charge that battery.

Of course, if we bump up that wattage to 200-watts of solar power, then it would only take 8-hours. So, a day and a half. If we bumped it up to 300-watts, then that battery could be charged in the five hours of sunlight that we get per day.

You can’t really bump the charge wattage up more than 300-watts. It could cause the battery to overheat which, of course, is going to end up causing long-term damage.

Let’s go through, step-by-step, how long you can expect your 12V battery to charge using solar power. This information is applicable, no matter the capacity of your battery, or the wattage of solar panel that you have:

• Determine how much of the battery needs to be charged. This will be a maximum of 80% of the charge capacity.
• For every 100-watts of solar power, expect that battery to receive 5amps of power per hour. So, multiply 5 by the number of 100-watts you have e.g. if you have 250-watts of solar power, then multiply by 2.5. This will give you the amount of power the solar panels can charge the batteries with per hour.
• Divide the amperage you need to charge by the hourly amperage provided by the solar panels. This will give you the number of hours it would take to charge that battery with full solar power.
• Divide this number by 5, and it will give you the number of days it would take to charge (assuming 5-hours of sunlight per day)

Yes, we know that it is quite a bit of math. However, it is the only way that you can work out how long that battery is going to take to charge. We will give you a quick rundown of the math for a 100-amp battery, though.

We, once again, want to reiterate that these are only rough figures, and they assume that you are only discharging your battery down 80%.

If you take it all the way down to 0%, then you will have to add about 8-hours to these figures as that is how long it will take for those battery cells to be reactivated.

What Can Make Battery Charging Slower?

The reason why we can only give rough figures here is that there is a lot that can influence just how fast a battery will be charging.

Obviously, the amount of direct sunlight that your solar panel is getting will determine how fast the charge is. So, if you are dealing with the following, then expect the charge time to be drastically slower:

• Clouds
• The solar panel is in the shade
• You are charging during the winter months

We did vastly underestimate that you will get 5-hours of sunlight per day, though. It is likely that you will have ever so slightly more, so the charge times shouldn’t be that far off.

As we said before, over time, you will start to get a feel for how long it takes your battery to fully charge in a variety of different situations.

The quality of the solar panel will also impact how long the battery takes to charge. For instance, a quality 300-watt solar panel will likely be producing more power per hour than a cheaper 300-watt solar panel. This is is because the technology is going to be a whole lot better. This means that when you are searching for a solar panel, you will want to pay attention to what the manufacturer claims is the charge rate for their solar panels.

You will then need to change the calculations we gave before to suit. However, once again, we did underestimate things to try and ensure that the charge time is applicable in most situations.

Will RV Battery Charging be Slower if You Are Using Power at the Same Time?

Of course.

If you are drawing power from the solar panel at the same time, then your battery charging is going to be a lot slower. This is because solar panels are only going to be able to provide so much charge at a time.

So, if you are drawing 5-amps of power an hour for whatever you are running in your RV, then the amount of power that the battery receives will have to be reduced by 5-amps.

So, if you had a 300-watt solar panel, you can expect it to charge the battery as if it were a 200-watt solar panel.

Again, a pretty rough calculation, but it should give you an idea of how much the charge speed will be reduced (i.e. the charge time will now be 30% higher!)

How Do You Choose a Solar Panel for Your 12V Battery Charging?

If you don’t have a solar panel yet, then the world is your oyster. You can get one that will charge the battery in the amount of time that you want it to be charged.

So, if you want a 100ah battery to be charged within a single day of sunshine, then you would need to buy a 300-watt solar panel. If you want to provide 5-amps of power to the rest of your RV while the battery is charging, then you would need 400-watts.

If you are charging two 100ah batteries and want them to be fully charged before the day is up, then you will need to be providing them with 600-watts of solar power.

By now, you really should know the wattage that you want.

Beyond this, you need to focus on the quality of the solar panel. The better it is, the more stable the power it delivers will be.

This means that your batteries are likely to charge a lot quicker. Always ensure that you read reviews to get an idea of how good a particular solar panel is.

SOLAR REGULATORS EXPLAINED! >> Check out the video below:

Do You Also Need to Buy a Charge Controller?

Yes. If you want to charge your batteries, then you will need to buy a charge controller.

The purpose of a charge controller is to ensure that there is not too much power flowing into the battery at once. It will also help to ensure that the battery is not overcharged.

The result will be a battery that lasts you a whole lot longer. Without a charge controller, your battery would very quickly overheat.

Conclusion

How long it takes to charge a 12v battery will be dependent on a variety of factors. You need to know the amperage of your battery, and the wattage of your solar panel.

For every 100w of power that the solar panel is able to provide, you can expect it to provide 5-amps of charge to the battery.

Once you know this information, then it should be pretty simple to work out just how quickly your battery is going to charge on an average day on the road.

References

https://www.renogy.com/blog/what-size-solar-panel-do-i-need-to-charge-a-12v-battery

Mike Gilmour

Hi, I'm Mike, co-founder, and editor of RV and Playa. My passion is traveling (with my RV) and enjoying the day at the beach (Playa)! Well, I originally created this blog as a way to share what I've learned by experimenting with the RV lifestyle, and I want to help others develop in life through new skills and opportunities.

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