If you have an RV then, chances are, that it will be loaded with deep cycle batteries. It is these batteries that will store any excess power produced by your generators or solar panels.
This means that when you are not running the generator, or there is enough sunlight about, you are still going to be able to charge your battery.
One question we get asked a lot here is how many watts of solar power energy it will take to charge a deep cycle battery? On average, you need a 300-watt solar panel to charge a 12 V 100 Ah deep cycle battery within 5 hours of sunlight. However, you must keep in mind that the wattage required depends on the amount of sunlight and the battery capacity.
If you use a 100-watt solar panel, you will need 15 hours of sunlight or an average of 3 days to charge your battery.
Perhaps more importantly, how quickly that deep cycle is able to charge depending on the number of watts that you give the battery. We are going to try and answer all of your most burning questions on this page.
Table of Contents
What is a Deep Cycle Battery?
First things first, what is a deep cycle battery, and how do they differ from your typical battery?
Well, a deep cycle battery is designed to offer sustained power over many hours. It is designed to be fully charged, and then discharged down to about 20% capacity. Basically, you will be deep cycling their power.
No other battery type is going to be able to put up with the constant charge and discharge cycle that a deep cycle battery has. This is why this is the only type of battery that you should be using in your RV.
How Many Watts of Solar Panel to Charge Deep Cycle Battery?
In theory, you can charge a deep cycle battery from a 5-watt solar panel. It wouldn’t be that effective, and your battery would likely take over a year to charge, but it can be done.
To be honest, the main concern here isn’t how many watts it would take to charge a deep cycle battery. Any wattage can do that.
It is how many watts it would take to charge the battery in a reasonable amount of time. In our opinion, this would be within a day of charging.
|Amperage||Solar Panels wattage (watts)||Time to charge (Hours)||Amount of sunlight a day (Hours)||Time to charge (Days)|
In order to work out how many watts you need for your battery, there are a few pieces of information that you will need to hand. Let’s go through them.
Battery Capacity (AH)
The battery capacity is the amount of power that your battery can hold. For most deep cycle batteries in RVs, this will either be 100ah or 200ah.
Now, because you are only supposed to discharge a deep cycle battery up to 80%, the effective capacity is much lower than this. In the case of 100ah, your real capacity would be 80ah. With a 200ah one, it would be 160ah.
We are going to be working on the assumption that you are discharging your battery down to the 20% mark here. Obviously, if you are not fully discharging your battery, then the charge time will be a lot quicker than the numbers that we give.
Your battery voltage is likely going to be 6v, 12v, or 24v.
It is 12v that will be the most common.
Both the AH and the voltage will be listed on the battery, so it shouldn’t be that much of a problem finding out that information.
Hours of Charge Time Per Day
Generally speaking, there should be about 5-hours of effective charge time per day. This is where the sun is giving the solar panel a full amount of juice that it can convert to energy. Some days may have a little bit more than this, and some days less.
We are going to go with the assumption that there will be 5-hours, just so that we can get an average here.
With this information to hand, we can start to work out how many watts we need to charge that battery fully within 5-hours i.e.,the amount of effective power that the solar panel is giving us.
To start with, we are going to need to work out the number of amps that we need to charge that battery by per hour.
For simplicities sake, let’s say that we are charging 80ah (i.e. a fully depleted 100ah battery), although you can change up this information a little bit based upon the battery capacity that you have.
With 5 effective hours a day, we need to divide that 80ah into 5. This means that to charge it fully, it would need 16ah of power.
We now take that 16ah of power, multiply it by the voltage (let’s assume 12v) and this gives us the wattage. In this case, it would be 192.watts, but to ensure that the power is stable, we would want 250-watts of power delivered.
So, to charge a 12v 100ah deep cycle battery in 1-day, you would need 250-watts of solar energy.
Let’s break that down a little bit for you. This way you can work out the calculation yourself:
- Divide the total amperage that needs to be charged into 5.
- Multiply by the voltage of the battery
- The number you get is the minimum wattage you need for the solar panel. However, it is recommended that you opt for a wattage slightly higher.
What Conditions Can Impact How Fast the Battery Charges?
As we said, there are a variety of different situations that can impact the speed at which the battery charges. That is what we want to take a look at in this section.
Wattage of the solar panel
Basically, the more watts you throw at the battery, the faster it is going to be charging. Although, there is a cap here. You can’t expect to throw thousands of watts at a battery and expect it to charge in 10-minutes.
The manual that came with your battery should indicate what the maximum wattage it can take is. Cheaper batteries tend to charge slower as they have a strict limit on the amount of power flowing into the battery.
The number of batteries being charged
Most RVs have more than one battery in. Obviously, the solar panel is only going to be able to produce a set amount of power.
This is going to be split across all of the batteries. So, if you had 2 x 12v 100ah batteries, then it would take 10-hours to charge (i.e. 2-days) with a 250-watt solar panel.
Whether you are using power in the RV at the same time
Same thing as with the multiple batteries being charged. If you are using power in the RV at the same time, then the battery is going to discharging quickly.
You need to factor this into the equation. For example, if your RV was pulling 15ah of power and you had a 350-watt solar panel coupled with a 12v 100ah battery, then the battery would never charge. Every time that it gets a little bit of power, you are going to be draining it away.
How much sunlight you are getting
Solar panels work at their peak when they are in direct sunlight. If even the smllest amount of shade is on that solar panel, it won’t work anywhere near as effectively.
If there is even the smallest amount of cloud in the sky, then the solar panel won’t work that effectively.
When we spoke about the 5-hours charge time before, we tried to work in the inconsistencies of sunlight. So, in most cases, you should expect about 5-hours of chargetime per day. However, if the following occurs, expect charging to be much slower:
- You are parked in the shade
- It is very rainy
- It is very cloudy
- It is in the midst of winter
It isn’t uncommon for those that do most of their RV usage during the winter months to require any extra 100w-200w of solar panels to make up for the short fall in the amount of sunlight that they are getting.
Will the Number of Solar Panels Decrease Charge Time?
In most cases, no. If you have 2 x 150-watt solar panels, that is still going to be delivering 300-watts of power to the batteries. This doesn’t change.
That being said, there are some situations where having multiple solar panels as opposed to one big one could benefit you.
As we said before, shade is a bad thing for solar panels. If you are parked in the shade, then the solar panels won’t be working to their maximum capacity. Because of this, a lot of people will actually place solar panels at different positions on their RV.
This means that if one solar panel loses its ability to charge effectively, the other solar panel may be able to work at peak performance. Charge capacity would still be reduced, but by nowhere near as much as having a single solar panel set up.
Do You Need a Charge Controller?
Yes. We know that there are some people that will try and cut costs by skipping the idea of a charge controller. However, this is an absolutely ridiculous idea. You may be cutting costs on that charge controller, but you will be increasing costs by having to replace your batteries a lot more frequently.
The idea of a charge controller is to help prevent the battery from being overcharged. It will also help to ensure that the battery is not receiving too much current at once.
As we said before, pouring too much energy into the battery at the same time can lead to it overheating. If you connected thousands of watts up to a 10ah battery, it would melt if you didn’t have a charge controller in place.
The charge controller will also help to prevent your deep cycle battery from being discharged below that 80% mark. This, of course, helps to ensure that the lifespan of your battery is going to be as long as possible.
In theory, any solar panel could charge a deep cycle battery. However, only a few will be able to charge the battery effectively.
If you have a 100ah battery, then you will need to purchase a 250-watt solar panel, assuming that you are not using any other energy in your RV at the same time.
If you have a 200ah battery, then you would need 500-watts of power. This is assuming that you need the battery to fully charge during the hours of sunlight that you get per day.
If your energy usage is a lot lower than the maximum capacity of the battery, then you may be able to reduce the solar panel you need quite a bit. For example, a 50ah charge requires just 125-watts of solar panel.
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