Can I Connect A Solar Panel Directly To Battery? Actual Tests

Have you found yourself wondering if a solar panel can charge your battery?

Can I Connect A Solar Panel Directly To Battery Actual Tests

Maybe you want to know what happens if you plug your car battery directly into a solar panel? Well, you wouldn’t be alone! 

These questions plague many car or large battery owners as they look for new and often cheaper ways to charge their batteries.

Whether it is a deep cycle battery, lithium iron phosphate battery, or a lead-acid car battery, we bet you have tried to find out if you can charge them with solar panels or not. 

And we bet when you tried searching, you couldn’t find any answers! Believe us, we have been there too and know just how frustrating it can be!

That is why we started performing our tests and have the answers here for you today. Just keep reading to find out if you can connect a solar panel directly to a battery or not! 

Can I Charge A Battery With A Solar Panel? 

Let’s dive right into it! Yes, you can charge a battery with a solar panel. We know that is good news for many of you! But there are some factors you need to consider.

First, you can connect a 12-volt car battery to a solar panel directly, but you will need to monitor it if it’s more than 5 watts. 

Solar panels with a higher rating than 5 watts should not be connected to the battery directly. Instead, you should use a solar charge controller.

This helps to protect the battery from overcharging and ensures that you can use it and the solar panel again! 

We did tests using both a direct connection and a charge controller to see if this theory held up. We’ve got the results coming up later, so be sure to stick around for those.

But before we dive into those, we must walk you through the theory and answer some burning questions. After all, you don’t want to leave here today with only part of the information, do you?

Can You Charge Solar Batteries Without A Controller? (What Is Nominal Battery Voltage?) 

Before we address the first part of the question, we need to look at what nominal battery voltage is. You can think of ‘nominal’ as ‘designed for.’

What do we mean by that? Well, when you buy an appliance with a rating of 12V, that means it is designed to operate at 12V, drawing the correct amount of power it needs to run. 

Although the appliance is designed for 12V, it can run on 11V or 13V if needed. Generally speaking, appliances will run on a different volt, providing it doesn’t deviate more than 10-12%. 

If the voltage is too high, the appliance pulls more power than it needs and can overheat. No matter what appliance you are using, this is incredibly dangerous and can cause fires. If the voltage is too low, then the appliance won’t perform as it should.

Most commonly, it will run a lot slower than it should. 

Now car batteries also have a nominal voltage, usually 12V. Just like the appliances we mentioned, a car battery can be used below or above its nominal voltage.

The terminal voltage of a car battery is often an indicator of its depth of discharge. We’ve got a handy chart below that explains this relationship/ 

Lead Acid Battery Depth Of Discharge Chart

12V Lead Acid Battery State Of Charge (%)Battery Terminal Volts

As you can see from the chart, when the battery is fully charged it has 12.73V and at 90%, you have a voltage of 12.62.

Once the voltage drops below 12, the battery is discharged and you will need to recharge it immediately to avoid any damage. 

When reading the voltage of a car battery, you should wait at least 3 hours after the battery has been used. This way, there will be no chemical activity in the cells that could pose a challenge to you. 

What Is The Difference Between Regular Batteries And Deep Cycle Batteries?

It’s also worth clearing up the difference between regular and deep cycle batteries today. 

Regular, lead acid, or auto batteries are designed to provide lots of power to a cold engine or car until it fires. To do this, they need to draw hundreds of amps quickly.

Once the engine is powered up, the alternator will recharge the battery and keep it fully charged until it is needed again. 

Deep cycle batteries work differently. They are designed to provide light to medium currents for a longer period. Most of these batteries are rated for 20 hours or more. 

Although these batteries are different, both of their performances decrease when the current draw is high and they will not supply their rated capacity.

You should never allow your auto battery to be more than 15% discharged and your deep cycle battery should not be discharged more than 50 to 80%. 

Deep cycle lead-acid batteries can last up to five times longer if they are regularly discharged to 50%, meaning for a 100Ah battery, you only have 50aH to use! 

Are Lithium Batteries Better Than Lead-Acid?

Lithium iron phosphate batteries are pretty much deep cycle batteries. These batteries can be discharged up to 9%% without any damage, but the more you do this, the shorter the battery’s lifespan becomes. 

It’s worth knowing this so you purchase the right battery. One with a capacity of 60Ah will offer 57Ah usable capacity.  If you discharge a lithium battery to 80%, the battery lasts half a lifetime!

In many cases, these batteries only need to be bought once rather than other batteries that need to be repurchased. 

It’s worth considering this when deciding if lithium batteries are a better choice for you. 

Solar Panel Voltage And Current Output 

When we talk about solar panels, there are a few operating features we need to consider. The most important ones are the following: 

  • Pmax – maximum power 
  • Voc – open circuit voltage 
  • Isc – short circuit current 
  • Imp – maximum power current 
  • Vpm – maximum power voltage 

You can find most of this information out by checking the label or user manual of a solar panel.

The specifications are easy to read and the features we just listed will be displayed in watts, volts, and amps. 

To help you learn more about these features, let’s take a closer look at some of them now. 

What Is Voc?

Open circuit voltage, or Voc, is the value of DC volts measured on solar panel leads. For a 36 cell, 12 volts solar panel, you would have a Voc of 21 to 22 volts. As we mentioned earlier, this information will be displayed on the label of your solar panel. 

What Is Isc? 

Short circuit current, or Isc, is determined by connecting panel leads together with an ammeter. Doing this is less dangerous than when the circuit voltage falls to zero, as there is no power to cause any damage or overheating. 

How Much Current Can A Solar Panel Produce? 

The current your solar panel can produce will be stated on the label, usually in amps. For example, a solar panel could have a current of 5.75 ample when the voltage is at 17.4 volts.

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When we multiply these two numbers together, you get 100W, the rated power of the panel. 

You will want to know this number as it is the MPP, or maximum power point. 

What Is the MPP Of A Solar Panel?

We touched on this briefly, but let’s look in more detail now. The MPP, or maximum power point, is when the current and voltage combine to deliver the Pax, maximum power, that your solar panel can deliver.

This number will vary depending on your solar panel and is printed on the label or in the user manual. 

The MPP occurs when the load resistance matches the internal resistance, known as the characteristic resistance. For a typical 100W solar panel, you would expect a resistance of 3 ohms. 

Can I Charge A 12V Battery With A Solar Panel? 

Lead acid batteries internal resistance fluctuates depending on their discharge state, but for the sake of this question, let’s assume yours is 1 ohm.

We then need to consider the maximum power flows when your load resistance is equal to the characteristic resistance.

In this instance, the 12V battery might charge when directly connected to a solar panel, but it won’t be done very efficiently. 

We also wouldn’t know the voltage across the terminals. This means you can’t determine how much current is flowing into the battery. Not knowing this information isn’t very safe and could lead to overpowering or damage to the battery or panel. 

How Do I Connect A Solar Panel To A Battery? 

When charging your battery with a solar panel, unless the panel is very small, you should use a solar charge controller to connect the battery to the solar panel.

This is the safest and most efficient way to charge your battery using a solar panel. 

These days, controllers will have settings for all types of batteries, including lead acid and lithium, making them incredibly versatile. 

What Solar Charge Controller Do I Need?

There are a few different types of solar charge controllers out there, and it’s vital that you choose the correct one. We have MPPT charge controllers that adjust the internal resistance to match the power of the panel.

This allows the correct power to be drawn from the solar panel safely, helping you to charge your battery. 

To help you learn more about them, we have a brief chart with the results of an MPPT and PWM solar charger tests. We used a lead acid deep cycle battery when completing these tests. Check out the results below!

Solar Panel ConfigurationInput VoltageMPPT Power TransferredPWM Power TransferredMPPT Gain (%)
2 x 100 Series42V7322%
2 x 100 parallel21B5822%

MPPT solar charge controllers are 30% more efficient than PWM, allowing you to charge your battery 30% faster. However they are more expensive than PWM controllers.

When deciding between the two, it’s worth considering the results from our tests, and also your budget and needs. 

If you can afford to spend more, then go for the MPPT. But if your budget is an issue and you don’t mind waiting a little longer, then a PWM controller will serve you well. 

Charging A Battery With Solar Panel – Test Results 

Earlier, we mentioned that we ran tests charging our car battery directly with a solar panel and with a charge controller. Here we have the results for you!

When running the tests, this is what we did: 

  1. We measured the terminal voltage of a 90Ah deep-cycle lead-acid marine battery when it was partially discharged to get the State of Charge. 
  2. Next, we used a 100W solar panel and connected it directly to the battery, measuring its current and voltage.
  3. Then we used an MPPT solar charge controller, connecting it to the solar panel and battery, before measuring its amps and volts.
  4. We noted our readings and collated them in the table below for you to see. 
Battery Charge CurrentBattery Charge VoltageTime To Recharge
100W solar panel connected directly2.45amps12.7 volts12.25 hours 
100W panel and MPPT controller3.3 amps13.5 volts 9.5 hours 

What shocked us was how little difference there was! We used a 31Ah battery that was 35% discharged and it managed to pull the healthy current you would expect.

The MPPT controller manages to outperform the solar panel working on its own. As the controller adjusts the internal resistance, we get the maximum power point quickly. 

Now you are bound to ask, if the currents are so similar, why do we need a charge controller? Well, we need to leave the battery to charge fully for us to have our answer! Which, of course, we did. 

It took roughly 11 hours, but when the battery was fully charged using the MPPT controller, it had a current reaching of 0.5A and a voltage of 13.5V.

The voltage would creep up to 12.73V with hardly any current entering the battery. 

We then disconnected the MPPT and connected the solar panel directly to the battery. Our readings then gave us the following: 

  • Charge current – 2.5A 
  • Terminal battery voltage – 15.5V 

Even when the battery was fully charged, the solar panel was trying to add more current to the battery. If we had left the solar panel connected to the battery, the battery would have overheated and been damaged. Not what you want! 

So, it’s better to opt for a charge controller when charging your battery. As our table shows, the battery will charge faster and you will get more volts flowing into the battery.

While the use of a controller does mean you need to pay for one, it’s worth it if you don’t want to worry about the safety of the battery, or have time to sit next to the battery watching it charge! 

DIY Battery Maintainer 

Before you leave us today, check out our DIY battery maintainer. This allows you to connect your battery to a high-power solar panel without any damage or your battery over-charging.

We know, it sounds too good to be true, doesn’t it? This is a good option for those that don’t have the budget for a separate controller and are willing to turn their hand to some DIY! 

To make this maintainer, or buck-boost converter, you will need to ensure that it accepts DC voltage input and that you can limit the DC output voltage.

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If this sounds beyond your means, then you can grab an affordable buck-booster model from here.

The multi-pack is super affordable and means you will have plenty of buck-converters for all your solar panel or charging needs.

Simply set the output volts to 13.5V (a typical float voltage) and your battery will charge easily without any damage to it! 

You can turn the output up to 14V if you wish, increasing the charge current, but be sure to monitor it while you do this as it runs the risk of overcharging or overheating.

When using a buck-converter in this way, you won’t want to leave it connected to the panel and battery indefinitely. Instead, remove it once the battery is fully charged. 

Final Thoughts 

And there you have it! You can connect a solar panel directly to a battery to charge it, but you need to be mindful when you do this. You run the risk of the solar panel overcharging the battery, leading to damage if the panel is too powerful.

Instead, you should opt for a solar charge controller that will sit in between your battery and solar panel. 

The controller will monitor the input flow of current and voltages to ensure that your battery never has more current flowing through it than it needs.

It will ensure that the maximum current and voltage is reached and maintained, preventing overcharging which can cause damage to your battery. 

When using controllers or solar panels connected directly, ensure that you monitor your battery at all times and read the specifications to ensure that they are appropriate to be connected to one another.

Joe Danner