Solar Panels

5 Reasons Your Solar Panel is Draining Your Battery (& How to Fix It)

By:Michael Johns

Solar panel draining battery

Checked your solar battery and realized that it’s not fully charged?

Yeh, me too and it’s so annoying!

How can it even be possible, after all, you’ve not been using any power and the solar panel should have been charging all the time.

So now you’re wondering if a solar panel can discharge your battery?

Well, that could be the case.

In fact, you might even find your solar panels drain your batteries at night when there is no sun to produce electricity.

But don’t worry, it doesn’t have to keep happening and it should be a simple fix to remove the problem.

There are several common reasons why a solar panel drains a battery.

Let’s go through the issues and how to easily solve the problem. 

1. No Solar Charge Controller

A solar charge controller is a vital part in managing the electricity in your between your solar panel and your battery.

Although it is possible to connect a solar panel directly to a battery, you should never do this as it can lead to discharge. Plus, there are additional safety issues like overcharging.

A solar charge controller connects to the solar panel and a battery. It ensures that the current always flows in the correct direction.

If you don’t have a charge controller, energy can flow the wrong way in your circuit. This means that when no energy is being produced by the solar panel (like at night), energy will flow out of the battery towards the solar panel.

This is called reverse current. An electrical current will flow from wherever is highest to wherever is lowest when left unregulated. Use a solar charge controller to stop your battery draining, and to protect your solar panels and batteries from electrical faults.

Need to get one? Read my guide to the Best Solar Charge Controllers.

2. Incorrect or Faulty System

If you have a solar charge controller connected to your system but batteries are still draining, you need to investigate further.

Ensure all the components including panel, battery, and controller are working correctly. This includes checking that it is wired up correctly. You should check all connections for a snug connection on the positive and negative side of the circuit. For example, ensuring the battery is correctly wired to the output on the charge controller.

Many people cause damage to their components during installation as they don’t follow best practices. Often, this occurs from connecting the charge controller to the solar panel before the battery. If you make a mistake like this it can damage your equipment from the start. A faulty or incorrectly wired charge controller can lead to discharged batteries.

3. Inverter Stealing Energy

If everything is wired up and working correctly but your battery is still draining at night then it could be your power inverter.

Did you know that a power inverter draws energy even with no load attached? That means even if you are not running an appliance, it will still drain your battery. Even while you sleep!

When not in use, turn your power inverter off. Otherwise, it draws power to run internal systems like cooling fans and digital screens. Although it is not a huge amount of energy, it will drain your batteries over a period of time. Especially at night when there is no sun for your solar panels to charge your batteries.

4. Worn or Damaged Batteries

Deep cycle solar batteries have a shelf life. Eventually, they will be able to store less and less energy.

Generally, they will last 5 – 10 years or 500 to 2000 charge cycles depending on quality. The older a battery becomes the less charge it can hold. Adding to this, it is possible to damage lead-acid batteries in a number of ways including over-discharge and poor maintenance.

If your solar panels have been draining your battery and it becomes completely discharged, it can sustain irreparable damage. This will lead to a poor charge capacity!

Make sure your batteries are in good working order and that they can hold charge. Otherwise, you might think you are charging your batteries but in actual fact they are already dead.

5. Solar Panels Draining for Each Other

Solar panels can actually drain power from one another, especially when mounted in different light conditions. For example, if you have one panel in sunshine and one in shade it can lead to this issue.

This is another example of reverse current. As we discussed earlier, the electrical current will flow from high to low, including between solar panels. 

Fortunately, you should be protected against this with bypass and blocking diodes which only allow current to flow in one direction. 

  • Bypass diodes are found on most solar panels to stop backfeeding between solar panels.
  • Blocking diodes stop reverse current from your battery but these are rarely used as the charge controller protects from this problem.

FAQ on Solar Panel Draining Battery

Let’s answer your other questions about solar panels discharging batteries.

Do Solar Panels Drain Battery at Night?

A solar panel should not drain your battery at night, your charge controller will stop this from happening. It can happen due to a fault, incorrect wiring, or missing charge controller. For example, with no charge controller installed you are at risk of reverse current discharging your battery at night.

Can a solar controller drain a battery?

No a solar controller will not drain your battery. It is designed to control the flow of electricity from your solar panel to your battery. To run the charge controller can draw power directly from the 12V solar panel and stop any reverse current that might drain a battery.

Can a solar panel ruin a battery?

If you connect a solar panel directly to a battery without a charger controller it can easily damage the battery. You run the risk of overcharging which causes damage, swelling, and even explosions!

Can a solar trickle charger drain a battery?

A trickly charger should not drain a battery, it should only ever trickle energy into the battery. Your trickle charger should have sensors and diodes to prevent battery discharge.

How do I keep my solar panel from draining my battery?

To stop your solar panel draining your battery you must connect a solar charge controller. This component sits between the solar panel and battery to manage the flow of electricity and stop reverse current.

How long will a solar battery hold its charge?

A solar battery should hold its charge for a long time. The best batteries will slowly discharge at 3% a month when not in use. So if you have a battery with 100% charge and leave it unused and disconnected for a month it will still have 97% charge. If you always leave your battery connected to your solar panel it should continuously hold full charge.

Final Words on Solar Panel Draining Battery

By now you should understand that you have a problem to address if your solar panel drains your battery.

It is vital to connect a solar charge controller to manage the flow of electricity correctly. This stops the chance of reverse current and battery discharge.

You might also experience faults, incorrect wiring, and other appliances drawing power to discharge a battery.

If you have such issues, be sure to rectify the problem otherwise you will always be frustrated with your battery energy levels.