Can Solar Panels Drain a Battery?

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Solar panels and batteries work together, with the panels generating energy so it can be stored in the battery for storage and use. But in some instances, it seems like the battery gets drained even when there are no devices running from it. Could solar panels be draining your battery?

A solar panel is going to drain a battery if the voltages do not match, for instance charging a 24V battery with a 12V solar panel. If the solar panel is directly connected to the battery without a blocking diode or a charge controller, the battery will be drained.

If your battery is draining quickly even with little usage, the most likely reasons are:

  • The solar panel and battery voltages do not match
  • No charge controller
  • No blocking diode
  • Poor battery maintenance

If you perform proper maintenance and troubleshoot solar panel systems, usually everything will run fine. But if there are battery drain issues, it could be any of the four above. Let us take a look at each.

Make sure that you use only high quality batteries for with your system. Among AGM batteries we like the Weize 12V Battery . If you want to use lithium instead we recommend the Battle Born Batteries LiFePO4 100ah as it should work with most solar system configurations.

Incompatible Solar Panel & Battery Voltages

If you are using a PWM charge controller, you have to use compatible solar panels and battery voltages. If you have an MPPT charge controller, mismatched voltages are unlikely to occur.

When manufacturers say 12V solar panel, it means the panel is designed to work with 12V batteries. The solar panel voltage must be higher than the battery voltage to carry a charge. A 12V solar panel actually has 18V, but a PWM controller will adjust it to match the battery.

A 24V battery needs a 24V solar panel, and a 48V battery needs a 48V solar panel. Again this does not reflect the panel voltage but what batteries they can charge.

Charging a 12V battery with a 24V solar panel or a 24V battery with a 12V solar panel will at the very least, waste energy. in the former, the battery is too small and won’t be able to store the energy produced by the module.

In the latter, the connection probably won’t work at all because the panel voltage must be greater than the battery voltage. In all likelihood the system will drain the battery and damage it.

Make sure your batteries, charge controller and solar panels are compatible to avoid overcharging. In grid tied systems a battery bank is optional. But if you are off the grid or on a solar RV, batteries are indispensable. If you buy a solar panel kit, it should be bundled with a charge controller already.

No Charge Controller Results in Battery Drain

A charge controller performs several important functions, chief of which is to match the solar panel and battery voltages to get the best results. Both PWM and MPPT controllers can do this, but an MPPT allows you to use higher voltage solar panels with batteries.

A charge controller keeps the current flowing one way, into the battery. It prevents power from getting out of the battery unless you load the solar panel with AC or DC devices and appliances.

Without a charge controller the battery power could drain rapidly and suddenly even with little use. Without a controller, there is no protection against overloading, overheating, over charging etc. Any or all of these can drain a battery quickly.

If you are going to run a refrigerator on solar power, or any appliance for that matter, you need a controller and inverter that can match the power requirements.

How PWM Charge Controllers Work

When a solar panel charges a battery, the PWM (Pulse Width Modulation) controller pulls down the panel voltage to match that of the battery. So an 18V solar panel gets pulled to 12V to match the 12V battery it is charging.

As the battery continues to charge, the battery voltage goes up. The controller increases the solar panel voltage too. And this is why you have to match the voltages when using a PWM charge controller. If not, the system could suffer serious damage.

How MPPT Charge Controllers Work

An MPPT (Maximum Power Point Tracking) determines the panel VMP voltage and matches the battery and PV voltage. The power that goes in and out of the MPPT are the same, so current goes up when the voltage drops down.

This means you can use a bigger solar array compared to the battery. You could use a 20V solar panel to charge a 12V battery. You can also connect two 12V solar panels and charge a 24V battery. You can hook up three 12V solar panels and charge a 36V or even a 48V battery.

With an MPPT charge controller, the chances of battery drain due to the solar panel is very low. The only reason it could happen is faulty wiring or improper installation. Due to the high cost of MPPT charge controllers though, most solar power systems use PWM, so voltage matching is necessary.

No Blocking Diodes Installed

A blocking or isolation diode prevents batteries from charging back to the solar panel. Most solar panes have a blocking diode installed so you don’t have to add one. Modern charge controllers perform the same function as a diode, making it redundant in most setups.

If your solar battery drains at night, the most likely reason is it does not have a blocking diode or charge controller. Current moves from high voltage to low voltage, hence the need for solar panels to have a greater voltage than the battery.

During day time, the solar panel voltage will be greater than the battery, whether it is lead acid or lithium ion. The current moves from the solar panel into the battery as it should. At night time however, the panel voltage will be lower than the battery.

If the solar panel is connected to the battery and there is no blocking diode or charge controller, the current could move backwards. With the solar panel not transmitting current to the battery, the current might flow out into the panel, draining the system.

The chances of this happening are low but it is possible. The outflow will not be large, but you will notice it. In most cases of solar batteries draining at night, this is the most likely reason. Again, this is only going to happen if you don’t have a blocking diode or charge controller.

If you have a controller, there is no need for an isolation diode. But if the battery is still draining, check the cable as it might be loose. A defective battery or charge controller could also drain the energy out of the system.

Solar Battery Maintenance Tips

Use as Few Batteries as Possible

The more batteries in the battery bank, the more wires and cables are needed. This increases the resistance and may affect charge efficiency. Use as few batteries as possible, but make sure it is enough to meet your needs.

Rotating Batteries Helps

If you have a large battery bank, change the battery positions every now and then. Batteries at the back and middle may not receive sufficient power compared to those in front. By rearranging the position, you ensure each battery receives optimum charge from the PV modules.

Use Large Cables

Portable solar chargers charge best with short cables, while for solar panels, large, thick cables are ideal. Keep the distance between the solar panel and battery bank as short as possible. Just as important is the thickness. Thick, large batteries have lower resistance and are more efficient. Go with 2/0 for 48V systems and 4/0 for 12V solar panels.

Charge Batteries Regularly

Whether it is lithium, AGM, gel, FLA or SLA, charge them regularly. Charge lead acid batteries at 50% and top off at 85%. For lithium you can recharge at 35% up to 85%-95%. Do not let the battery drain to 0% then charge to 100%. That will only shorten its life cycle.

Follow Manufacturer Instructions

Lithium batteries need little maintenance, but lead acid batteries do. They need to be refilled with water every two weeks or so depending on the design. Check the manual for more details on how to properly care for the battery.

Mindful Battery Usage

Sometimes we don’t realize that devices still use power even when we think they are shut off. If your laptop and router are solar powered, putting it to sleep mode will not turn it off. The laptop will still use power. The system is still on and will check for any notifications from your programs.

Putting a solar TV on standby mode will still draw power too. Check any other devices you are running on solar and if you lave them on standby mode. Energy consumption is low, but it will add up. If you notice the battery losing small amounts of power at night, this is a good place to start looking.

For more tips on how to make batteries last longer, read our guide on how to make solar batteries last longer.


To sum everything up, solar batteries can drain rapidly due to improper connection or if the solar components do not match up. This goes to show why research is important and why you have to be certain each part matches with the rest.