Does a Charge Controller Drain the Battery?

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Is your solar battery losing power overnight? It is fully charged but when you check back in the morning the capacity level has dropped? Could your solar charge controller have something to do with it? This is what we are going to find out.

A charge controller is meant to be connected all the time to the battery, so leaving it there should not cause any drainage. If the battery loses power for seemingly no reason, it means there is a power draw somewhere or the battery and/or controller is damaged.

The most common problem seems to be when the battery drains at night. Because solar panels do not produce current at night, the blame gets pointed at the charge controller because it is the only other thing connected to the system, unless there is an inverter.

However it is very unlikely a solar controller is causing the overnight power loss. To better understand, we need to take a look at how a charge controller works and what it is for.

What is the Function of a Charge Controller?

A solar charge controller has many functions, but it is basically a voltage regulator for the battery and solar panel. It regulates the power coming into the battery to prevent overcharging.

A 12V solar panel generates up 18 to 20 volts, which could overload the battery. Most 12V batteries need 13 to 14.5 volts to be 100% charged, but 18 to 20V is too much. Without a charge controller the battery will be overcharged and become unusable.

With a charge controller, the high solar panel voltage is adjusted to match the lower voltage of the battery. From 18V or 17V, the panel volts is taken down to 14.5V or whatever the battery can handle.

There are two types of solar controllers, MPPT and PWM. PWM is less expensive and does a decent job, but energy losses can be significant. An MPPT controller does a better job of lowering PV voltage so you can avail of the higher power available. One of the best examples of this is the Top One Power 60A MPPT Charge Controller which can handle heavy battery loads without problems.

The battery voltage capacity is determined by battery type, temperature, controller mode and other factors. In other words, you can connect solar panels directly to battery , but you still need a charge controller. Unless you drawing very little power (say, 1 to 5 watts), a charge controller is always required.

So that is what a charge controller does, regulate current that flows into the battery from the solar panel. But why do some batteries lose power overnight?

Why Your Solar Battery is Discharging

There are two places you should look at immediately, the battery and the charge controller. As pointed out earlier, these are the most likely culprits, but there are a few other things to consider. Let us examine each one.

Damaged Solar Controller

A faulty controller will not be able to regulate current flow in the battery. No power enters the battery after sundown, but it is possible that the reverse might be happening, the power in the battery goes into the solar panel.

This is very rare but it could happen if the controller is not working correctly. This can also occur with an older battery. The current outflow can take place anytime, but it is most noticeable during the night because there is no solar power going into the battery.

Controller is Preprogrammed to Load

Most high end charge controllers like the Victron Energy SmartSolar MPPT allow custom programming. You could set it up to power up the terminals at sundown. If you put a load on the system, the battery will drain, no matter the load size.

If this is the cause, adjust the controller parameter settings to prevent an automatic load. Or you can reduce the capacity to limit battery power use at night.

Battery is Completely Drained

Lead acid batteries like AGM and gel should be recharged when the capacity falls to the 50% mark. Lithium ion batteries can be recharged at the 35% level.

Never let solar batteries go to 0% and recharge to 100%. It wears out the components and could cause various problems, including power leaks. Repeatedly charging and recharging from 0% to 100% may cause battery power loss overnight.

Damage due to completely discharge will not happen overnight. But it will occur. And one of the signs is inability to hold a charge. If you have been discharging the battery completely and it suddenly loses charge with no load, this is the likely reason.

Parasitic Battery Draw

A parasitic battery is when battery energy continues to get drawn even when the system is shut down. The term is often used in cars when the battery power keeps going down even when the the engine is off.

However it is also applicable to solar charge controllers. We might think that the system is completely off, but there may be power draws draining the battery.

  • Leaving the TV in standby mode does not turn it off. It will continue to draw power. That red light you see on the TV means it is still using a charge.
  • Sleep mode in computers are the same. Power draw is very little, but it is there. Putting a computer to sleep or hibernate mode does not shut it down.
  • Routers and modems are always drawing power unless you manually turn it off.
  • DVD and Blu-ray players never completely shut themselves down. Even if you turn it off, it is still active so it can receive a signal from your remote.
  • Other appliances and devices that consume power when turned off are set top boxes, video game consoles, cordless phones, printers, sound systems, coffee maker, clocks etc.

These are just some of the appliances and devices that use power even when “off”. If you are off the grid and your appliances run on solar power , then there will always be some power drawn from the battery bank.

Cold Weather

Batteries do not like cold weather (or extremely hot for that matter). If the temperature drops below the ideal operating range, the battery internal circuits could malfunction.

Modern solar batteries, especially lithium, can run under extreme heat or cold. However, misuse, age, wear and tear can weaken the battery. When an old, degraded battery is forced to run in cold temperature, it might not be able to sustain charge or even lose them.

Solutions to Charge Controller Battery Discharge

Judging from the number of people asking questions about this online, battery discharging at night seems common, and many believe their solar charge controller has something to do with it. There are many solutions you can try to find out.

Check the Load

If you have an off grid setup, there will always be something drawing from the battery unless you pulled the plug from all your appliances. Check how much power each appliance draws when in standby mode, as that will give you an idea of how much the battery should drain overnight.

If the overnight loss is within the range, then you know it is the appliances causing it. The only way to prevent this is to unplug those devices.

Some TVs can be completely turned off and all laptops can be shut down. However, modems have to be left on, otherwise you will have no Internet access. And most appliances just cannot be turned off short of pulling the plug.

If the power draw is higher than what it should be, you should check the controller and battery. This is where the problem likely is.

Reset the Charge Controller

If you suspect a faulty charge controller, the first thing to do is reset it. A soft or hard reset is usually enough to fix any issue. Most controllers do not need to be reset, but if it carries a heavy load or you added a new component, it can help.

If resetting does not resolve the draining problem, check the cables connecting the controller and battery. If it loose or worn out it will affect the battery charging capacity. Faulty wires cold also cause solar arcing so have that fixed quickly.

Inspect the Battery

The last thing you can do is check the battery. If you have a single solar panel connected to a battery, this will be easy. Use a multimeter to determine if the battery carries a charge. Assuming it does not, the battery is damaged or dead and needs to be replaced.

If you have a solar array connected to a large battery bank, inspection will take longer. You have to check each battery individually for problems. You also have to look at the connection, the wires, cables etc.

Aside from the batteries, you also have to go over the controller connection. Is the unit being overloaded? You also have to evaluate the inverter because the problem might be there. It is unlikely, but with a large system you can never be too sure.


There are many reasons why a solar battery may lose charge, but it is unlikely to be due to the charge controller. The only exception being if the controller is malfunctioning, but other than that, there are other places one should look at.