Can You Overcharge Batteries with Solar Panels?

Solar power systems are known for their safety and ease of use. But that doesn’t mean we should get careless. A case in point are batteries. Technology has gone far in making them more efficient to use, but it is possible to overcharge them with solar panels.

A solar panel can overcharge a battery if it generates more voltage than the battery can handle. A charge controller can prevent overcharging by reducing the current that goes into the system.

How Can Solar Panels Overcharge a Battery?

Battery overcharging can happen because solar panels produce more current than their rated voltage. The ratings you see on solar systems – 12V, 24V, 48V – are nominal and do not reflect their performance.

A 12V solar panel can produce up to 20 volts when exposed to sunlight. A 12V battery can accept a charge up to 14.4 volts. Without a charge controller, the panel will send 18 or 20 volts into the battery, overcharging its circuits.

The easiest way to avoid this is to use a charge controller. We recommend the EPEVER 40A Solar Controller as it can handle up to 150 PV and 600 watts on 12V batteries or 1200W for 24V systems.

For a smaller solar system you can use a PWM charge controller like the Renogy Wanderer 12V Controller. This is a good option if you only need a small system or use solar power infrequently.

The good news is most solar panel kits are bundled with a charge controller. This is ideal if you are beginner so you do not have to worry about the sizing or if the controller is compatible. kit is the most ideal solution especially for someone new to solar power.

How a Charge Controller Prevents Battery Overcharge

A charge controller sets a limit to the panel output. If a solar panel produces 20 volts, a PWM controller drops this to 14.6 or whatever the battery can take. Battery voltage intake depends on type, temperature and the controller type.

Grid tie / high voltage solar panels rated above 140W are not 12 volts. They’re usually 24V though it can be anything from 21 to 60V. You can use any type of controller for grid tie panels provided the controller’s maximum voltage input is maintained. For best results you should go with MPPT though.

The problem with a PWM controller is up to 60% of a solar panel’s capacity is going to be wasted. The controller receives the panel output at 14.4V and sends it to the battery. Voltage can drop from 24V to 13.6V, and the panel amps can’t go beyond the rated amps. You can have a 200W panel but with a 7.6 amp/23V rating, only about half – 90W – gets in the battery.

This is where an MPPT charge controller is important. With it you can get the highest possible output from the solar panel without overcharging the battery.

In a previous post we explained why you shouldn’t connect a battery directly to a solar panel. Those same reasons apply here. A battery is built for a specific voltage / current capacity. Too much of it and the system malfunctions. This could lead to permanent damage and require replacement.

Do All Solar Panels Need a Charge Controller?

A charge controller should be used for any solar panel above 5 watts. It is better to be safe and ensure your deviec is protected.

Some may think that a 15W solar panel is too small to cause battery damage, but that is incorrect. Hook that up to a 100ah battery and over time the voltage will rise. Even without load this is not good for the battery’s circuitry and will make it inoperable. The general rule is: if the solar panel churns out 2 watts or less per 50 ah you don’t need a charge controller.

Battery System Monitor/ This device does exactly that, monitor a battery’s performance. Different types are available for solar panels and it’s a good idea to have one. This monitors your battery’s performance, condition, charge state and other system information. You should have one if you installed a solar array or use several batteries.

Why Do Solar Panels Produce More Volts Than Their Rated Output?

This is a fair question. Why do solar panels produce more volts than what they’re “supposed’ to? If it’s 12 volts, why can the output reach 16V or 20V?

If solar panels were limited to their nominal voltage rating,, it would absorb power only under ideal conditions, when the sun is at its peak. That’s not something you can or should count on. Solar panels must have additional voltage for overcast days, low sun angles or when the temperature is high.

The last point is worth repeating, as solar panels operate better at cool conditions. The sun needs to be out but high temperature isn’t necessary. In fact higher temperature leads to a performance drop. A 100W solar panel running in room temperature drops to 83 in 110 F.

Should I Buy a PWM or MPPT Charge Controller?

A PWM controller will be fine for small solar panels. The only issue is they are less efficient than MPPT. With an MPPT you can maximize the charge. There is always energy lost during the energy transfer from solar panel to battery. However an MPPT ensures the battery’s charge state and solar panel power are equal. A well designed system also prevents an inverter from overcharging batteries.

When the battery is fully charged, an MPPT controller settles to a 13.8 voltage, which is good for batteries in the long term. You won’t get the same benefits with a PWM controller, but for low power charges it will be sufficient. Knowing what type of charge controller you need is going to make buying easier.

Is My Solar Panel Too Powerful For My Battery?

If you have a charge controller you don’t have to worry about this. The controller will limit the current going into the battery to the maximum acceptable level. But if you don’t have one or want to know the figures behind it, read on.

First you have to note the battery capacity (amp hours or ah) and voltage. Let’s say you have a 12V, 200ah battery.

Second you have to find the watt equivalent of amp hours. Multiply amps x volts. In this case 200 x 12 = 2400.

So your battery needs 2400 watts to charge fully. If your solar panel can produce more than 2400 watts, the battery will overcharge if it isn’t unplugged from the panel when full. Obviously there’s no overcharge risk if your panel can’t reach 2400W.

But again this is unnecessary if you have a charge controller. Once the controller is installed you don’t have to do anything. The controller works like a voltage regulator and cuts off the power when the battery is fully charged. It’s an automated process, though efficiency depends on the controller used.

Conclusion

Solar panels and batteries are easy to use, and they’re safer than traditional power sources. Overcharging a battery might damage the unit but unlikely to cause other issues. And with the tips provided in this guide you can be sure it won’t happen to your system.