Can You Overcharge Batteries with Solar Panels?

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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.

12V solar panels produce up to 20 volts, and without a charge controller the battery will overcharge. A charge controller keeps the excess current and voltage from the battery. 14.5V is enough to charge most solar batteries.

How to Prevent Battery Overcharge

Solar panels rarely reach their maximum rated output. You can’t expect a 100W panel to produce 100W every time. On the other hand they generate more volts than their stated value. When the sun’s energy is absorbed, a 12V solar panel produces up to 20 volts. More powerful solar arrays generate even more current which your battery may not be able to handle.

The easiest way to avoid an overcharge is to use a charge controller. 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.

Should I Buy a PWM or MPPT Charge Controller?

With a 12V system, a PWM controller will run fine. The only issue is they’re less efficient than MPPT. With an MPPT you’ll 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.

How a Charge Controller Prevents Battery Overcharge

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

High Voltage Panels, Batteries and Controllers

Grid tie / high voltage solar panels rated above 140W are not 12V. 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.

The problem is up to 60% of the solar panel’s capacity is wasted. Controllers receive the panel’s output at 14.4V and sends it to the battery. Voltage drops from, example, 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.

Do All Solar Panels Need a Charge Controller?

We recommend you use a charge controller, but for 1W-5W trickle chargers, it’s all right to directly connect the battery. The voltage is too small to cause any problem, but the slow charge rate limits its usefulness. For anything over 5W a charge controller should be used with solar panels.

Some may think that a 15W solar panel is too small to cause battery damage, but that’s 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 Monitors

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 to do that, 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.

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.