As you probably already know, you can wire solar panels either in series or parallel.
If you change the wiring you can change the way voltage or current behaves:
- In series, the current stays the same while the voltage adds up.
- In parallel, the voltage stays the same while the current adds up.
Here are some reasons why you might want to add another charge controller:
- You want to add more solar panels to your system.
- You want to add a panel with a different specification than the one you already got.
- You want to separate panels from each other because they receive shade at different times of the day. For example on a boat.
How do charge controllers interact with each other?
Charge controllers sense the internal resistance of a battery and send their current to the battery terminals based on the resistance of the battery. If the battery is at a low state of charge, the resistance will be low and the charge controller will charge in bulk mode.
If the resistance becomes higher, the charging current will decrease because the battery is almost full.
If two or more charge controllers charge the same battery, the battery will be charged quicker. The multiple charge controllers will not compete with each other because they all sense the same internal resistance of the battery. If charge controller A puts in 100Watts, charge controller B will also put in 100Watts.
When you program your charge controller you can add the cutoff voltage in the software. That means if your battery reaches a pre-defined voltage, it will stop charging. You can tell charge controller A to charge to 12 volts and charge controller B to charge to 12.8 volts. This is not needed but is good to know because one charge controller might stop charging if it reaches that point, and you will be wondering why that is. Also, if the wires of charge controller A are a bit longer, it might enter float mode earlier than charge controller B because of the voltage drop.
If the charging current becomes too high, the internal resistance of the battery will change because of the heat generated. The charge controllers will then reduce their power input to the battery.
Wiring two strings to two charge controllers
In this example, there are two strings or arrays of solar panels that go to every charge controller. This setup is ideal if you have multiple solar panels that do not have the same rating. Refer to the article about series and parallel wiring solar panels.
You can also use this kind of setup on a boat. If one charge controller is in the shade of a sail, but the other is in full sunshine, the sunny panel will still deliver its max power. The shaded panel will deliver reduced power. This is very similar to wiring your panels in parallel.
Synchronizing Multiple Charge Controllers
If you are using two or more charge controllers they do not need to be able to communicate with each other.
There might be a problem with flooded lead-acid batteries where they equalize every month. If you have multiple charge controllers attached to the battery bank they equalize more frequently, Which is not good for the battery. You need to turn off the equalizing function of the other charge controllers so you only have one charge controller performing the equalize function.
Some models are able to communicate with each other like the Victron BlueSolar and SmartSolar models. The communication happens with the built-in Bluetooth module for the SmartSolar model. For larger systems, they recommend using the VE.can port.
The reason for communication between charge controllers is so that only one charge controller will balance the cells, not all of them.
Both charge controllers will deliver their maximum amount of current to the battery.
You can wire charge controllers in parallel to support an expanding solar system. You do not need to have charge controllers that are able to communicate with each other but you should only enable the equalizing function in one of them if you have flooded lead-acid batteries.