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When a solar panel charges a battery, you need a charge controller to prevent overloading. If you have a 500W solar panel for instance, the charge controller must be the right size to match the current coming into the system and provide protect for the panels and the battery.
A 500W solar panel needs a 30A charge controller. Divide the watts by the battery voltage and add 25%. In this case, 500 watts / 24V battery voltage + 25% = 26 amps. Round it off to 30A and you have the ideal charge controller size.
How to Calculate Charge Controller Size for Solar Panels
In the following examples we will use 500 watt solar panels. But these calculations work with any solar panel size.
To calculate charge controller size, divide the solar panel watt output by the battery voltage. Add 20-25% to the result and you have the charge controller size you need in amps. In our example we will add 25%.
It looks like this:
Watts / battery voltage +25% = charge controller size in amps. Charge controller sizes are measured in amps (A). The most common are 10A, 20A, 30A, 40A, 50A, 60A, 80A and 100A. Round off the figure you get to the next largest size available.
Suppose you have the AC POWER 500W Solar Panel Kit , which is optimized for RV, home and other off grid applications. Now you just need to figure out what battery voltage to use.
The most commonly used are 12v and 24V. If you pair a 500W solar panel with a 12V battery, you will need a 60A charge controller.
500 / 12 + 25%= 52
Round off 52 amps to 60, and that is your charge controller.
If you use a 24V battery with your 500W solar panel, you can use a 30A charge controller.
500 / 24 + 25% = 26
Rounded off that would be 30 amps. In this case, you can use a 24V battery like the Ampere Time or connect two 12V batteries in a series to get 24V.
You can use this approach for any solar panel or battery size. Once you know the charge controller size, the nest step is to decide what controller type to use. This is what we discuss below.
What Type of Charge Controller PWM or MPPT?
An MPPT charge controller is the best for a 500 watt solar panel system. It is up to 30% more efficient than a PWM and offers greater flexibility. A PWM charge controller is only suitable for small solar panels.
To understand this, we need to explain the difference between nominal and actual voltage.
Solar panels and batteries are classified as 12V or 24V, but these are nominal voltages. A 12V solar panel can charge up to 18 volts, while a 12V battery charges from 12.5 to 14.4 volts.
PWM vs. MPPT
Suppose you have a 100 watt solar panel, a 12V battery and a PWM charge controller.
A PWM charge controller pulls the power up to the maximum battery charge limit, which is 14.4 volts. Most of the time, 12V batteries charge at 13V. To find out how much solar power is being charged, multiply volts x amps. Assuming the solar panel lmp LNK is 5.7 amps:
13 x 5.7 = 74.1
Only 74.1 watts out of the possible 100 watts goes into the battery. Solar panels rarely draw up to maximum power, but even at 90 watts, that is 16 watts an hour wasted. Multiply that by 5 hours and 80 watts is wasted per day.
Now imagine if you have 5 x 100 watt solar panels hooked to a PWM controller. Using the same calculations the battery will get only up to 370 watts out of a possible 500 watts per hour.
An MPPT charge controller like the Victron 30A is better optimized for these situations. Using the same system specs, more power goes into the battery.
Assume the battery charges at 13 volts. With a PWM controller the rest goes to waste. But an MPPT turns them into amps. With an 18 volt solar panel charge:
18 / 13 = 1 .38
Now we multiply the lmp by 1.38:
5.7 x 1.38 = 7.8
Now we have 7.8 amps. Multiply this by the battery volt charge:
13 x 7.8 = 101.4
That is 101.4 watts. It is higher than the solar panel maximum output, but since panels are not perfect the result will be less than 100 watts. But this goes to show why MPPT controllers are superior to PWM. For small systems, a PWM is fine, but for a 500 watt solar system and larger, MPPT is a must.
How Many Batteries Can My Charge Controller Handle?
You typically need just one charge controller for a solar system. Large solar arrays benefit from multiple controllers, but for a 500 watt system, one is enough to handle the batteries.
Recall that you have to divide the solar panel watts by the battery voltage to get the charge controller size. In our example we used 12V or 24V batteries. But you are not limited to one 24V battery. You can connect two 12V batteries in a series and get 24V.
Series and Parallel Configurations
Two or more batteries can be joined together in a series or parallel connection.
Batteries connected in parallel will add up their amps, but not the voltage. If you have 2 x 100ah 12V batteries in parallel, you will get 200ah capacity but the voltage will remain at 12V.
Batteries connected in a series will add their voltage but not the amps. If you have 2 x 100ah 12V batteries in a series, you get 100 ah capacity at 24V.
To configure batteries in parallel, connect the positive terminal of one battery to the positive terminal of another battery. Repeat this with their negative terminals.
To configure the batteries in a series, connect the positive terminal of a battery to the negative terminal of another battery. Repeat with the rest.
Going back to our example: you can use several batteries with your solar panel without exceeding the capacity of your charge controller. If you have 2 x 12V batteries connected in a series, you get 24V, which means you need a 30A charge controller. But if you add another 12V battery, the minimum charge controller size goes down to 20A.
Most of the time though we configure batteries in parallel to increase capacity. In this case the voltage does not get added up so you can use several. With an MPPT charge controller, you can use parallel or series configurations and get maximum output.
If you are going to run AC appliances you will need an inverter too. The inverter size will depend on how much power load it has to carry so keep that in mind.
Why a 500W Solar Panel Needs a Charge Controller
If you are still thinking of setting up your solar system without a charge controller, don’t. There are a lot of reasons why you should always have one installed.
Automatic Low Voltage Disconnection. Your charge controller, as the name makes clear, controls the amount of current and power going into the battery. A solar panel and battery setup without a charge controller can result in an overload and destroy the components.
Overload Prevention. Solar panels continuously receive energy from the sun. There is no off button here. If you don’t have a charge controller installed, the panel will keep transmitting current into the battery. This can overload the system and cause serious problems.
Reverse Current. Solar panel current normally flows into the battery. But sometimes the opposite can happen whereby the battery current flows back into the solar panel. Reverse current is also the reason why some charge controllers appear to drain batteries at night. This can be prevented with a charge controller installed between the solar panels and battery.