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Avoiding Common Mistakes: Why Dividing by 14.4V is Key in MPPT Controller Sizing
Maximizing Efficiency:
Why dividing by 14.4v is Key in MPPT Controller Sizing
While it’s common practice to divide the solar panel wattage by the battery voltage to determine the minimum size of your charge controller, this is slightly misleading. Many blog posts will tell you to divide your solar panels wattage by your battery’s voltage.
For example, let’s say you have a 200-watt solar panel array and a 12-volt battery bank. Using the formula above, the minimum size of the MPPT charge controller would be:
MPPT Controller Size (in amps) = (200 watts) / (12 volts) = 16.67 amps
This is wrong!
When sizing an MPPT charge controller for a 12-volt battery, it’s crucial to use the charging voltage of 14.4 volts, not the nominal 12 volts. This is because the MPPT controller adjusts its output to match the battery’s charging voltage requirements. That requirement is 14.4v.
You might be wondering why not at 12v?
A 12-volt battery doesn’t operate at a fixed 12 volts; instead, its voltage fluctuates within a range, typically between 11V and 14V, depending on its state of charge. To understand this better, there are three key voltage stages to consider:
Empty Battery:
A deeply discharged battery will have a resting voltage closer to 11V (for lead-acid) or around 10V (for LiFePO4).
Fully Charged (Resting Voltage):
A fully charged battery will have a resting voltage of approximately 12.8V–13.6V, depending on whether it’s lead-acid or LiFePO4.
Charging Voltage:
During charging, the battery requires a higher voltage (around 14.4V) to overcome internal resistance and ensure a full charge.
What does that mean in non-chinese?
Think of current flow like water flowing in a river. Just like water flows from a higher altitude to a lower altitude, electricity flows from a higher voltage to a lower voltage. It’s like water flowing downhill – the higher the altitude, the more potential energy the water has, and the faster and stronger it flows. Similarly, the higher the voltage in an electrical circuit, the more potential energy the electricity has, and the faster and stronger it flows through the circuit.
So, just as water flows from high to low, electricity flows from high voltage to low voltage in a circuit. This is why it’s important to have the right voltage in a circuit, so that the electricity can flow to the device or equipment you need to power.
A fully charged 12-volt lead-acid battery typically has a voltage of around 12.6 volts, and the charging voltage needs to be higher than this to overcome the internal resistance and ensure a full charge. The recommended charging voltage for a 12v lead-acid, therefore, is typically around 14.4 volts, and this is the voltage that should be used when calculating the size of an MPPT charge controller for a 12-volt battery.
Correctly calculating the size of an MPPT controller
To calculate the minimum size of an MPPT charge controller for a 12-volt battery, you should use the following formula:
Minimum MPPT Charge Controller Size (in amps) =
(Maximum Solar Panel Wattage) / (Charging Voltage of the Battery)
For example, let’s say you have a 200-watt solar panel array and a 12-volt battery bank. Using the formula above, the minimum size of the MPPT charge controller would be:
Minimum MPPT Charge Controller Size (in amps) =
(200 watts) / (14.4 volts) = 13.89 amps
In this case, you would need a charge controller with a minimum rating of 13.89 amps to effectively charge the 12-volt battery bank using the 200-watt solar panel array. That’s better for you as you don’t need to fork out for such a large controller!
If you are thinking of getting an MPPT controller, check out Voltanic controllers below. They are some of the best priced highest quality controllers available on the market today.