PWM charge controller
4.1 Need-to-know glossaries
At the beginning, we will go through some glossaries – refer to the following table.
- Voc, open-circuit voltage, is the maximum voltage across a PV cell, when you measure a solar panel in theoretically standard test conditions (STC) with only a voltmeter connected. The voltage the meter receives is the Voc.
- Vmp, voltage at maximum power, is the output voltage of solar panels when connected with the PV system.
- Nominal Voltage is a reference voltage used to categorize solar equipment in an off-grid system. In a grid-tied system, the nominal voltages (12v, 24v and 48v) are meaningless.
Although charging a battery requires higher voltage, nominal voltages can help you find out corresponding equipment (such as a battery bank) with which a solar panel can match.
a 12V solar panel actually has Voc of 22V and Vmp of 17V, with 36 pieces of silicon squares on the front side.
the 24V solar panel has Voc of 44V and Vmp of 36V, with 72 pieces of silicon squares.
You might be wondering: why a 12-volt panel is not 12 volts?
here’s the deal.
4.2 Why 12-volt panels are 17 volts
A fully charged 12V battery is actually approximately 12.6 volts; to charge a 12V battery, we need higher input voltage – about 13.7-14.4 volts from a solar panel. But why did we design the solar panel Vmp to 17V and not just 14V?
A Voc of a solar panel is measured under Standard Test Conditions or STC, and Vmp is also measured under STC, where the ambient temperature isn’t too hot, the intensity of sunshine is perfect – no clouds, no haze. However, we are not always that lucky. If we encounter some bad weather – for example, hazy or cloudy days – the voltage of a solar panel will drop; so, the panels should be designed with some extra voltage so that your system can still receive enough voltage, even if the weather is not ideal; i.e. no sunlight.
4.3 Charge controller types:
There are 3 types of solar charge controllers:
- Shunt controllers
Shunt controllers: We mentioned shunt controllers when we talked about current regulation – they act just like a switch, turning on and off the flow of current to a battery. You may still see a few on old systems, although they have already been taken off the market. PWM and MPPT are the 2 main types that prevail today.
Let’s go into PWM first.
4.4 What is PWM solar charge controller?
PWM (Pulse Width Modulated), literally, works by modulating its current pulse width.
PWM sends to the battery intermittent charging pulses rather than a steady power output. It operates more like stage-3 float charging that generates trickles of currents to battery.
But how fast (frequency) and how long (width) the pulse should be produced is determined by the state of the battery it detects. If the battery is already fully charged, and the loads in the system are not working, the PWM will only send a very short pulse every few seconds; for a discharged battery, the pulses would be near to continuous. This is the basic working principle.
Although PWM is less expensive than MPPT, because of the sharp pulse the PWM generates, when processing a rapid on and off switch during working, your TV, radios or telephone signals may often be interfered with. That’s the inherent downside of PWM.
When your system chooses PWM as the charge controller, it is important to make the nominal voltage of solar panels the same as the nominal voltage of a battery bank;
if your battery bank is 12V, you must select 12V solar panel as well.
And if your battery bank is 24V, then you must use a 24V solar panel, or wire two 12V solar panels in series, to make it a 24V.
But if your battery bank is 48V, then you need to wire four 12V solar panels in series, or two 24V solar panels in series, to get 48V.
And so on.
Meanwhile, make sure the PWM specifications match that of your battery bank, too.
4.5 How big of a solar charge controller do I need: PWM
How to size a PWM when we design an off-grid PV system?
Step 1, Get the Isc (Short Circuit Amps) and the Voc (Open-Circuit Voltage) solar panel from its nameplate, and figure out how many strings in parallel are in the solar array.
From the nameplate,
we read the Voc 22.1V and Isc 8.68A, and we confirm that it is a nominal 12V solar panel from Voc.
Let’s start with a simple example and assume we only have 1 string in parallel.
Step 2, Multiply Isc by the number of strings in parallel.
8.68Isc x 1 string = 8.68A
Step 3, multiply by 1.25 safety factor. (Why the factor is 1.25: refer to NEC 690.8(A)(1) Photovoltaic Source Circuit Currents)
8.68Isc x 1 string x 1.25 = 10.85A
So, we can choose a PWM that’s current load capacity should be larger than 10.85A.
Now, let’s check another example with 2 strings in 2 parallels using the same 140w panel mentioned just now.
But do remember – we are using a PWM charge controller, so we need to pay attention to how many panels are in strings so that the voltage of the solar array matches the voltage of the battery bank.
In this example, we have 2 parallel strings and 2 panels in series, so the solar array is for 24V battery system.
8.68Isc x 2 strings x 1.25 = 21.7A
A 25A PWM solar charge controller would be enough.