In a solar electrical power system you have your solar panels that convert solar energy into raw unusable electrical power. This electrical power needs to be converted into usable energy which then can be used to charge your batteries and electrical devices. This task is performed by what is called a solar charge controller.
The voltage off the solar panels can range from just a few volts to hundreds of volts depending on how you have them wired. But your batteries and electrical devices only work within a small range of voltages. This most typical is around 12 – 15 volts for most DIY 12 volt solar systems.
Your batteries along with the charge controller will maintain this 12 to 15 volt range to be used by an inverter to produce 115 – 120 AC volts to power your house hold electrical devices.
These charge controllers also have computer programs that regulate the charging of your batteries. The quality of these programs determine the life of your batteries. This is why only quality charge controllers should be used, because batteries are the most expensive part of any solar off grid system.
MPPT and PWM are the Two Most Common Charge Controllers
The two most common types of solar charge controllers are MPPT or maximum power point tracking, and PWM or pulse width modulation.
MPPT Charge Controller
Maximum power point tracking or most commonly known as MPPT charge controllers work with the voltage and current of the solar panels to maximize power transfer to your batteries and electrical devices. This can equate to hundreds of watts that would have been lost, depending on your setup. It does this by converting excess voltage into amps.
Power, measured in watts, is voltage multiplied by amps. So if the charge controller determines your batteries need 13 volts at this stage of charging and your system at this time is producing 15 volts at 3 amps, let say, than it will convert 15 volts @ 3 amps to 12 volts @ 3.75 amps, there by giving you the maximum power available from the solar panels. This is a very crude example but you get the point.
An added benefit of MPPT technology is that, depending on the design of the MPPT controller, your input voltage from the panels does not need to be close to the battery voltage. There are MPPT controllers that are able to handle hundreds of input volts from the solar panels in which this excess voltage will be converted into amps. This allows you to use lighter gage wires from the solar panels to your charge controllers.
PWM Charge Controller
Pulse width modulation or most commonly called PWM charge controllers work by rapidly turning the power on and off to control the voltage and current to your batteries. The voltage from the solar panels needs to be close to the proper voltage of the batteries. For a 12 volt battery system the solar panel voltage should be around 22 volts VOC or the open circuit voltage of the panel. For a 24 volt battery system it should be around 42 volts VOC.
PWM charge controllers do not do any converting of voltage and current, which means any mismatch in the power transfer from the solar panels to the batteries will result in a loss of power, and depending on your setup could result in the loss of hundreds of watts of power that could have been used by your batteries and electrical devices.
Why Use PWM if MPPT is Better?
If MPPT is so much better than PWM why would people use PWM technology? Price. A 250 watt PWM charge controller costs around 30 bucks where as an MPPT charge controller may cost hundreds of dollars for the same wattage.
I also use PWM for my cloudy day solar panels. I have sunny day panels that are angeled at the sun and are managed by an MPPT charge controller. My sunny day panels produce all the power I need on a sunny day. But, on a cloudy day the voltage from the panels is very low which diminishes the benefit of an MPPT controller – no extra voltage to convert to current – so rather than spend hundreds on MPPT controllers I spent less than a hundred on PWM controllers to give me the extra wattage I need on a cloudy day. Works out just fine.
Mixing and Matching Charge Controllers
Charge controllers should not be mixed and matched. You will find that consumer solar charge controllers max out around 250 watts reasonable prise wise. Most will find that 250 watts is not enough juice to do much, so multiple charge controllers will be needed to increase the power of the system.
Each make and model of charge controller has its own algorithm for charging the batteries. When these different algorithms start competing with each other damage could result to the batteries, either by over charging them or undercharging them. So it's important to use the same make and model of charge controller to properly charge your batteries.
Like I said above, I use an MPPT for my sunny day panels and a PWM controller for my cloudy day panels, but I do get conflict. In the early morning my MPPT want to give the batteries a boost at 12.6 volts but there isn't enough sun yet so it can't, my cloudy day panels don't kick in because their boost voltage is 12.45, so my batteries don't get the full kick they would have if I had all the same charge controllers.
In the summer when there are many hours of sun light this is not a problem but in the winter when the days are so short that extra boost voltage is welcome. Problem being, lead acid batteries are very slow to fully charge so in the winter months most solar system are running with less than fully charged batteries.
My Recommendations for Solar Charge Controllers
For my MPPT charge controller I use the Tracer-2215rn found on Amazon. Many of the so called MPPT controllers in the 100 to 200 dollar range are fakes. This is a real MPPT controller and it does a great job at keeping my batteries charged. I've been using it now for over two years with no problems.
For my PWM charge controller I use the LMS 2430 intelligent solar controller. For a PWM charge controller this unit is exceptional. Its battery charging algorithm brought a 15 year old lead acid battery back to life. It also has read outs of your panel voltage, charging current, battery voltage, load current and much more. And very different from other controllers it allows you to change various settings, one of them being the load shut down voltage. Most controllers shut down the load at around 11 volts, but by then you have already done damage to your batteries. This unit allows you to raise it to 12 volts which is around 50% discharge rate there by not causing damage to your batteries.
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