"PWM" versus "MPPT "Solar Power Controllers. (PWM first).

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When using any Solar System with TM batteries, a "Solar Charge Controller" is always required to lower the operating voltage (from one or more solar panels) downwards, to create a "battery charging voltage" which the batteries can actually accept.

"12 Volt" Solar Panels actually run at 17.9 Volts, or more. If a Solar Panel was connected directly to a water-filled Lead-Acid battery, it would pretty quickly overheat and ruin the battery. (The acidic "water" would start to boil away. A "sealed" battery would crack and leak as it began to overheat. AGM and Gel batteries would also overheat, being destroyed even a bit faster. Lithium batteries would suffer fatal damage to each of the internal cells.

"Solar Charge Controllers", like the 3-stage (and so-called 4-stage) "Power Converters" which charge from 120V shore power, must choose how much voltage the batteries can safely accept. For "sealed" or wet cell batteries, the upper limit is set at about 14.4 volts, and usually limited by time (to also help prevent battery overheating). Charge Controllers have switches, or programming ability, to optimize settings for other battery types. (Lithium batteries can be charged somewhat higher, and kept at that charging voltage until nearly full. AGM and Gel Cell batteries require slightly LOWER maximum voltages.)

The lower limit, the "float voltage", is typically preset to about 13.6 Volts. While the Solar Controller is charging batteries, it will shut down charging, for very short periods of time, every few minutes to measure the current Voltage of the battery connection - and it will adjust it's voltage accordingly, moving from the upper limit towards the "float voltage" in one or two steps.

Both types of Solar Controllers contain this "Voltage-Tuning" capability. (Although cheaper Solar Controllers may only have a switch for "Sealed or Wet Cell" versus "AGM or Gel Cell".) They should make approximately the same decision for choosing what voltage the batteries can accept. But, there is a big difference in the method they use to lower the Voltage from "Solar Panel Input" down to a Voltage which the batteries can safely use:

'PWM' SOLAR CHARGE CONTROLLERS use a very rapid switch on the Input "Solar Panel "+" connection, disconnecting and reconnecting the Solar Panel(s) many thousands of times per second. At any particular instant, the incoming Voltage is either above 17.90 Volts, and full panel power is being used (if not limited by the Controller's maximum current value). Or it is zero, and zero power is being accepted from the Solar Array. The effective charging voltage is the "weighted average" amount of time which the Solar Controller spends within the two alternating states. ("PWM" means "Pulse Width Modulation", the creation of an intermediate Voltage Value by varying the the width of each "ON" pulse, relative to the remaining "OFF" portion of each ON/OFF cycle.)

Gory details (you can skip this part): On the "Battery Charge +" wire going into the TM 12V system (either directly or indirectly reaching the batteries), the Source Voltage from the PWM Solar Controller is changing very rapidly. But "smoothing" capacitors, wire inductance, and the batteries themselves are all responding to the rapid shifts in received Voltage by sending power back into the wires during "zero voltage" periods, and offering greater resistance to incoming power at the end of each "full voltage" period. When viewed on a volt meter or measured by the Controller, the "zero voltage" and "full voltage" periods result in a "weighted average" - the "weight" corresponding to the proportion of time that the original Solar Panel in charged versus uncharged state.

SUMMARY: The important things to understand are: #1, During the very rapid and short "PWM" disconnect cycles, power is being left behind in the disconnected solar panels, totally unavailable for charging batteries or running 12V electrical loads inside the TM trailer. But also, #2: PWM Solar Charge Controllers are very simple, reliable, smaller in size, and much less expensive.

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An "MPPT" Solar Charge Controller is a more complicated and expensive device, full of electronic parts. (The main "working" parts are inductors and capacitors.) You can think of a "perfect MPPT controller" as a magical box, which converts the incoming power (lower current at too-high voltage) into MORE CURRENT at the lower voltage which is optimal for the batteries, preserving nearly all of the incoming power for use by the batteries and TM appliances.

Instead of "throwing away" Solar Power which the panels generate at higher-than-battery Voltage, an MPPT Controller can send nearly all of the power into the Trailmanor - limited only by the ability of the batteries and TM to use it.

What goes on inside (you scan skip this part): When the batteries (or running appliances) can accept ALL of the Solar Panel Array output power, at the correct Voltage for the measured battery State-Of-Charge, an MPPT Solar Controller will try to keep the the Solar Panels connected continuously - absorbing all the power the panels can generate at the optimal (high) voltage of the panels. But instead of delivering "too much" Voltage into the batteries, an MPPT controller uses the input power to drive the "primary" side of one (or more) large inductors. Then, using rapidly firing switches, output power is sent from the "secondary" side of the inductors, through large capacitors, with HIGHER amounts of current.

That part of the MPPT "deal" is great. The not-so-great parts of the "deal" are:

1. Being full of electronic parts, MPPT controllers cost much more money.
2. With so many more electronic parts, MPPT controllers are far more likely to break down.
3. In performing actual power conversion, they generate considerable heat.
4. In order to disipate that heat, they are built with large heatsinks. They are much larger than PWM controllers, and much less easy to mount inside a TM Trailer.

When operating in MPPT mode (and converting Voltage from "input" to "output") the conversion process generates about 6-10% waste heat. They must also have adequate ventilation, which further limits your choices for a TM installation location. The heat also degrades them over time. They slowly become less efficient, and may eventually fail. I do not know the lifespan of the models I recommend. I kon't know of any which have failed yet, but but adequate ventilation is really important.

When the batteries (and appliances) in the TM can no longer accept as much current as the Solar Array is creating, MPPT controllers will switch into "PWM" mode acting as "PWM controllers. While doing this, they leave unneeded power on the roof, within the Solar Panels.

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As discussed in the previous post, an 'MPPT' Solar Controller can utilize power which a 'PWM' controller must always leave within the panels by rapidly disconnecting and reconnecting to the panel array. The power increase typically varies in the range of 18-35%, depending on the number of panels in the Solar Array, their "maximum power point" Voltage values, and their configuration).

The main advantage occurs in all configurations. Even when Batteries are being charged at 14.4 Volts, the Voltage of "12V" panels operating under good sunlight is typically between 17.8 and 19.5 volts. A 'PWM' Controller will only accept power equivalent to 14.4 Volts, while the MPPT controller can accept all of it. The MPPT controller starts with an advantage of about 25-30%, just from the advantage of operating at the "maximum power point" Voltage instead of the average battery charging voltage.

But this initial advantage must be discounted by power consumption and waste heat within the MPPT controller itself, typically 4-10%. We therefore obtain the lower limit of about 18% "better", with typical "12 Volt" panels operating at a maximum power point voltage of about 18 volts.

How do some configurations enjoy greater benefits? They use higher Voltage panels, or they use "12V" panels wired in Series. Large Solar panels tend to run at high voltages 20V and even higher - and they enjoy a greater advantage.

But even with typical "12 Volt" panels, the change in Voltage between "Solar Input" and "Battery/TM Output" can be made very large.[/B]. With an MPPT controller, Solar Panels may be wired in Series (while a "PWM" controller must always work only with panels wired in a parallel configuration). In Series wiring (with all identical panels), the CURRENT remains constant, while the VOLTAGE of the panels is increased with each additional panel. In Parallel wiring, the VOLTAGE remains constant, while the CURRENT is increased with each additional panel.

The amount of power which is lost in the Solar wires is proportional to the amount of current, squared. Double the current makes 4x the wire losses, and triple the current creates 9x larger losses in power - lost before even reaching the Solar Controller.

Example: In an MPPT-compatable array of solar panels like mine (currently 4 panels wired together in Series), the amount of current in the wires is about 5.5 Amps. The VOLTAGE increases with each panel, from about 19.6V (at the end of the first panel) to 39V, 59V, and finally about 78 Volts at the end of the Series "String". My lost power is only about 0.3%.

If I were using a PWM controller and forced to wire in Series, my Voltage would be only 19.6 Volts. But my total current would be about 5.5 Amps * 4, or 22 Amps total. I would lose about 4% of my power within the Solar wires, before ever getting to the Controller.

The larger the number of panels, the larger the additional MMPT "parallel wiring" advantage becomes.