Theory on the working of  Solar Charge Controllers & Regulator

Solar regulators or solar charge controllers are a very important part of a Solar solution. This is the link between Solar panels and the  Deep Cycle Solar batteries.

In order to understand the importance of the regulator and the differences of same, it is firstly important to understand the characteristics of a Solar Panel.

A solar panel is sold on a rating. I.e 100 Watt panel. On the solar panel will be specifications as follows:

These are the ratings of the solar panels at what is referred to as STC- STANDARD TESTING CONDITIONS, which are an international standard- which would typically be ideal conditions of 25C, 1000Watt/sqM light intensity etc.

The Solar Panel cannot deliver 22V and 9A at the same time  (When 22V I=0 or When I=9A V=0) , if the Solar panel is connected to a battery which is for instance flat and at 11V, the Panels voltage will drop to 11V and it will try to put maximum current into the battery which is 9A. But this is not the optimum power ability of the panel.. and as the battery become fuller charged ..the less power the panel would deliver.  There is an optimum point of loading at which the Panels voltage and current reach a point where the Panels actually deliver is maximum power…this is typically around the point of around 70% of the open circuit voltage and or 70% of the Short circuit current. See Graph below

In short you must understand there is a lot of variables which influence the working and performance of a Solar panel, Sun Intensity, Panel angle, Panel temperature, State of the deep cycle battery etc.

Solar Panel Power Performance Curves

Fig 1 - Solar Panel Power performance Curves

At first the” primitive” solar regulators were simple units with a relay control. One would for example have a 12V battery, connected to a 12V solar panel and basically a switch (relay) in between. The solar panel would charge the deep cycle battery to a certain preset voltage – ie 14.5V.  When it reached this point, the solar regulator will disconnect the Solar Panel and no further be connected to the Solar Panel- like a switch which just opened. Until such time that theDeep Cycle battery has been discharged a bit again. Currently many solar  regulators like this are still being sold is does not use a Physical relay anymore, but a Mosfet or an electronic switch of some kind. ( See below- Voltage Controlled Switch)

Solar Panel Old Type Switch Regulator

Figure 2 Old Type solar regulator- Voltage controlled switch

The next “generation” of solar  Regulators which came out was much more intelligent in the way they referred to as PWM solar regulators. PWM stands for PULSE WIDTH MODULATION. The concept is however the same as per discussion above, but instead of disconnecting the solar panel from the solar deep cycel  battery, it will switch the solar panel very fast at a high frequency but with small pulses and as the battery get closer to the rated voltage, the pulses with narrow (PULSE WIDTH will reduce” ) to such level in order to maintain the solar deep cycle battery at the required voltage.  From a normal multimeter it would seem as if the voltage are straight just DC, but is fact is  fast switching pulses feeding the battery.  The fats switching had various other advantages to the solar deep cycle battery, the PWM would also “move” the sulfate around a bit etc. This was a good improvement on solar regulators

PWM Solar Regultors - the working of it

Figure 3 PWM Controlled Switch solar regulator

Although these units work pretty well, they have a few serious shortfalls of which the “mismatch of solar Panels versus solar Deep cycle Batteries are the biggest. Let us look at an example.

A few years ago a few companies such a Outback, Morning Star and later Victron various others, started with the manufacture of a new generation of solar regulators called MPPT- so called MAXIMUM POWER POINT TRACKERS. This is in many ways a game changer

MPPT solar regulators allow us a variety of options.  Let me explain the concept.  A MPPT solar regulator – as the name says it tracks the maximum power point of the Solar Panel array. The unit is fitted like a regulator between the panels and the batteries; the MPPT will always LOAD the solar Panel and charge the deep cycle battery in such a way that the power drawn from the Solar Panel is optimum. It will always work at the Maximum power point of the Solar panel, independent of the actual Sun Intensity, or the Panel temperature or even the Angle or partial shading.  MPPT solar regulator come with various methodology op working, but the more advanced units actually use a logarithm which actively always searches for the optimum power point in real time.

However in order to make use of a MPPT solar regulator, the Solar Panel voltages have to be higher than the battery voltage- this puts a limit on 1 panel or 2 solar panel systems, but for anything bigger the positive gain can be as much as 30% per day.

The MPPT solar regulators are usually designed to be able to go to a much higher input voltage, ie 75V 100V or 150V, for instance if you using 12V panels, you can now put 6 panels in series..etc. Unfortunately the technology also comes at a price as they are more expensive than normal PWM or Switch solar regulators, but if you compare the energy advantage and installation simplification which is bring, the advantages outweighs the cost – although I personally think they still too expensive.

MPPT also have limits and practical applications and the efficiency graph is not linear, as the transfer of power between the solar Panels and the solar battery is still done by an Electronic stepdown converter with limitations.  I.e. as per Figure 4,  if you do connect a system such as that 100V to 12V, the efficiency of the  MPPT is only around 85-90%, so one also have to optimize the design of the system (In this case the switching devices have to work at 10% duty cycle to transfer power), if however in this case the input voltage were only 36V, the Ratio/Duty cycle would have been 33-40% but the MPPT efficiency is up to 95%.  To be honest- to me-  it doesn’t make sense that the designers knowing in certain configurations the efficiency would drop, would allow such configurations – but that is part of the game I suppose

Connections of a MPPT Solar Regulator

Figure 4 MPPT Solar regulator , the way of connecting the Solar panels

Copyright : Dr Gawie van der Merwe ; PlanMyPower (PTY) Ltd


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