How Does A Solar Charge Controller Work? Which Is Best PWM or MPPT?

What is a solar charge controller featured image

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A solar charge controller is essential for charging batteries efficiently and safely. A solar panel connected to the battery directly may continue to charge until internal damage occurs.

It isn’t too useful to understand the electronics inside a controller, but it is important to understand how and why they are connected electrically, and to have some understanding of the different types.

A Solar Charge Controller works by regulating the voltage and current flow from solar panels to a battery. It detects and monitors the battery voltage, reducing the current when the battery is fully charged. The controller maintains a float charge to keep the battery ready for use.

Video – How does a PWM solar charge controller work?

 

How To Connect A Solar Charge Controller To A Battery

Simple Diagram Of Solar Charge Controller Connections

How to connect a PWM Solar Charge controller

PWM solar charge controller connections

Most batteries for auto and marine operation have a terminal voltage of around 12 volts.

The two broad groups of batteries in common usage are lead-acid in various forms and lithium based. Different type of batteries need different charging characteristics – more of this later.

Whichever type of battery you have, the basic connections are the same. The solar panels connect to positive and negative terminals of the controller, the output connects to the battery terminals and a load may or may not be connected in parallel with the battery as well (see above.)

Some solar charge controllers also have extra USB outputs and perhaps a separate set of terminals for a DC output load. However, this is normally small capacity, for a light, for example.

How To Charge Different Battery Types

The Four Main Battery Types In Common Usage

How Does A Solar Charge Controller Work With Different Battery Types?

The best charge controllers have settings for different battery types

Not so many years ago, the only type of battery for most applications were heavy lead-acid type with electrodes immersed in liquid acid.

In the age of electric vehicles there have been huge changes in energy-storing technology – storage capacity density is now key.

The four types of battery shown above all have particular charging characteristics:

Battery Type

Full Charge
Open Circuit Volts

Lead-acid
(deep cycle)

12.72

Lead-acid
(auto)

12.72

LiFeP04
(lithium phosphate

13.4

Li-ion – 4 string
(lithium polymer)

14.4

 

Luckily for us, modern solar charge controllers either have switch settings or a digital menu for setting the battery type.

Solar Charge Controller Types And Connection Tips

Although relatively sophisticated, solar chargers are not all the same. The two main types are call PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking). MPPT vs PWM.

PWM are the simplest and cheapest type, although the higher power models can be expensive. The model shown is by Victron, a very reputable solar charge controller manufacturer.

Image – Victron PWM Solar Charge Controller

PWM Solar Charge Controller

How PWM charge controller works – Victron charger shown

How Does A PWM Solar Charge Controller Work?

Solar panel voltages are generally higher than battery voltages, which is good if the battery needs a lot of recharging, but not so good if it’s approaching full charge.

The PWM solar charge controller monitors the battery voltage and decides how much current it can safely feed into it.

The circuit then chops up the voltage into pulses of the same voltage. If the pulses are long, then the average voltage is higher and so is the charge current.

If the pulses are shorter, then the voltage and charge current is lower.

Battery charging controller voltage pulses

PWM battery charge controllers modulate voltage pulses

The pulses are long at the beginning of a charge cycle when more charge current is required for faster charging and shorter towards the end of the charge.

You can find a more in-depth explanation here.

How Does An MPPT Solar Charge Controller Work?

The Maximum Power Point of a solar panel is the voltage at which they can produce the maximum charging power to the batteries.

The controller compares the battery voltage with the solar panel and converts the panel voltage into the value which maximises the current into the battery.

In this way, the rising voltage limits the charge current to it’s most efficient value at any given time.

Image – Graph Showing MPPT Voltage/Current Curve Courtesy electricalacademia.com

How solar charge controller works for maximum power point tracking

How an MPPT solar charge controller works

MMPT solar charge controllers are more expensive but much more efficient than the cheaper PWM models.

They produce more power from the solar panels in cloudy weather, for example, or in the winter season. This is because they extract the maximum possible power in the prevailing conditions.

For a large DIY domestic system using 400 watt solar panels, where there is plenty of space to mount an extra panel or two, it may not be worth while paying extra for a MPPT controller.

Solar panels are becoming cheaper every year and more innovations are sure to follow.

If mounting space is limited, such as when building a DIY solar kayak or canoe (my favorite pastime!)

MPPT can make a lot of difference as they are between 20% to 30% more efficient than PWM.

Table – MPPT vs PWM Solar Charge Controller Compared

Solar Panel Configeration

Input Voltage

PWM Power Transferred

MPPTPower Transferred

MPPT % Gain 

2 x 100 Parallel

21 volts

58

22% 

2 x 100 Series

42 volts

73

Solar Charge Controller Ratings

Different brands of solar charge controller have a wide range of input voltages, output voltages and output current.

Many basic models can accommodate battery voltages from 12 to 24 but the more expensive ones often have an upper range limit of 72 volts.

These are essential for large battery banks needed for off-grid living. PV (solar panel) input voltage also varies widely, from 24 volts to 250 volts.

If installing a smaller system on a budget you might be tempted to buy a cheap solar charge controller with a maximum input voltage of 24 volts.

This is fine if you connect 2, 3 or more in parallel. The voltage stays the same and the current production would increase.

How to connect solar panels in series to a solar charge controller

Solar connections can change voltage output to solar charge controller

Solar Panels Connected In Series Double The Output Volts

However, in some circumstances it makes sense to connect solar panels in series. In this case, the solar panel output volts would be around 42 i.e. double that of just one panel alone.

Higher current rating models are more expensive too, as you would imagine. It’s very important to choose well before you buy.

Specifications For Midnite Classic 250 MPPT Solar Charge Controller

Solar charge controllers are rated according to voltage and current they can handle

What size solar charge controller do you need?

Solar Charge Controller Tips

  • Always connect the batteries before the solar panels – this provides reference voltage
  • Keep the solar charge controller well ventilated
  • Buy the best quality you can afford – very cheap ones can fry your batteries!
  • Buy a solar charge controller with 50% more capacity than you need – all for expansion
  • Make sure it’s a smart type that detects panel and battery voltages

Related Questions

Why do I need a load connection on a charge controller?

It isn’t strictly necessary. The main load should be connected to the battery terminals so that the charger feeds them both. The auxiliary d.c. load output terminals are provided for connecting up local lights, for example, or other small d.c. loads.

Why do I need a solar charge controller?

For most applications you need a solar charge controller to limit the voltage and current to the battery so that it won’t overcharge. In some circumstances a low-power solar panel can be safely used if it’s maximum current output is less than the normal float charge current for the battery.

What size charge controller for 200w solar panel?

You will need a 15 amp solar charge controller at minimum. It’s always best to buy a bigger capacity than you need, so you can expand the system in the future. A 200 watt solar panel delivers between 10 and 12 amps in full sunshine, depending on inclination and insolation values.

Why do I need a load connection on a charge controller?

Many charge controllers have a seperate load connection which is provided for convenience. Only connect low power DC devices that pull 1 or 2 amps.

The main load of course is the battery that is being charged. The AC inverter, if required, will connect directly to the battery terminals, also any main DC load.

Does a charge controller drain the battery?

No, a charge controller incorporates blocking diodes that only allow current to pass in one direction only.

How does PWM solar charge controller work?

PWM stands for Pulse Width Modulation, which is basically a description of its operation.

Direct current from the solar panels is chopped up into pulses at a certain voltage. The charger detects the voltage of the battery connected to it and calculates the correct charging voltage to apply, which determines the charging current.

The spaces between the voltage pulses are adjusted electronically. The average voltage depends on the time between the pulses.

PWM charge controllers actually waste any surplus voltage from the solar panel and are not very efficient.

How efficient is a solar charge controller?

The most efficient type of solar charge controller are MPPT (Maximum Power Point Tracking) at 97%.

A solar panels delivers maximum power when the load resistance matches the panel’s internal resistance, called its Characteristic Resistance.

An MPPT solar charge controller matches the panel’s Charcteristic Resistance to maximise the available power. It then converts extra voltage into more charging current, so the battery charges faster. This is why they are more efficient.

Can I use solar panel without charge controller?

It’s never a good idea to connect a solar panel to a battery without a solar charge controller. Even a 10 watt solar panel can generate 0.7 amps and overcharge a battery if left connected.


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