Solar Charge Controllers

More About Solar Power System Charge Controllers

An Explanation of Deep Electronic Wizardry For

In the world of off-the-grid home solar power systems, charge controllers make it possible for your solar panels to charge your batteries in the most efficient way possible. There are simple ways to do this – which are cheap but not very efficient – and there are sophisticated ways to do it – which are expensive and highly efficient. Let’s go over the different types, from simplest to most sophisticated.

Single Stage Charge Controller

This is a device which just dumps power in bulk into your batteries at a uniform rate until the charge controller’s internal detection device determines the battery has achieved a specified voltage.

This kind of charge controller could be an electrical device but it could also be YOU. You can hook up your solar cell directly to the battery, let it charge and test it periodically with your multi-meter every so often to that it doesn’t over- or undercharge. It’s cheap, it’s simple but it’s not good for the battery.

Lead-acid batteries, the kind most used in solar power systems, don’t like to be fed at the same rate all the time. Here’s why:

  • As they get more full, they prefer to be fed more slowly.
  • If they aren’t fed more slowly, they don’t ever get fully charged.
  • If they don’t get fully charged, they begin to get more and more sulfate deposits on plates.
  • As they collect more sulfate on the plates, they accept less and less charge.
  • And so on until you have a very dead battery
  • The other problem – which is even worse – is that if the voltage is too high, it will cook the battery!

So the wizards of electronics came up with the …

Two-Stage Charge Controller

This device divides the task of charging your battery into two sections:

Stage 1 – Bulk Charging: This is the same as the first stage above. It quickly fills the battery at a high voltage and high amperage except that it is also smart enough to know then it has filled the battery to only 90% capacity.

Just to review, using the analogy of a water hose, the voltage is like the water pressure; the amperage is like the amount of water passing through the hose in each second.

Stage 2 – Float Charging: What happens after the 90% full mark is reached, the charger lowers the feeding voltage to just above the maximum voltage of the battery (usually around 13.5 volts). It will also lower the amperage.

In this way battery cannot be over charged. And as mentioned before, the battery will get more fully charged if it is fed more slowly as it nears its maximum recharge state.

Three-Stage Charge Controller

This design is even more sophisticated. It can charge the battery faster than a two-stage yet still prevent it from overcharging. It can also keep the battery topped off. Here’s how it does it:

Stage 1 – Bulk Charging: This is the same as in the two-stage charger; it fills the battery quickly to 90% full.

Stage 2 – Absorption Charging: In this stage the charger maintains the same voltage as in Stage 1 but tapers off the amperage.

To taper off the amperage, the three-stage charge controller has a special circuit that detects how full the battery is and automatically adjusts the amperage to fill it quickly yet not overfill it.

Stage 3 – Float Charging: This is a way to keep the battery topped off even if it  has a constant load on it. In other words, if you have a battery that has been fully charged but is connected to a device that is constantly using some of its power, the float charging part of the charge controller will continue trickling power into the battery in order to compensate for the drain from the device as well as any “natural” discharge the battery experiences over time. (All batteries eventually discharge even if not connected to anything.)

Electrically, float charging involves sending the required amount of electrical charge (amperage) to the battery at a pressure (voltage) just slightly above the battery’s maximum voltage. This will keep the battery fully charged at all times without causing any damage.

Pulsing Width Modulation (PWM) Charge Controllers

While researching how to get the most charge possible into batteries, the electronic wizards discovered two very interesting phenomena:

  1. In the final stages of charging, if they sent the electricity to the battery in tiny, short bursts instead of continuous chunks, the battery could take on more charge.
  2. Pulsing also helps breakdown the sulfate crystals forming on the plates.

So a “PWM Charge Controller” is really a three-stage controller with this extra little twist. During the FLOAT stage of charging, the power is sent in extremely rapid pulses. There are special electronic circuits inside the device to check continuously how charged the battery is and adjust the length and strength of the pulses accordingly.

You can think of it as topping off your gas tank. As you near the top, you probably have discovered that you can squeeze more into tank if you slow down the flow. But also, if you are very careful to squeeze a little in and then stop and let it settle, over and over, you can fill it right to the top without it overflowing. This is a bit like what PWM does for your battery.

Incidentally, it might be important to know that the electrical pulsing of a PWM charge controller might interfere with very sensitive electronic equipment if you put them close together. When planning your setup, you might want to keep this in mind.

Maximum Power Point Tracking Charge Controller

The next level up in charge controller sophistication is called Maximum Power Point Tracking or MPPT. This feature has to do with not charging the battery but getting the most power out of the solar panel itself.

Most explanations of exactly how this works depends upon understanding something about electronic formulas and so forth. For the purpose of keeping this as non-technical as possible, envision it this way:

To produce the most electricity possible, the solar panel has to have two rules satisfied:

  1. It has to have as much sun as possible. The more sun, the more electricity it can produce (up to its maximum). This is obvious.
  2. It has to have a load attached to it. This is not so obvious. What does this mean?

Rule #2

“Load” is anything the electricity can make happen, something to resist it, something to push or some work to do. Like run a motor, a heater, or … charge a battery.

From the point of view of the electricity being generated by the panel, it wants to feel it has something to do. If the wires from the panel are not attached to anything, it has no where at all to go & nothing to do. The electrical geeks would say that there is an infinite resistance in the line, so of course nothing can flow.

Think of child wanting something to do. If she has no way to get out of the house; she just sits home and pouts.

On the other hand, if there is no resistance at all in the line, if the positive and negative wires are touching each other (shorted out), all the electricity made by the panel is just running around doing nothing.

This would be like our child running out the front door and returning through the back door without really having done anything. Sure, she is busy but not much of anything is really gets done.

Now, if you put some different levels of resistance between the negative and positive wires of the solar panel, all of a sudden something very interesting happens. There is going to be some amount of resistance that perfectly provokes the panel to generate the maximum amount of power it is capable of.

In our analogy, this would like the child having just the right amount of things to do without getting overwhelmed or too bored.

Of course we still have to remember that the maximum electricity production of the panel also depends upon how much light is hitting it, which leads us back to  …

Rule #1

This rule says the more sunshine, the better. So if the weather is nice, the child wants to keep as busy doing things outside. The cloudier it gets, the less energy she has, the more she wants to say in the house and not do too much.

So, as it gets cloudier and cloudier (or as night time comes), we have to adjust how much there is for her to do. If we don’t reduce the amount of things she feels she has to do outside – but doesn’t have the energy for – she’ll get frustrated.

In the case of solar panels, as it gets darker outside (maybe because a cloud just passed overhead), we have to reduce the amount of load or resistance between the positive and negative wires of the panel. As the cloud moves away, we again have to put more load on the line.

If we keep adjusting up and down the amount of resistance in the circuit as the clouds pass, we can get extract the maximum amount of power that the panel is capable of at whatever level of sunlight we happen to have at the moment.

That’s is why its called “tracking the maximum point of power” for a solar panel, or “Maximum Power Point Tracking” or MPPT. Having this kind of capability in a charge controller means that if the panel gets shaded because of clouds, leaves, dust, Uncle Jake sitting on it or whatever, the system will adjust for the change in light and keeping producing the maximum amount of power it possibly can.

Pretty neat, eh?