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This is a blog post I made on Grit Magazine's website. Just thought I'd repost it here in case anyone was interested.

The Off-Grid Power Series: Charge Controllers

In the last two posts we looked at energy conservation and the importance of solar panels getting full sunlight. In this post we'll look at charge controllers.

Charge Controllers: Their Function and Purpose

The only reason charge controllers exist is to make your batteries last longer and perform at their maximum capacity so we need to understand a few things about batteries before we go farther.

Batteries consist of a series of "plates" that change chemical composition during their charge/discharge cycles. When they discharge they do so more quickly on the outer surfaces. The charge in the inner part of the plate takes longer to leach out to the surface where it too, is siphoned off as electrical energy. That's why you can run the battery of your vehicle down trying to start it then let is sit for a few minutes; and when you try to start it again the battery will once more spin the starter (but not for long!).

When you recharge the battery(ies) the process is reversed. The outer parts of the plates charge first (called a "surface charge") then the inside of the plates slowly catch up. The better charge controllers are programed to recharge your batteries in the most efficient way possible.

Remember: the purpose of a charge controller is to protect your batteries. They do this by regulating the amount of power going to your batteries. (In the "old days" they also prevented the batteries from discharging back through the solar panels at night but most newer solar panels have diodes to prevent this.)

The cheap, single stage, controllers often used in RV's and "solar power kits" sold by some retailers are very similar to the voltage regulator in your vehicle. All they'll do is keep the battery(ies) from being overcharged.

The better controllers like those used in off-grid power systems are little more complicated and are designed to get the maximum power and life from your battery bank. These have three stages, (bulk, absorption, and float) and also perform what's called an equalization cycle.

First is the Bulk Stage: At this stage the controller applies maximum voltage to the batteries in order to bring them to a full charge as rapidly as possible.
It then moves to the Absorption Stage: Once the batteries reach full voltage the controller holds them at the bulk voltage setting for a predetermined time (usually one hour) to ensure that the batteries are fully charged.

Once the preset time period is over, the controller moves to the Float Stage: At this point the battery should be fully charged so the controller reduces the charge rate to a level needed to maintain the batteries at full charge.

If your electrical use is greater than the charge rate of your panels and you draw the batteries down below a predetermined voltage the charge controller begins the cycle all over again.

There's one more cycle your charge controller puts the battery through. This is the Equalization Cycle (used only with standard electrolyte, vented batteries).

The equalization cycle increases the charging voltage in order to overcharge the batteries. Remember, when a battery is discharged the outside of the plates transfer their electrical energy faster than the inside of the plates. The result is that the outer surface of the plate is usually discharged to greater degree than the inside. Over time the inside and outside tend to equalize with the power from the inside leaching to the outer surface of the plates. This is one reason your battery banks should not be run low for extended time periods. (As may happen in winter when charge rates are low and electrical usage is high.)

When a battery is charged the reverse occurs and the outside takes a charge at a faster rate. This is called a "surface charge." The plates will try to equalize but it takes time for the charge to reach the interior of the plate. If the batteries are discharged before the plates equalize it's possible that the inside of the plates may never reach a state of being fully charged.
When a controller goes into the equalization cycle it increases the charging voltage to higher levels for a specified time period in order to "force" the electrical energy deep inside the plates. This creates a lot of heat so this cycle is often referred to as "boiling" the batteries. Most charge controllers do this about once a month.

PWM vs. MPPT Charge Controllers

Charge controllers are usually classed as Pulse Width Modulated (PWM) or Maximum Power Point Tracking (MPPT).

The PWM controllers have been the tried-and-true standard until recently. The main use of solar power in the early years was for off-grid applications. Solar panels were manufactured in voltage outputs designed for 12, 24, 36 and 48 volt systems. For example, a panel designed for a 12 volt system might produce 17 to 18 volts at the maximum charge rate. The charging voltage at the "bulk" setting will be around 12.5 to 14.5 volts. If you run the equalization cycle you'll need to add one more volt making it 15.5 volts. An extra volt or two from the panel is often needed to overcome resistance in the line so a panel designed for a 12 volt system should have the ability to produce 16 1/2 volts minimum.

The larger (higher output) panels today are sized for grid-tie applications and the output voltage may be 24 volts or more. The problem with a 24 volt panel is that it's not powerful enough for a 24 volt system (which requires up to 31 volts for the equalization cycle or 29 volts for the bulk charge rate), yet it's too big for a 12 volt system.

A PWM controller will limit the voltage to 14.5 volts at the bulk charge stage. Let's say your solar panel's maximum output is rated at 24 volts/200 watts/8.33 amps. If your controller limits the voltage going to your batteries to 14.5 volts/8.33 amps your output in watts is reduced to 120 watts.

The reason this happens is that a PWM controller hooks the solar panels directly to your batteries. When the voltage from the solar panels reaches the voltage setting on the controller the controller simply blocks the extra power in order to protect the batteries. But the end result is that you've cut the total output of your solar panel by 40 percent!

So why not just purchase panels designed for 12 volt systems? One good reason is that the higher output panels often cost half the amount per watt of electricity produced than the lower voltage panels. The drawback to using them in an off-grid situation is that they tend to come in voltages that don't mesh well with a 12, 24, 36 or 48 volt system.

(For example: I checked online a few minutes ago and a 130 watt/17.4 volt panel cost $250.00 for a per-watt expense of $1.92. The same site offered a 250 watt/30.3 volt panel for $244 which is under $1.00 per watt. You get almost twice as much power generation for six-dollars less! That savings of 92 cents per watt adds up to over $900.00 when you're building a home-power system charging a thousand watts. That's a lot more than it will cost you to upgrade from a PWM to an MPPT controller.)

An MPPT controller has a DC (Direct Current) to DC power converter built in. Instead of blocking the excess current it converts and regulates it to safe levels. (How it does this is a little complicated!) This allows it to use the entire 24 volts from the panel in the example above. The end result is that you utilize the full output of your solar panel.

You can also wire your panels in series instead of in parallel. The advantage of wiring in series is that the voltage is higher which means that your wire size is smaller (and cheaper!) and you can move the solar panels farther away from the controller and batteries. (I'll tell the reasons why this is the case when I do the post on resistance.)

Another positive of MPPT controllers is that they are more efficient when your panels are less efficient like in mornings and evenings and on cloudy days. For example: If you have four panels hooked up in parallel circuits (as is often the case with PWM controllers) and each panel is producing only five volts then the power produced by those panels is useless. You need at least 13.5 volts to charge a twelve volt battery.

With an MPPT controller you can wire the panels in a series circuit. Then if each panels is producing five volts the total output to the controller is twenty volts. This is enough power to begin charging your batteries.

In northern climates you may get 40 percent more power by using an MPPT controller.

The downside of an MPPT controller is cost. They are considerably more expensive initially than a PWM controller. For those of use who've been off the grid awhile and have the lower voltage panels they may not be worth the cost but if you're just getting set-up with your own solar power system you should give them some consideration. In many cases they'll pay for themselves fairly quickly.

If all you want is a small system to keep a couple of batteries charged up you'll probably do fine with a PWM charge controller. If you plan on a larger system for powering your home you should consider an MPPT controller. The MPPT controller cost more initially but you can often cover the extra expense there with the money saved by purchasing larger (with a lower, per-watt cost) solar panels.
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