Part 7 - The Charge Controller

The charge controller's job is to take the electricity produced by your solar array and put it into your battery bank.  It should prevent that electricity from bleeding back out of your panels at night, and it should also prevent your array from overcharging or undercharging your battery bank.  But there are several kinds of charge controllers, some that are good at these functions, and some that are not so good.  There's also the question of whether or not you need a controller that has MPPT (Maximum Power Point Tracking) technology.  The answer to that question is "maybe."

The Morningstar Tristar TS-45 Charge Controller

Night Time Power Loss

Virtually all charge controllers prevent power loss through your panels at night, so that's not really an issue in deciding which controller to get.  And many of today's solar panels include diodes that prevent that anyway.  Back to the plumbing analogy - a diode is to electricity flow as a check valve is to water flow.  It allows current to flow in only one direction.

What Size?

Note - if you're planning on using higher than 12v nominal panels or wiring panels in series to increase array voltage, then reducing that voltage back down to 12v nominal with an MPPT controller, scroll down to the bottom of the "Do you Need MPPT" section and read the note regarding sizing the controller.  Otherwise keep reading here.

The first thing you need in terms of a charge controller is one that is big enough to handle the amps your solar panels can throw at it.  The number you want to look for is your panels' short circuit current rating, or Isc.  That number will be listed in the specs for the panel.  Add each panel's Isc to get the Isc for the entire array, then add another 25%.

That extra 25% is to account for a phenomenon called "edge of cloud effect," which at times could cause your panels to produce up to 25% or so higher than their combined short-circuit current rating.  Taking that into consideration, my panels (which have an Isc rating of 3.16 amps each, totaling 12.64 amps for the 4 of them) could potentially produce 12.64 + (12.64 x 0.25) = 15.8 amps, so I would need a charge controller that could handle at least that amount.  

The one I have will handle 45 amps, which obviously is overkill.  But I wanted a controller that would allow me to add more panels at a later date if desired without having to upgrade the controller.

Which Charging Algorithm?

The type of charging algorithm a controller uses is important.  Very inexpensive controllers often use an "on/off" algorithm meaning they will allow the electricity to flow into batteries until they reach a certain voltage and then they simply turn off.  That itself is a problem, but to add to the troubles, too often the voltage at which they turn off is too low, before the battery is anywhere near a full charge.  To charge these batteries fully the electrical current should be tapered down slowly after the batteries reach a certain voltage so they can absorb the power over several hours, until they are truly full. 

PWM (Pulse Width Modulation) charge controllers are great for this.  They utilize three stages in recharging your batteries:  Bulk, Absorption and Float.  And they often include an Equalization mode (which was described earlier).  

1. Bulk Stage - During this first stage, the charge controller will allow all the electricity your panels can provide to be directed to the batteries as battery voltage increases.  This brings their state of charge up rapidly.  Once the state of charge reaches 90% or so (when battery voltage reaches the controller's "set point"), bulk stage is complete and the charge controller switches to 
2. Absorption Stage - Electricity is still allowed to flow to the batteries, but at a gradually diminishing amperage while the voltage is held steady at the controller's set point.  Eventually the amperage is reduced to a small amount, at which time the batteries are essentially full and the controller switches to
3. Float Stage -  In float stage, voltage flowing to the batteries is reduced to somewhere in the neighborhood of 13.4v (depending on the particular algorithm) and batteries are trickle charged, just to maintain the full state of charge.

Depending on the particular type and brand of batteries you have, the recommended set point varies.  The better charge controllers will allow you to set this number yourself instead of having it hard wired into the system.  

For my 6v golf cart batteries, set point is recommended to be 7.4v (14.8v for the two wired together in series). For many 12v deep cycle batteries, that number is lower, 14.4v, for example.  If the controller has a non-adjustable set point of 14.4v, it will work fine with batteries made to be charged at that voltage, but will not fully charge my pair of 6v golf cart batteries which could shorten their lives.  

Some inexpensive on/off charge controllers have a non-adjustable set point of as little as 13.2 volts, which is essentially a float charge and grossly inadequate for recharging deep cycle batteries fully.  Long term charging with this type of algorithm is almost guaranteed to shorten battery life if other measures are not taken to counteract the negative effects (regularly charging batteries fully via some other method and equalizing them).  This is why it is important to match your controller's set point with that of your batteries.
 
To learn more about PWM technology see this document produced by Morningstar Corp.

Do You Need MPPT?

Now to the question of MPPT.  Do you really need it?  Charge Controllers that use Maximum Power Point Tracking make your solar system more efficient by maximizing the number of watts of electricity that flow into your batteries.  It does this by forcing your solar panels to operate at the voltage which produces the maximum number of watts.  This voltage is called Vmp (maximum power voltage).  A PWM charge controller, on the other hand, essentially connects your solar panels to the batteries, which forces them to operate at the battery bank's voltage which most certainly would be quite a bit less than the panels' maximum power voltage.  That translates to a loss of watts.  

But the decision on whether or not to get an MPPT controller is not so cut and dried, and here's why.  MPPT works best when there is a big difference between your panels' Vmp and the battery voltage.  There are two things that typically affect this voltage difference:  Battery depth of discharge, and solar panel temperature. 

If your batteries are regularly deeply discharged before being charged back up again your charge controller will be in Bulk Stage longer, and Bulk Stage is where MPPT really shines.  When the charge controller switches to Absorption Stage, however, it is cutting back on the amps (and by definition, the watts) being allowed to flow into the batteries, so the benefit of having MPPT at this point is greatly diminished.  If you don't usually draw your batteries down too far on a daily basis (I fall into this category) your controller won't be spending much time in bulk stage, so the benefits of MPPT are not going to be as great.

Regarding temperature, Vmp is higher when solar cells are cool, as they typically are in winter.  A higher Vmp means more watts and a greater difference between panel and battery voltage, especially if your batteries are low, which means a bigger benefit to having MPPT.

So this combination of regularly deeply discharging your batteries (but not more than 50%) and camping in cooler climates makes MPPT a big benefit.  But if not, MPPT won't help you as much.

Add to the equation the fact that MPPT controllers are more expensive than non-MPPT controllers.  The MPPT version of the controller I have is about two and a half times more expensive.  In small solar systems it can be cheaper to just buy an extra panel to increase watts rather than buying an MPPT controller.  But, having said that, the price of MPPT controllers has been dropping, so you really have to get current prices and do the math yourself to see if it's worth it to you.

If you absolutely want to maximize the efficiency of your solar panels, get MPPT.  But realize that it could be an unnecessary extra expense, especially if you don't deeply discharge your batteries regularly, mostly camp in hotter climates, or have a sufficient number of watts on the roof to charge your batteries without the benefit of MPPT.  

For me, having 190 watts on the roof with tilting capability, a 220 amp hour battery bank and typically discharging my batteries only down to 85% or so, MPPT would make no sense at all.  My PWM controller more than meets my needs and has never let me down.  But your needs may be different.  This is another area of solar technology that needs to be evaluated based on personal usage.

Note - if you're planning on wiring panels in series or using 24v or 48v panels to charge your 12v battery bank, you must use an MPPT controller that has the capability to reduce that higher voltage down to nominal 12v before sending it to the batteries.  To size the controller properly in this situation, you need to read the specs on the panels and controller and make sure voltage limits of the controller are not exceeded.   This involves adding the open circuit voltage (Voc) in each string of your panels and multiplying by a factor determined by the expected temperature where they will be used, since temperature affects panel voltage.  See the video which explains how to size PWM and MPPT controllers on AltE's website:  AltE's Solar Videos

A good article that explains how MPPT works is this one from Northern Arizona Wind and Sun.

Temperature Sensing

Having temperature sensing capability on the charge controller is crucial, in my opinion.  Batteries are sensitive to fluctuations in temperature.  A cold battery is harder to charge and requires more voltage than a warm battery.  A warm battery is easier to charge and requires less voltage than a cold battery.  

Without temperature sensing, your controller ignores temperature and sends the same amount of energy to the batteries whether they are at 25 degrees or 95.  Batteries can easily be over or undercharged, possibly resulting in damage, just because of normal temperature variations by location or season.  

With temperature sensing, the controller will moderate the rate of charge according to temperature to protect your batteries and ensure they are charged properly.

Some charge controllers come with temperature sensors, and some don't.  Of the ones that do, some are built into the controller itself, while others have connections for a remote temperature probe which can be placed right at the batteries for accurate measurements.  Of the latter, some include the temperature probe as a standard item, some offer it optionally at extra expense.

Other Thoughts on Charge Controllers

If I were to design my system all over again I might do things a little differently.  These days there are some very inexpensive PWM charge controllers on the market, some that include in-unit temperature sensing.   Many of them have a static set point of 14.4v, which would work fine if I had batteries that had that recommended set point.  

You have to be careful, though, and read the specs.  Some of these cheap charge controllers are advertised as PWM controllers, but the fine print states the duration of absorption stage is as little as ten minutes, which is worthless.  Why they would even bother to design a controller that only spends ten minutes in absorption stage is beyond my comprehension.  It should be several hours at a minimum.  

Also beware of controllers that don't say what the specs are.   If they don't tell you the details, you can bet it's not good news.  The specs should tell you what charging algorithm is used (on/off, PWM, MPPT), what set point is (sometimes referred to as "boost voltage" which is confusing since many use the term "solar boost" as a synonym for MPPT), what system voltages (not to be confused with input voltage) the unit will work with (12v, 24v, 48v), whether or not temperature compensation is included, whether or not it will accept a remote temperature sensor, and if it's an MPPT controller it should specify how high the input voltage can be, etc.  If you still have questions after reading the specs it may be best to look elsewhere. 

One good inexpensive PWM controller I know of is Landstar (see ebay) which includes in-unit temperature sensing, connections for a remote temperature sensor and a user-adjustable set point.  I may have gone that route if it were available back when I put my system together.  It costs about 1/5 what I paid for my controller, but it comes from China so it takes a while to arrive.

If you'd prefer a well known, high quality brand and are not so worried about price, look for brands such as Morningstar, Xantrex, Blue Sky, or Rogue, to name a few.  

See part 9 of this series for a list of places to shop for solar equipment.

So, What's Next?

After deciding the type and number of batteries, number of solar panels and the charge controller you want, you need to hook it all together.  That means you need wires, fuses and switches.  Continue on to Part 8, Wires, Fuses and Switches.