Words of wisdom: “Any Solar is better than no solar, but having too much solar is difficult. The best solar systems are those which can keep the batteries the happiest. Happy batteries are those which are recharged quickly after depletion.”1) - SternWake
Counterintuitively, the primary job of the charge controller (CC) is to keep the batteries from being overcharged. Solar panels run at higher voltages than batteries, often at voltages high enough to damage the batteries. The CC provides the correct amount of power the battery needs at the correct time; this is called smart or three stage charging.
Charge controllers are generally rated by the amount of output they can provide. This output is shared by charging circuits and LOAD circuits. For example, a 20A controller might be using 18A for charging and have 2A available for LOAD.
Controllers operate based on factory- or user-defined setpoints (values) stored internally.
[note from frater secessus: PWM vs. MPPT debates can get overheated in forums and comments. It's your money and your build so do it the way that meets your needs.]
PWM (pulse width modulation) controllers charge by connecting panels to battery until a given voltage setpoint is reached. When the desired setpoint is reached the controller shuts current on/off to the battery in very fast cycles and in such a duration needed to keep voltage from rising. Excess power is given off as heat. This time slicing power delivery is called is pulse width modulation, or PWM.
This runs the panels at2) battery voltage (Vbatt). Vbatt is usually much lower than the panels' Vmp3) and so PWM generally cannot capture the panels' maximum available power4) under typical conditions. A side effect of this is the PWM controller will have highest power output when Vbatt is highest: Absorption (Vabs) and Float (Vfloat). Setpoints can be tweaked for longer duration Absorption and higher Vfloat to maximize power output of a PWM controller.
PWM controllers are simple, inexpensive, and sufficient for many uses, particularly if ambient temperatures are fairly high, paneling is ample, and batteries are not deeply cycled.
During Bulk phase when maximum power is required5) the controller will run the panels at Vmp (the maximum power point). At other times the controller will find a less-than-maximum power point to match panel output to system needs.
Since panel voltage at a given power point is usually too high for system needs6) the controller performs a DC-DC conversion to bring the voltage down to a directly usable level. Because current through a conductor is directly proportional to voltage7) this downconversion effectively turns excess voltage into increased amps.(minus conversion losses).
This ability to decouple panel and bank voltage can result in 10%-30% more power harvested from 12v nominal panels than with a PWM controller. Marketers like to call this “boost” or “gains” but it really just full capture of whatever power the panel[s] can provide under given conditions.
MPPT controllers are most efficient when panel output is less than 3x battery voltage.13)
Shunt controllers are very simple devices that charge to a given voltage setpoint (Vdisconnect) then stop charging. If/when voltage falls to some lower voltage (Vreconnect) charging begins again. This is sometimes called charge and stop charging.
…PWM [and] shunt controllers apply full panel voltage, or something close to it, across the battery terminals, at a duty cycle (fast for PWM, slow for shunt) that keeps the battery happy, and the controller monitors the battery voltage and then adjusts the PWM duty cycle accordingly, or in the case of a shunt, it clamps the panel output. – Tx2Sturgis14)
They may be hardcoded with setpoints or allow user configuration. If configurable, you can get better results by choosing setpoints to fit your situation.
[note: this is a placeholder for a possible new entry.]
There are DC-DC converting charge controllers (DDCCC) appearing on the market that claim to be MPPT but do not actually track power points (maximum or otherwise). They do downconvert some excess voltage into amperage. See this blog post.
PWM is the default choice in most situations because they get the job done in most scenarios and are inexpensive. PWM controllers can cost half or a third of their MPPT workmates for any given rated output.15)
If more power is needed (and there is physical space) additional PV can be added to match the charging output of an MPPT charger, often at a lower cost. [There are no prizes for fanciest or most expensive charge controller! Do what is best for you – frater secessus]
MPPT controllers also tend to consume more power to run themselves than PWM models due to additional processing and electronic components.
MPPT may be a better fit when:
MPPT is effectively required when:
Shunt controllers are useful for shallow-cycling configurations or for battery chemistries that are not damaged by partial state of charge (PSoC). Their relative inexpense makes them useful for even very small systems and test configurations.
Their simple ON and OFF setpoints may also make them useful for charging lithium chemistries.
Further reading: an excellent and readable whitepaper by Victron (PDF).
Multiple controllers (with separate arrays) can be used to charge a common bank. The controllers should be configured with similar charging setpoints for the greatest efficiency.17)
As with inverters, sizing the controller correctly will help system efficiency. An oversized CC will have unnecessarily high parasitic drains as it powers itself. An undersized CC will not be able to put all the rated solar wattage to use and will leave no room for expansion.
For PWM controllers, the formula is something like (panel wattage / 13) * (1.2 oversize for safety) = charge controller amps.18)
MPPT controllers have more leeway in sizing since they can control the output of the panels independently of battery voltage.
See also Sizing a Solar Installation
this section has been moved.
It is common for charge controllers to have a LOAD output for powering (or switching) loads. One can run loads heavier than the controller is rated for by connecting the heavy load[s] to a relay, which is in turn connected to the LOAD output. This still allows for Low Voltage Disconnect because the CC will turn off power to the LOAD output, which turns off power to the relay, which turns power off to the load.
You may also be able to use the LOAD output to create a separate 12v circuit for non-essential ("opportunity") loads.
Note: in wind and hydro power applications the output can be sequenced so that LOAD is activated only when batteries are fully charged. This is called a “dump load” because those power sources need to be able to “dump” excess current to prevent damage to themselves. Dump loads are not necessary in solar because panels can be open- or short-circuited without damage.
Some controllers are labeled or described as “positive ground”, often by their competitors. The term is incorrect and misleading:
… “positive ground” terminology is wrong. There is nothing connecting positive supplies to chassis ground, earth ground, or any other ground. – Trebor English19)
A more accurate term would be Low Side Switched Controllers (LSSC hereafter). A how and why of low side switching is found at the end of the article.
These LSSC can be incorporated into your camper's electrical system as long as:
If you are not using the LOAD output, communicating with the controller, or using the vehicle as an electrical path for the solar panels, LSSC (so-called “positive grounding”) doesn't matter.23)
If some basic steps are not taken the device may not turn off/on as expected.
Here are some appropriate ways to use the LOAD output with LSSC:
LSS means switching (turning on-off) is done on the negative (or low) side rather than on the positive (or high) side. Referring to a low side switched PWM controller, Trebor says:
All three plus connections, solar panel, battery, and load, are tied together. Loads are turned on with a switch (transistor) between the load minus terminal and the battery minus terminal. Charging is turned on (bulk) or pulsed with a switch between panel minus terminal and battery minus terminal.25)
He goes on to explain why this is the case:
The reason for the low side switching is the intersection between physics and economics. N-channel field effect transistors are faster, better, cheaper than P-channel parts. The designer can A) use good parts in a simple circuit or B) use more expensive not so good parts or C) make a complicated circuit that uses 18 volts to control a 15 volt switch.26)