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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

Solar charge controllers

A solar charge controller (CC) regulates the charging of a battery bank from the solar panels.

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.

types of charge controllers

[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.]


images-na.ssl-images-amazon.com_images_i_41pwjorizhl._ac_us160_.jpgPWM (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.


images-na.ssl-images-amazon.com_images_i_51bi9ijcsrl._ac_us160_.jpg MPPT (maximum power point tracking) controllers have two defining abilities:

  1. discover (track) and utilize various power points along the panel's power curve. Sometimes this is the maximum power point (MPP); often in a 'dweller context the system needs less power and the controller runs the panels at some other power point. It might be more accurate to call them PPT controllers.
  2. DC-DC downconvert excess voltage to amps

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.

images-na.ssl-images-amazon.com_images_i_512x3hbc6jl._ac_us160_.jpg 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.

"boost" effect

When compared apples-to-apples on identical systems with only the controller being different, the “boost” effect is most pronounced:

  • during bulk stage and the early part of absorption stage when the battery can take in the most power
  • when the battery is most depleted (ie lowest resting voltage). This is the result of a cascade of factors:
    • When a battery is deeply discharged it will go into Bulk charging mode until it reaches the Absorption voltage (Vabs). For the purpose of illustration we will assume the bank is depleted to 12.2v (~50% state of charge), a commonly recommended lower limit for deep cycle batteries.
    • PWM controllers can only run the panel at whatever voltage they are charging at right now. In our example that is 12.2v.
    • Nominal 12v panels usually have max power output (Vmp) around 17v.8)
    • when a typical 100w panel9) is run at 12.2v in lab conditions it puts out ~68.32W. The same panel run at Vmp (~17v) would put out 100W. MPPT is capturing more power than the PWM when the battery needs it most.10)
  • during times of greatest PV efficiency11) (clear, cold weather)
  • during times of low irradience (low light levels due to low angle or overcast conditions)12)13)

designing for MPPT

There are benefits from using higher voltage panels that can be realized when a system is designed for an MPPT controller. An MPPT controller can be the centerpiece of a DIY converter.

MPPT controllers are most efficient when input voltage (Vpanel) is about 2x nominal battery voltage (Vbatt). When Vpanel rises above 2x Vbatt more heat is generated when downconverting.14) At less than 2x the controller may not have enough “headroom” to work efficiently.


images-na.ssl-images-amazon.com_images_i_41gwyw0bt5l._ac_us160_.jpg 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. – Tx2Sturgis15)

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.

how to choose

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.16)

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:

  • battery banks are cycled deeply on a regular basis so more time is spent in Bulk
  • nominal 12v panels have a relatively high Vmp (>=18v)
    • high latitudes
    • overcast or rainy weather
  • if the camper will be used in areas of partial shade17)
    • canyon or “urban canyon” areas
    • forested areas
  • if temperatures are very low (greater harvest)
  • the increase in cost is not an undue burden

MPPT is effectively required when:

  • using panels with nominal voltages higher than 12v
  • space available for panels is limited, as on a sailboat or small vehicle

We say “effectively required” because one can use 24v panels with PWM and 12v banks – it just doesn't work very well. The 24v panels will be run at 12v voltages, yielding about half the power you would get with MPPT.

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 charge controllers

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.18)

sizing your charge controller

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.19)

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.

Using LOAD output

It is common for charge controllers to have a LOAD output for powering (or switching) loads. One benefit to doing this is you can define a Low Voltage Disconnect (LVD) setpoint and Low Voltage Reconnect to protect the battery from excessive discharge.

Not all types of loads should be run from these terminals, though. Morningstar says:

Heavily inductive or capacitive loads such as pumps, motors, compressors, and inverters should not be wired to the controller’s Load terminals. In addition, loads exceeding the Load Current Rating of the controller should not be connected to the controller’s Load terminals.20)

elsewhere they say:

Inductive loads can generate large voltage spikes that may damage the controller’s lightning protection devices.21)

although an exception is made for the SunSaver MPPT which “Handles inductive loads without problems.”22)

One can run loads heavier than the controller is rated for (or inductive/capacitive loads) by connecting those load[s] to a relay23), 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.

positive ground controllers

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 English24)

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:

  1. anything powered by the LOAD output does not ground the negative wiring to the vehicle.25)
  2. any communications i/o (like ethernet or other jacks) do not connect to devices that are grounded to the vehicle.26)
  3. solar panels are wired directly to the controller and do not use the vehicle as an electrical path27)

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.28)

use of LOAD output on an LSSC

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:

  • run all positive and negative wires back to the LOAD output; or
  • run them through buses that lead back only to the LOAD output; or
  • put a relay on the LOAD output and run all the loads through that.29)

low side switching

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.30)

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.31)
2) or very close to
3) unless the panels are very hot
4) adjusted for temperature, insolation, etc
5) or at other time when loads + charging >= panel output
6) particularly with higher than nominal 12v panels
9) Iop = 5.6A
10) there are minor inefficiencies not considered here
11) i.e. highest voltage
12) this is a function of higher input voltages
16) 10A, 20A, 40A, etc
24) PM correspondence with frater secessus
electrical/solar/charge_controller.txt · Last modified: 2018/06/30 16:43 by frater_secessus