Product FAQs

Charge Controllers: Definitions, Settings & Features

What are the key differences between PWM and MPPT charge controllers and which is best for my application?

PWM (pulse width modulation) charge controllers are simpler charge controllers compared to MPPT controllers. A PWM controller uses very fast switching, many times per second, to control the current flow from a PV panel to a battery for charging. PWM controllers work best when the nominal voltage of a solar array matches the nominal voltage of a battery bank.

MPPT (maximum power point tracking) controllers use conversion technology for charging. An MPPT controller does not require the solar panels to be of the same nominal voltage as the battery bank. It can convert PV power at a high voltage to charge power at a lower voltage for a battery bank. MPPT controllers can be up to 30% more efficient than PWM, but they are often more expensive and unnecessary for small systems.

Read our document “Comparing PWM & MPPT Charge Controllers” to learn more.

What is low voltage disconnect (LVD)?
The LVD feature on a charge controller turns off the load of a system automatically when the load drains the battery bank to a low voltage. LVD protects your batteries from reaching a depth of discharge that may damage them and reduce their lifespan.

Phocos has developed three types of LVD to protect your batteries. One type is voltage controlled LVD. When the load drains the battery to a specific voltage, the controller switches off the load within a few minutes. Another type is SOC controlled LVD. The controller considers the battery state of charge and load current to dynamically determine when to switch off the load. The controller typically takes about half an hour to switch off the load. A third type is emergency LVD, or undervoltage protection. This is a very fast acting LVD usually triggered due to errors or fault conditions when the battery voltage suddenly drops to an extremely low level.

What are the differences between 'middle of night' and 'dusk to dawn' settings in CIS-N, CIS-MPPT and CXN controllers?

The difference is reference points for load or dimming timers to give you more options save energy and improve user experience.

CIS family and CX family controllers intelligently detect day and night using the PV array voltage. Night is detected when PV voltage drops below a low level during dusk, and day is detected when PV voltage rises above that low level during dawn. For example, CIS family controllers in 12V systems detect night when PV voltage drops below 8V, and they detect day when PV voltage rises above 9.5V. Two slightly different levels ensures a smooth transition during cloudy weather.

These Phocos controllers also intelligently calculate the middle of the night as halfway between night detection and day detection. This is updated each night for high accuracy throughout the seasons. There is no real-time clock so there may be variation between true midnight and what the controller measures as the middle of the night.

When “dusk to dawn” is selected as a reference, the load timers can be set to turn the load on (or dim) for a selectable number of hours after dusk and a selectable number of hours before dawn. Alternatively, the load can be on the entire night.

When “middle of the night” is selected as a reference, the load timers can be set to turn the load off (or dim) for a selectable number of hours before the middle of the night and a selectable number of hours after the middle of the night.

For example, in a CIS controller, if evening hours are set as “3” and morning hours as “2” with a “middle of the night” reference, the controller will turn the load off three hours before the middle of the night and back on two hours after the middle of the night. (see screenshot of CISCOM setting below).

Even if the hours selected exceed the length of night, the controller will still shut off the load at dawn and turn on the load at dusk.


How do I adjust the settings on my charge controller, such as the low voltage disconnect (LVD) threshold or dusk to dawn hours?
With CIS family controllers, use CIS-CU remote or MXI-IR with CISCOM software.
For CXN family, use CXI with CXCOM, CXM, or the onboard button and LCD. The MPM system settings can be changed through MCU with MXI and MODCOM as well as DIP switches. With other Phocos charge controllers such as the ECO series, settings cannot be changed.

Check the datasheet or user manual for more information, or contact Phocos technical support.

Are AGM, flooded or gel batteries my only energy storage options compatible with a Phocos charge controller?
Most Phocos controllers are designed specifically for lead acid batteries such as AGM, flooded, or gel batteries. Sometimes programmable Phocos controllers can be made compatible with other chemistries such as lithium ion. If you would like to use a battery chemistry other than lead acid, please contact Phocos technical support with a datasheet for the battery you want to use.

CXNup controllers come with a built-in profile for lithium iron phosphate (LiFePO4) batteries.

What do the load terminals on my charge controller power?
The load terminals on a charge controller power connected DC devices. The DC device must be compatible with the battery nominal voltage, and the battery bank capacity should be appropriately sized for the load. Loads can be many things from a DC refrigerator to a DC LED light.

Some loads should not be connected to the controller’s load terminals and instead connected directly to the battery. Highly inductive loads with high inrush currents may damage the controller’s load terminals. DC motors and inverters are examples.

Are Phocos charge controllers protected against lightning?
Phocos charge controllers are CE compliant which includes rigorous surge protection. Phocos charge controllers have internal surge protection which will guard the PV output and battery input, but not from a direct lightning strike. The controller may withstand indirect strikes that occur nearby.
Is there any other equipment or hardware that I need to purchase for safety of the PV system?
For safety purposes, it is highly recommended that the user places a fast acting fuse or DC breaker between the ungrounded lead of the charge controller to the corresponding battery terminal as close as possible to the battery terminal. This will protect the conductor, device, and user from overcurrent. It is recommended installers use electrically insulated tools when wiring a system and follow applicable laws for the installation region.
Does Phocos have instructions for CXup and CXNup load settings? Can I use my CIS series controller with a motion sensor?

Yes. Find out how in this Tech Bulletin

 System Design & Sizing

Why am I getting less power than expected from my PV system?
This can be due to several factors. First, your battery bank may not need it. If your battery bank is full or nearly full, the controller must limit PV power to prevent overcharging.

Second, standard test conditions (STC) don’t always match actual environmental conditions at the install site. Thermal effects, atmospheric conditions, tilt, azimuth, and irradiance change PV performance. Check if your module manufacturer lists “NOCT” data on the datasheet. For many installations, NOCT data is closer to actual performance than STC data.

Also, your PV array and controller may be mismatched. If you have a PWM controller, the nominal voltage of your panel should match the nominal voltage of your battery bank. If it’s higher, the PWM controller is essentially “throwing away” the extra voltage it cannot use. For example, if you have a 12V battery bank, choose a 36 cell module. The Vmp will typically be 17 to 18V.

Other causes of power losses include loose wiring connections, module mismatch, dirty module, and orientation of the module to the sun.

What is the optimal distance for wiring between my charge controller and my battery? When does a significant voltage drop occur?
It is highly recommended that the charge controller be within one meter (approximately 3.25ft) of the battery bank and in the same room or enclosure. Be sure all rooms and enclosures are well ventilated. Lead acid batteries produce flammable hydrogen gas.

Significant voltage drops occur not only with excessive distance but with improper wire size. Check the back page of the Phocos catalog to view a quick guide for wire sizing.

Measure the voltage at the controller, and measure the voltage at the battery terminals. If there is a 0.5V difference or more, reduce the wiring distance or decrease the wire gauge.

Can I use a 60 cell module (commonly used for grid-tied applications) with my battery bank?
High voltage panels, or 60 cell modules, can be used with MPPT controllers. Be sure to check that your panel is within the other specifications of the controller you are using. These specs include max power input, max voltage input, and max current input.
How do I know that I’m receiving enough sunlight on my PV module to adequately charge my batteries?
Check the battery voltage. If the voltage is at a target charging voltage, then the module is adequately charging your batteries. For example, if you measure 13.7V on a 12V battery, then it has reached the Float charging voltage, and the battery is being adequately charged. Remember that the target voltages may be higher in cold weather and lower in hot weather due to temperature compensation.

Check the battery voltage over several minutes. If the voltage is increasing, the battery is being charged. If the voltage is increasing very slowly, then the module may not be receiving enough sunlight or another problem might be to blame. Check the terminal connections and any wiring connections of the PV module to the charge controller and the charge controller to the battery bank. Clean your panels and remove any obstacles causing shading, no matter how small.

Use a controller accessory with current measurement (if applicable) or a digital multimeter to measure the module current. Compare the measured value to the module datasheet. Check the battery manufacturer datasheet or user guides for recommended charge current or charge rates (C-rates).

Sizing MPPT and PWM Charge Controllers to PV Panels

How do I correctly size my charge controller for wintertime low temperature effects on PV panels?
Cold winter temperatures can damage controllers due to low temperature effects on PV panels. Damage occurs when the cold weather panel voltage increases above the Standard Test Condition (STC) ratings.

Below is a quick, five-step worksheet to size your controller appropriately for low PV cell temperatures. You’ll need to have the manufacturer’s product specifications for the PV panels and the charge controller to complete the worksheet.

In Step #3, NEC Article 690 Table 690.7(A) Voltage Correction Factors for Crystalline and Multicrystalline Silicon Module is the source for the factor of 1.25 used to calculate a worst case panel voltage condition in -40°C weather.

There are other methods and NEC factors that might be applicable, but this is the quickest, easiest check. If you do not know about worst case ambient temperatures at the install location, then use the worst case 1.25 factor or consult the NEC.

5-Step Worksheet for Sizing Charge Controllers for Minimum PV Panel Temperatures
Step #1 Panel Voc
Enter panel Voc at STC from your panel datasheet or nameplate here:

Step #2 Series String
Enter number of the above panels wired in series here:

Step #3 Array Voc @ STC
Multiply #2 by #1:

Step #4 Array Voc with Min. Cell Temperature
Multiply #3 by 1.25:

Step #5 Controller Check
Is #4 ≥ maximum controller solar input voltage specified on the datasheet of your selected controller?

If #4 is greater or equal to maximum controller solar input voltage, the selected controller is not suitable for the system as designed. Select a controller with a max solar input voltage ≤ #4 and/or change system wiring configuration and start over.
If #4 is less than maximum controller solar input voltage on the datasheet of your selected controller, it is suitable for the system as designed.

What are tips for sizing a MPPT charge controller to my PV panels?
Here are 2 important reminders for sizing MPPT controllers:

Tip #1 PV panel Vmp > controller’s minimum solar input voltage.
The panel voltage at maximum power (Vmp) should be higher than the controller’s minimum solar input voltage specification. For 12V systems, this is typically 17V. For 24V systems, this is typically 34V.

Tip #2 Don’t pair one 60 cell panel to a 24V battery bank.
Using one 60 cell panel to charge a 24V battery bank is not recommended. The Vmp is typically too low to charge the batteries sufficiently.

What are tips for sizing a PWM charge controller to my PV panels?
Here are 3 important reminders for sizing PWM controllers:
Tip #1 PV panel Vmp > controller’s overvoltage protection level for the battery.
The panel voltage at maximum power (Vmp) should be higher than the controller’s overvoltage protection level for the battery. The battery overvoltage protection level is typically 15.5V for 12V systems and 31V for 24V systems.

Tip #2 Pair 12V battery banks with 36 cell panels.
12V battery banks perform best when paired with 36 cell panels. When larger panels such as 60 cell panels are used, the power output will be far less (<50%) than the maximum rating even in full sunlight. This is not recommended.

Tip #3 Pair 24V battery banks with 72 cell panels.
24V battery banks perform best when paired with 72 cell panels or strings of two 36 cell panels in series. When a smaller panel array is used, such as one 60 cell panel, the voltage is typically too low to keep the batteries charged properly, resulting in reduced battery life. When a larger panel array is used, such as two 60 cell panels in series, the power output will be far less than the maximum array rating even in full sunlight. This is not recommended.

 Charge Controllers and Alternatives to PV Modules

Can I use a power supply rather than PV modules to charge a battery with a Phocos controller?
No. There is a very high likelihood of breaking the controller, power supply, or both. This is not covered under warranty. Phocos controllers are designed for use with PV modules only.
Can I use a solar simulator rather than PV modules with a Phocos controller?
Do NOT use a solar simulator with a PWM controller. They are extremely likely to break PWM controllers. This is not covered under warranty.

Solar simulators were designed for MPPT controllers only. Some solar simulators may not be compatible with Phocos MPPT controllers. Contact Phocos tech support if you have questions about compatibility.

CISCOM Software Frequently Asked Questions

CISCOM values are programmed for 12V systems; how do I program for 24 or 48V?

To program CISCOM settings for a 24V system, multiply the 12V values by a factor of 2. To program for a 48V system, multiply the values by a factor of 4.

Can I program multiple dimming levels with CISCOM?

CISCOM only allows for single level dimming. So, you can dim your light to, say, 50%, but you cannot dim it to another level after – it can only operate at 100% or an arbitrary dimming value.

How do I use CISCOM with my CIS charge controller?

You can use CISCOM with a MXI-IR USB cable or with a CIS-CU. CISCOM needs one of these units to send information. These components are both available through Phocos distributors. See our website for a list of distributors near you, or contact our Sales Department.

How do I know if my CISCOM settings were successfully transmitted to the controller?

The CIS controller family has a LED display that will show if it is communicating with the CIS-CU or MXI-IR. Two red LEDs will steadily be on, and CISCOM will notify you if the controller successfully saved the settings or not.

Will my CISCOM settings be erased if the controller turns off, then is reconnected?

CISCOM settings that are successfully transmitted to your controller will not be erased if the controller is disconnected from power. The microcontroller has non-volatile memory and does not require a constant source of power to retain information stored on it.

My MXI-IR cable is not reading or sending the settings from the controller. What should I do?

Ensure that the drivers for the MXI-IR are installed properly on your computer. If you have done this and still need help, please contact our Technical Department for assistance.

What’s the default load programming setting?

The light/load behavior for CISCOM PRO is set to a state of charge (SOC) level of 4. The light will turn off at SOC level 4. This is typically around ~11V for a 12V nominal battery.

How are dimming settings controlled?

Dimming is controlled through battery SOC or through dawn to dusk/middle of night settings. These settings are compatible with one another, so you can dim your load with SOC/battery voltage and with dawn to dusk/middle of night mode. This dimming combination can make your lighting system even more energy efficient.

What do each of the values correspond to under the “battery charge regime settings” section? What does the terminology mean exactly?

Customized settings are disabled in this section because our engineers have determined that these settings best extend battery life. Depending on the battery type you choose, these values will change for different lead acid chemistries (flooded versus sealed).

Equalize voltage is specific to flooded batteries. Flooded batteries require a regular equalization charge to remove sulfation that accumulates on the internal plates of the battery, which is a natural consequence of repetitive battery cycling. Non-liquid, lead acid batteries do not require equalization, but do require a regular boost charging voltage.

Boost is a target charging voltage for a lead acid battery, which your charge controller will regulate to when there is initial input power or when a battery has reached a low SOC. This will happen at the beginning of each day (when the panels first receive sun) and when the controller first turns (when it is initially connected or reconnected to a battery).

Float is a target voltage that occurs once the battery has reached sufficient SOC.

The temperature compensation value is used in conjunction with your charge controller’s temperature sensor wire. Battery voltages will change when environmental conditions are hot or cold. The battery voltage decreases with higher temperature because temperature has a large effect on resistance. To compensate for fluctuating environmental temperatures, a value of 24 millivolts/Kelvin is used. This is a generalized temperature compensation value, but if you are living in extreme weather conditions, contact our Technical Department for more appropriate recommendations that best suit your environment.

What do each of the values correspond to under the “load disconnect settings due to low battery SOC” indicate?

This section controls load behavior, such as low voltage disconnects and dimming percentage. A low voltage disconnect is imperative to not permanently damage your batteries. “LVD load 1 offset” controls LVD relative to voltage or SOC.
If you do not have a CIS-N-2L, you only can operate one load per controller. Load 1 refers to a load that is not being dimmed. “LVD Load 2” is for a load that is going to be dimmed. If you do have a CIS-N-2L, load 1 cannot be dimmed, but load 2 can. This does not imply that load 2 has to be dimmed. If you have a CIS-N-2L, go back to the main menu and select “CIS/CIS-N Dual Load Controller” instead.
“LVD: Base + Offset (V)” changes LVD relative to voltage, from a base level of 10.98V. When you select “voltage” under LVD indicator type, the offset can be adjusted to 10.98V to 17.35V. Base is the value you cannot go under and offset adjusts the voltage relative to the base.
Emergency high/low-voltage values dictate when the controller will turn off based upon extreme battery voltage levels.