Solar FAQ (pt2)

Solar Info Solar FAQ (pt1) > Solar FAQ (pt2) <

[What is a blocking diode, and what does it do?]
[What would I lose if I didn't have a blocking diode?]
[Are there ways other than blocking diodes to prevent night time losses?]
[What other losses are of concern?]
[Why does the controller turn off when the battery is not fully charged?]
[Why does my controller turn on and off really fast?]
[How can the voltage drop error be avoided?]
[Can PV controllers be used to regulate AC chargers?]
[How do I check the output from my solar panels?]
[What size battery can the PV controllers charge?]
[I have a 50 watt solar panel, what can I expect to run?]
[I have a digital meter, what is the remote shunt?]
[What can you use a remote shunt for?]
[When I turn on my lights, the load current displayed goes down. How can that be?]
[ When a controller fails, what will happen?]
[What is temperature compensation?] [How do I mount the modules?]
[What wire do I use to wire up the modules?]
[Do I need to use the jumpers included in the module junction box?]
[Do I need blocking diodes within the array?]
[What type of diodes should I use?] [Where do I put the diodes?]
[What is a combiner box?] [What fuses do I use?] [Where do I put the fuses?]
[Can I wire the modules in parallel or in series?] [What happens if I scratch the module?]
[Can I use solar modules in northern climates or in hazy areas of the South?]
[UNI-SOLAR® modules with standard controllers and utility line-tie inverters?]
[Do I need a charge controller?] [How do I size a charge controller or voltage regulator?]

 

What is a blocking diode, and what does it do?
A blocking diode is like a check valve for the solar system. It allows current to go from the panels to the batteries, but prevents current drain from the battery into the solar panels at night.

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What would I lose if I didn't have a blocking diode?
The losses through a solar panel at night without a blocking diode amount to about .03 amps per solar panel in parallel. So if you have a 4 panel system and the night is 10 hours long, you would loose 1.2 amp hours. If you generate about 12 amps for 6 hours, this is less than 2%.

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Are there ways other than blocking diodes to prevent night time losses?
The charge controller can be designed to automatically disconnect the solar panels at night.

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What other losses are of concern?
Voltage drops through the controller and in the system wiring is a concern. For example if the max. power voltage of the solar panel is 16 volts, and you need to charge the battery up to 14.5 volts, if you have a voltage drop of 2 volts through the entire system from panels to battery, you will not get the battery fully charged. Other losses include current consumption of the controller and inverter. These are typically small, but should be considered.

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Why does the controller turn off when the battery is not fully charged?
If a voltage drop occurs between the controller and the battery, the controller will see a voltage that is higher than actual battery voltage. This will cause the controller to turn off too soon.

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Why does my controller turn on and off really fast?
This is related to voltage drop also. What is happening is that the controller sees the high voltage because of the line loss, but as soon as the charging stops, the error disappears, so the controller turns back on.

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How can the voltage drop error be avoided?
The correct size of wire must be used to minimize voltage drop in the wiring, but most importantly, the connections must be good. Connections that are crimped but not soldered often corrode over time, and components like fusses and switches often contribute to voltage drops. Where possible, use a controller that offers remote battery voltage sense for higher current systems.

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Can PV controllers be used to regulate AC chargers?
No.

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How do I check the output from my solar panels?
With a multi-volt meter, check the open circuit voltage, the voltage reading you get between the panel plus and minus with the panel in the sun (should be about 19 volts for a 12 volt system) and the short circuit current, measure the current when array plus is shorted to array minus. Make sure your meter is rated to handle the current that your panels produce.

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What size battery can the PV controllers charge?
PV controllers usually do not need to consider the capacity of the battery, only the current and voltage output from the solar panel.

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I have a 50 watt solar panel, what can I expect to run?
Depending upon your location, you can expect about 3 amps peak for an average of 3 to 8 hours per day. This gives you 9-24 amp hours per day. If you have two lights that draw 1 amp each, you could run these lights for 4-12 hours per day, depending upon your location.

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I have a digital meter, what is the remote shunt?
The remote shunt is a precision resistor that creates a voltage drop in exact proportion to the amount of current passing through it. If you monitor the voltage drop across the shunt, you can tell how much current is flowing through it.

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What can you use a remote shunt for?
The remote shunt can be installed to monitor any current in the system, for example current to an inverter or from a charging source like a wind generator.

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I installed a remote shunt to monitor my usage, but when I turn on my lights, the load current displayed goes down. How can that be?
The shunt is probably installed where it is reading net battery current, not just load current. What is being displayed is the charging current minus the load current. This would cause the current reading to go down when a load is turned on.

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When a controller fails, what will happen?
It depends on whether the controller fails in the charging mode or non-charging mode. The function of the controller is to prevent overcharging, therefore a good design would have the controller fail most often in the non-charging mode, so overcharge does not occur.

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What is temperature compensation?
Temperature compensation is a feature of charge controllers that automatically changes the charging set points based on temperature. A good design will monitor battery temperature via a remote sensor and vary the set points accordingly. An ambient sensor is better than nothing but often the battery is at a different temperature. Monitoring the controller temperature is often a bad idea because the temperature of the controller will vary with amount of charge current, charge mode and location.

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How do I mount the modules?
Modules are mounted using the existing holes located on the bottom of the module frame. The holes will accept ¼ inch diameter fasteners. To prevent wire chafing, clearance between the modules and mounting surface should be minimal. When installing modules on a building, use stand off rack methods to discourage water or ice damming.

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What wire do I use to wire up the modules?
Module interconnects are made with AWG # 10-16 single conductor or dual conductor cable. The wire or cable should be specified for outdoor use and sunlight resistance; such as USE. Follow the NEC guidelines. Specifying the wire or cable running from the module strings to the combiner box will be similar, ranging from AWG # 10-16. (See wire sizing chart)
The wire or cable running from the combiner box to the power conditioning equipment depends on the distance between the two components, and other factors such as the sum of the Isc, temperature and environment. Typically, this wire ranges in size from AWG # 2-10.

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Do I need to use the jumpers included in the module junction box?
Not necessarily. Only terminals 1 and 5 are connected to the active material in the solar module. However, when connecting more than two wires in the module junction box, the jumpers are useful for activating terminals 2, 3, and 4

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Do I need blocking diodes within the array?
It is recommended that blocking diodes be used in conjunction with fuses to protect the array, and increase daily energy output where shadowing is an issue.Back to the Top

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What type of diodes should I use?
There are two different types of diodes that can be used; the Schottky diode and the silicon power diode. The advantage of the Schottky type is that there is a lower voltage drop (less power loss) across the diode when compared to silicon power diodes. The advantage of the silicon power diode is that they are more tolerant of transient lighting strikes (available in higher rated voltages) and may be more durable in areas that have a higher of lightning activity.

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Where do I put the diodes?
For single string systems, the diode in the charge controller will be adequate for blocking current flow from the battery into the solar array at night. For multiple strings of modules in parallel, diodes are normally located in a combiner box near the array.
Diodes require a heat sink and should be mounted in a combiner box (w/ heat sink) along with the array fuses. Do not put blocking diodes inside of the module junction box.

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What is a combiner box?
A combiner box is an electrical box where array fuses and diodes can be mounted. The combiner box includes a negative bus bar and compression terminals where the positive wires from the array strings can be connected to the array fuses and diodes. Individual array strings can be parallel wired in the combiner box as well. The combiner box is also a good place to mount lightning protection devices like SOVs (silicon oxide varisters)

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What fuses do I use?
For high voltage systems (>48 voc), Class R fuses can be used up to 250 volts. For high voltage systems between 48 VDC and 333 VDC, Class T fuses can be used. For low voltage systems (<48 v DC) AGC or ATC fuses can be used. However, ATC fuses have not been approved by the NEC for use in homes (Class R or T can also be used).

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Where do I put the fuses?
Fuses can be mounted inside of the module junction box for 12 v DC systems (ex. in a small in-line fuse holder) or they can be mounted inside a combiner box where strings of modules can be brought together and wired in parallel.

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Can I wire the modules in parallel or in series?
Yes, modules can be wired in parallel or in series. Open circuit voltage cannot exceed 600 volts so the maximum number of modules that can be wired in series is 20. Any number of strings of modules can be connected in parallel using the appropriate combiner boxes.

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What happens if I scratch the module?
Scratched modules, where the top layer of Tefzel has been cut open and active solar material is exposed to the air, can be repaired with common silicone. A small dab of common clear silicone is applied to the hole or scratch and lightly pushed into the hole. Because the silicone may inhibit sunlight from penetrating the module, do not spread an excess amount of silicone around the hole.

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Can I use solar modules in northern climates or in hazy areas of the South?
Yes, solar modules will absorb sunlight in any part of the world. As you get further from the equator (latitudes 40 to 60 degrees), there will be fewer hours of overhead sunlight, causing a reduction in the total amount of energy generated daily. Closer to the equator, some areas will have high humidity and a haze may form in the atmosphere. This may have a small effect on the intensity of the sunlight and the amount of amps produced by the solar modules. However, this effect does not have as noticeable an impact as the reduced number of hours of overhead sunlight seen in higher latitudes.

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Can I use UNI-SOLAR® amorphous-silicon modules with standard controllers and utility line-tie inverters?
Today's controllers work well with amorphous-silicon modules and arrays. When designing a utility line-tie system (a system with no batteries) you will need to know the nominal DC input range for the inverter, and the Vmax voltage of your module string must be within that range. Make sure to take into account the voltage drop due to high cell operating temperatures under typical conditions at your site.

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Do I need a charge controller?
If the maximum current (amps) is less than 1.5% of the overall battery capacity (measured in amp hours), a controller may not be necessary. For example, if you are charging a 12 volt 210 amp hour battery with a US-42 (max power current = 2.4 amps), a charge controller may not be necessary. Generally, using a controller in the system is encouraged as modern controllers can regulate the charge rate and optimize the charge on the battery. Additionally, many controllers are equipped with a blocking diode, system indicators or meters, and other features such as a low voltage disconnect for attached loads or automatic equalization charge function.

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How do I size a charge controller or voltage regulator?
The system voltage and the maximum amps produced by the solar array under short circuit conditions, determines the size of the controller. For example, a solar module may produce 3.88 amps at maximum power and 4.8 amps when measured under short circuit conditions. If you have a two-module system, you would have a maximum of 9.6 short circuit amps at 12 volts. A 10-amp controller that can be used with a 12-volt system would be appropriate for this system.

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