Mar 10, 2008 03:32 pm
Re: 24 volt panels to charge 12 volt battries
Hi Steve,
I would agree with James that MPPT is the way to go if your charging a 12v battery with a 24v panel. Here is some more infomation about the different ratings that solar modules have. This info will be in our How To section shortly.
PV modules’ power output is rated in Watts. A Watt is an instantaneous measurement of electrical power. Power is a rate, much like miles-per-hour on your speedometer shows the rate at which your car is traveling. Power is not the same as energy. Electrical energy is expressed in Watt-hours (Wh) and is a quantity. Electrical energy is like the number of gallons of gas in your fuel tank. Your electric utility bill shows the number of kilowatt-hours (kWh, or thousands of Watt-hours) you use per month. So how does the wattage rating of a solar panel affect the amount of energy the panel produces? Simple-- with all else being constant, a higher-wattage solar panel will produce more energy over time than a lower-wattage panel. A 100-Watt module will produce twice as much energy as a 50-Watt module at the same location during the same period of time. It’s worth mentioning also that solar modules’ rated wattage is based on what the modules produce under laboratory-controlled conditions, called Standard Test Conditions (STC). STC allows solar panels to be compared to each other using the same metric. However, because these rated wattages represent ideal laboratory conditions, it is likely that the module will produce less wattage in actual use.
PV modules also have voltage and current ratings. The rated wattage of a panel is equal to its operating voltage multiplied by its operating current: Watts = Voltage x Amps. The amount of energy in Watt-hours that a panel will produce is a product of the wattage of the panel and the number of hours of full-intensity sunlight, or insolation, that it receives. For example, a solar panel that outputs 100 Watts for two hours will produce 200 Watt-hours of energy. The actual number of Watt-hours a panel produces will very likely be less than this theoretical value due to many factors that affect the efficiencies of the system components. There are standard factors used to correct for real-world energy losses, but to the present focus is the basic electrical principles.
Voltage
PV modules have three different voltage ratings that it’s handy to understand. The nominal voltage of a panel could also be called the “conversational voltage.” When we talk about the voltage of the panels and the other components of the system, we’ll most often use the nominal voltage. Nominal voltage actually refers to the voltage of the battery that the module is best suited to charge; the term is a “leftover” from the days when solar panels were used only to charge batteries. The actual voltage output of the panel changes as lighting and temperature conditions change, so there’s never one specific voltage at which the panel operates. Nominal voltage allows us, at a glance, to make sure the panel is compatible with a given system without having to look at the exact voltage. The second voltage rating is the maximum power voltage (Vmp). This is the highest voltage the panel can produce while connected to a system and operating at peak efficiency. The third voltage is open circuit voltage (Voc). This is the maximum voltage that the panel can produce when not connected to an electrical circuit or system. Voc can be measured with a meter directly contacting the panel’s terminals or the ends of its built-in cables.
Current
Panels also have two different current ratings: current at maximum power (Imp) and short circuit current (Isc), both listed in Amps. The maximum power current is similar to Vmp: it’s the maximum current available when the panel is operating at peak efficiency in a circuit. Similar to Voc, the short circuit current is the current measurement your meter would show when in contact with the positive and negative terminals of the panel while not connected to a system or load.
All these electrical characteristics are used to help determine the size of the PV system and components. These specifications can be found on the label on each solar module as well as on manufacturers’ specification sheets.
