Batteries in Series and in Parallel
Welcome to this information page of MPPT Solar. In this page we will illustrate in a schematic way the various types of batteries suitable for the solar / wind systems and will teach how to connect them together in series and in parallel, so as to have in output a higher capacity or a higher nominal voltage, according to the needs . So we will get a large energy storage system; energy generated by your wind turbine or solar panel system.
What kind of battery can we choose?
During the design phase of a wind turbine or PV off grid system it's important to choose the battery that will compose the battery bank for storage. On the market there are many types of batteries. Below we list the most popular:
• Lead-acid batteries: these are the batteries used to supply the electrical system of motorcycles, cars and trucks. Have a low cost, deliver very high currents, are reliable and of long life and work well even at low temperatures; counter are quite heavy, dangerous because the lead is a toxic metal, lose capacity as a result of mechanical stress and are not suitable for discharge too long due to the phenomenon of sulfation.
• Gel batteries: these batteries are lead-acid batteries in which the electrolyte is not liquid, but gelatinous. They are also called maintenance free batteries and they are suitable for very deep discharge cycles. Long-lasting up to three times longer than lead-acid batteries and they can withstand a high number of charge-discharge cycles. In contrast have a higher cost of lead-acid and if loaded wrong, they lose life very quickly.
• AGM batteries: compact batteries, free from short circuits and very resistant to mechanical stress. Can be mounted in any position, have an average life of 10 years, do not suffer the high temperatures and in case of breaking, the leakage of acid is limited, have high inrush currents, low self-discharge, greater speed in the load; against the cost little more than those to the gel (BATTERIES RECOMMENDED BY MPPT SOLAR).
How can I measure the charge state of a battery?
The most accurate is the measurement of the density of the electrolyte. If you do not have a battery hydrometer, thanks to the following table we can know roughly the percentage of battery charge, by measuring the voltage at the load terminals with a common digital voltmeter.
|Specific Gravity||Open Circuit Voltage||State of Charge|
|1,277||12,73 V||100 %|
|1,258||12,62 V||90 %|
|1,238||12,50 V||80 %|
|1,217||12,37 V||70 %|
|1,195||12,24 V||60 %|
|1,172||12,10 V||50 %|
|1,148||11,96 V||40 %|
|1,124||11,81 V||30 %|
|1,098||11,66 V||20 %|
|1,073||11,51 V||10 %|
How can I connect multiple batteries together?
First of all, it is essential that all batteries are the same (twins) and that they all have the same level of charging. Then it's important to use in the connection between the batteries, short electric cables, with equal length and with suitable section. Below you will find very simple images to easily understand the connections of the battery terminals.
The parallel connection of two same batteries allows to obtain an output with double capacity than the individual batteries, maintaining the same nominal voltage. Following this example where there are two 12V 200Ah connected in parallel, we will have an output voltage of 12V (volts) and a total capacity of 400Ah (amp hour). The capacity is the maximum amount of charge that can be stored. Bigger the battery's capacity, greater the amount of charge which can be stored. It is measured in Amperes per hour. In this case means that with a capacity of 400Ah, the battery bank can theoretically provide a current of 400A continues for one hour of time, or 200A for two hours continuously, or 100A for four hours, and so on ... Lower the maximum current drawn and longer the lifespan of the batteries.
The series connection of two equal batteries allows to obtain an output twice the rated voltage of individual batteries, maintaining the same capacity. Following this example in which there are two 12V 200Ah connected in series, we will have an output voltage value of 24V (volts) and a capacity of 200 Ah unchanged (Amp hour). In the wind turbines and photovoltaic panels, greater the DC voltage for charging the batteries, and smaller the energy losses along the cables. So for example, a 24V system is better than a 12V system.
Combining the parallel connection with the series connection will be a doubling of the nominal voltage, and capacity. Following this example we will have two blocks from 24V 200Ah joined in parallel, thus forming a total of 24V 400Ah. During the connection is important to pay attention to the polarity, use cables of suitable section and as short as possible. Smaller the length of the connections, lower the resistance that will be formed in the cables to the passage of current and therefore lower the energy loss in it.
In the planning phase of a stand-alone photovoltaic or wind system is essential to have a large and efficient storage system. To ensure proper charging of batteries we recommend high quality and efficiency solar charge controllers. Our wind solar charge controllers are designed to provide the best charging the batteries, ensuring maximum efficiency thanks to innovative MPPT technology. For those who want to transform DC voltage of battery into AC domestic voltage to supply electric devices is enought to buy a sine wave inverter. There are two types: Modified sine wave inverters (suitable for resistive loads, while with capacitive and inductive loads it can generate noise), pure sine wave inverter (suitable for all loads).
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