Connecting Batteries in Series and Parallel
Welcome to this informative article.
In this page we will illustrate the different types of batteries used into most wind and solar power systems and we will teach you how to wire them together in series and in parallel, in order to get a greater capacity or a higher rated voltage, depending on your needs.
In this way we will get an excellent energy storage system; energy generated by our MPPTSOLAR plant.
Are you ready? Let's begin!
Choosing the right battery type
During the design phase of an off-grid solar power system, it is important to choose the right batteries that will form the battery bank. There are many types of batteries on the market. Below we list the most common:
• Lead-acid batteries
These are the batteries used to power the electrical system of motorcycles, cars and trucks. They are low cost, deliver very high currents, are reliable and work well even at low temperatures. On the other hand, they are quite heavy, dangerous as lead is a toxic metal, they lose capacity due to mechanical stress and are not suitable for too long discharges due to the sulfation process.
• Gel batteries
They are lead-acid batteries in which the electrolyte is not liquid but gelatinous. They are also called maintenance-free batteries and have a better depth of discharge range. They also last three times longer than lead-acid batteries and withstand a good number of charge-discharge cycles. On the other hand, they are more expensive than lead-acid batteries and if they are not loaded correctly they lose their life expectancy very quickly.
• AGM batteries
They are lead batteries in which the electrolyte is absorbed by a spongy mass of fiberglass. They are compact batteries, immune to short circuits and very resistant to mechanical stress. They can be mounted in any position, have an average life of 10 years, perform well also under high temperatures and in case of breakage of the casing, the leakage of acid is limited. They have high inrush currents and low self-discharge. On the other hand, AGM batteries cost more than the gel ones and it is not recommended to discharge them over 50%.
• LiFePO4 batteries
LiFePO4 stands for Lithium Iron Phosphate. These batteries have no lead or corrosive liquid. They are therefore very light, compact, environmentally friendly and can be mounted in any position without risk. Even if they are 100% discharged, they are not damaged. At the same size, they store and offer more energy than lead batteries. In addition, they boast charge-discharge cycles unreachable by lead batteries. LiFePO4 batteries can be charged in a very short time and are normally equipped with an internal BMS which guarantees maximum safety and the correct cell balancing. On the other hand, they still cost much more than AGM batteries.
How to measure the state of charge of a battery?
The most accurate method consists of measuring the density of the electrolyte. If you do not have a density meter, thanks to the following table you will be able to know the state of charge of lead batteries by measuring the open circuit voltage at their terminals with a common digital multimeter.
|Density meter value||Voltage at the terminals||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 to connect multiple batteries together?
First of all, it is essential that all batteries involved are identical and have the same state of charge. Secondly, it is important to use short electrical cables, of the same length and with suitable cross-section for the connection of the batteries. Below you will find some very clear images in order to easily understand the battery connections.
The parallel connection of two identical batteries allows to get twice the capacity of the individual batteries, keeping the same rated voltage.
Following this example where there are two 12V 200Ah batteries connected in parallel, we will therefore have a voltage of 12V (Volts) and a total capacity of 400Ah (Ampere hour).
Capacity identifies the maximum amount of charge that can be stored. The larger the capacity, the greater the amount of charge that can be stored.
In this case it means that with a capacity of 400Ah, the battery bank can theoretically deliver a current of 400A for a whole hour, or 200A for two continuous hours, or 100A for four hours, and so on... The less current is delivered by a lead battery, the longer the battery lasts.
The series connection of two identical batteries allows to get twice the rated voltage of the individual batteries, keeping the same capacity.
Following this example where there are two 12V 200Ah batteries connected in series, we will have a total voltage of 24V (Volts) and an unchanged capacity of 200Ah (Ampere hour).
In off-grid wind and solar power systems, the greater the direct voltage for charging the batteries, the lesser energy is lost along the cables. So for example, a 24V system is better than a 12V system.
Combining the parallel connection with series connection we will double the nominal voltage and the capacity.
Following this example we will have two 24V 200Ah blocks wired in parallel, thus forming overall a 24V 400Ah battery bank.
During the connection it is important to pay attention to the polarity, use cables as short as possible and with an appropriate section. The shorter the length of the connections, the lower the resistance that will be formed in the cables when the current flow and therefore the lesser the energy loss will be.
When designing an off-grid solar power system, it is essential to have a large and efficient storage system. To ensure a correct battery charging, we recommend relying on high quality and efficient charge controllers. The MPPTSOLAR charge controllers are designed to guarantee the best charging process for any type of battery (LiFePO4 included), exploiting all the energy produced by solar panels thanks to MPPT technology.
For those who want to convert the direct voltage of the battery into alternating voltage for domestic use, a sine wave inverter is sufficient to power any appliance. There are two types: the modified sine wave inverter (suitable for resistive and capacitive loads; it can produce noise with inductive loads) and the pure sine wave inverter (suitable for all loads).
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