Installation of a 12 V/24 V off-grid system with DC and AC loads

Solar systems ensure an individual energy supply almost everywhere. An off-grid system for independent power supply with solar energy usually consists of several components, depending on the area of application:

 

Components of a solar island system

1  One or more solar module(s)
2  A solar charge controller
3  One or more solar battery(ies)
4  Optionally: a 12 V or 24 V direct current consumer (e.g. LED lighting)
5  An inverter if alternating current (230 V) is required
6  Alternating current consumer (e.g. 230 V AC lamps, TV set, tools)

Design and dimensioning of an off-grid solar system

• The solar module (1) is connected to the solar controller (2). The two cables (±) used should be sufficiently sized to avoid line losses.
• The charge controller (2) is connected to the solar battery (3) and, if necessary, to the DC consumers (4).
• The charge controller should be dimensioned at least 10 % higher than the maximum current of the modules.
• The battery cable and the cables to the loads should always contain an appropriate fuse.
• The inverter (5) is always connected to the battery (3), never directly to the charge controller (2), as this can destroy the controller. This cable should also be fitted with a fuse.
• The 230 V AC consumers (6) can be connected to the inverter (5).
• The safety regulations for electrical installations must be observed during installation.

Please note: The solar batteries should be installed in a closed, dry room.

Solar controllers are used to supply solar energy to a suitable energy storage device in a controlled manner.

The sun's radiant energy is converted into electrical energy using a solar cell or solar module. The charge controller then ensures that this electrical energy is fed precisely and gently into a battery.



Please note: Use suitable connection and connection cables to avoid losses on the lines.

Rechargeable batteries are used to store electrical energy. There are many different technologies on the battery market for constructing such an energy storage device. However, the most characteristic features are always the nominal voltage (V) and the capacity (Ah).
 

Why does it make sense to store surplus electricity?

When the sun is shining intensively, excess energy is often generated which should be stored for later use.

Benefits:

• With solar power, you are independent of public power grids or where a reliable power supply cannot be guaranteed, e.g. when travelling in a motorhome or caravan, remote cabins and much more.
• Power storage units provide self-generated solar power exactly when it is needed, usually in the evening and at night
• Solar power is environmentally friendly due to the saving of fossil fuels

Which type of battery makes sense?

Lead-acid batteries are often used in the field of solar technology due to their favourable acquisition costs. Depending on the design, a distinction is made between classic, open lead-acid batteries, lead-gel batteries and lead-flow batteries or lead-AGM batteries. Many charge controllers are specially designed for this type of battery.

LiFePO₄ batteries are the latest generation of energy storage systems and are suitable for replacing existing lead-acid battery systems. When switching to a LiFePO₄ battery storage system, the charge controller settings must be checked and adjusted if necessary.

If you consider the costs over the entire period of use, a LiFePO₄ system is more favourable. This is why the trend is increasingly moving towards LiFePO₄ battery storage systems.

 

What is a solar module or solar cell?

A solar module consists of several interconnected solar cells and is used to convert the radiative energy of the sun into electrical energy. Direct voltage is provided on the terminals of the radiated solar module. If the module is operated in a closed circuit, direct current flows.
 

Brief description of the charging process

Current/voltage characteristic of a solar module

(1) Short-circuit point
The solar module’s terminals are shorted, this means that the electrical resistance between the terminals is extremely low. The highest possible short circuit current I_K of the solar module flows at this point.
(2) Maximum power point (MPP)
The solar module provides the highest possible power. This is the product of current I_MPP and voltage U_MPP in MPP.
(3) Idle point
At this point the solar module’s terminals are open, this means that the electrical resistance between the terminals is extremely high. The idle voltage of the solar module can be measured on the terminals.

The working point moves between point 1 and 3, depending on which electrical consumer is connected to the solar module.

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Charging with constant voltage (U-charging)

With constant voltage charging, the charging/cut-off voltage U_end is kept constant throughout the entire charging process. As a result, a higher current I_max flows at the beginning of the charging process than at the end.

Due to the decreasing current towards the end of the charging process, the battery is charged gently.

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Charging by pulse-width-modulation (PWM)

When charging according to the pulse-width-modulation principle at the beginning of the charging process the battery is charged with maximum current I_max. As soon as the cut-off voltage U_end is reached, the current flow is stopped in order to avoid overcharging.

After this first charging step the battery is not fully charge most of the time. A decrease of the battery voltage is to be expected. For this reason, the charging current restarts if the voltage U_start falls short of a certain value. This process is repeated until the battery is completely full. The charging current phases become shorter as the battery fills up.