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Sunday, 26 April 2015

How to convert your existing battery inverter system into a solar PV /battery inverter system?

Renewable energy such as solar energy is fast becoming a reality in the Indian domestic scenario. People are keenly interested in installing the roof top solar PV system:

  • to reduce their dependence on the grid electricity, 
  • to reduce their bill on electricity and 
  • to contribute from their end towards the greener environment (In India more than 70% of the electricity is produced through thermal power plants). 
In such a situation, people who already have a battery and inverter system in their house and office are in a dilemma as they want to switch to the newer and emerging trend at the same time concerned towards the investment already made in the battery/ inverter system. The answer to them is the hybrid solar charge controllers available in the market.
How a solar charge controller works?

A solar charge controller can be connected to any existing home UPS /inverter to make the simple battery inverter system into a solar PV/ battery inverter system.  

When the battery or battery bank is fully charged and the solar power is still available, it automatically disconnects the mains supply from the inverter and the power to your electrical load is fed from the battery and solar PV system. The inverter is now working in the back-up mode and thereby forcing energy conservation; which is the much serious agenda now a days. This automatic operation of hybrid solar charge controller helps in utilizing the solar power to the most.

When the battery is discharged below a specific limit, the electrical load is automatically transferred to the mains supply i.e. grid supply, and the battery is now connected to the mains through the inverter for the charging process. The limit to which the battery energy is to be used is also user defined i.e. user can set the battery discharge limit to one of the available limits.

There are several hybrid solar charge controllers available in the Indian market, for example Solarcon by Su-Kam, which converts any existing domestic or commercial home UPS/ inverter into an intelligent solar PV system inverter.

Fig.1: Su-Kam Solarcon charge controller

Charging of the battery:

The charging of the battery is carried out by these controllers in 3 modes and the modes are:

1)      Boost mode,
2)      Absorption mode and
3)      Float mode.

Boost mode is the first stage charging where the battery is charged with maximum available charging current and that too at the highest voltage. This voltage is usually 14.2 V for Flat Plate Lead-acid battery and 14.6 V for a Tubular Lead-acid battery.

Once the battery is charged to its 80% charge level, the controller shifts to absorption mode. At this stage most controllers will maintain a steady voltage and a reduced current. This lower current charging increases the charge on the battery till it reaches 100% level. This stage takes comparatively more time as the charging current is low. The controller remains in this state for a pre-defined time after which the operation is shifted to float mode.

In the float mode, the charging voltage is reduced to the float level, about 13.7 V, and this level is maintained. The current is also reduced to a low value, called the "trickle". Charge is going into the battery all the time but only at a rate safe enough to ensure full state of charge.

Some charge controllers even have three additional stages of charging which enhances the performance of the battery, and hence the overall PV system. The six stages are:

  1. De-sulphation stage,
  2. Soft start,
  3. Bulk or Boost charging,
  4. Absorption charging,
  5. Float charging,  
  6. Equalization stage.

In the De-sulphation stage, a low value of current but at a high voltage is given to the battery so as to detach the sulphate crystals from the battery plates. De-sulphation helps the battery in producing optimum power.

During soft start, as the name suggests, charging is done at a low voltage and current until the battery attains 10-15% state of charge. Equalization happens once in a month or so as programmed by the manufacturer, during which a high voltage is applied to the battery which shakes the battery electrolyte. This enhances the charging efficiency.

How they improve the battery life:

Thus, these intelligent controller prevents overcharging and overheating of batteries and hence enhances the life to a greater extent. Different types of batteries such as lead-acid, tubular etc. are used with the inverter system and these controllers have the ability to easily adapt to the type of battery used and changes the charging profile and thereby enhancing the battery performance.

Several controllers come with the monitoring feature i.e. it continuously monitors and displays the generation through solar PV system. It also displays other essential parameters of the system such as battery voltage, PV voltage etc.

The Su-Kam Solar Charge Controllers  are available in 12 V, 24 V and 48 V option with the current range of 10 A to 45 A. Normally these controllers are equipped with short circuit protection. A 12/ 24 V 20 A charge controller cost about 3,300 INR whereas a 45 A is available at 4,400 INR (as on Dec 2016 in Bhopal.)