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Friday, 29 May 2015

Know your Solar Batteries

Lead-acid batteries are commonly used as an energy storage device for the stand-alone or off-grid solar PV system. Batteries are rated according to their voltage, Ampere-hour (Ah) storage capacity and their ability to deliver the stored energy over a given period of time called the C-rating. Figure 1 shows a 150 Ah, 12 V, Flooded Lead-Acid, Tall Tubular Solar battery.


Fig. 1: 150 Ah, 12 V, Flooded Lead-Acid, Tall Tubular Solar battery.

When a voltage, higher than the battery, is applied to the battery through an inverter or a charge controller of the solar PV system, a charging current will flow into the battery and will charge it. The flow of charging current is opposite to the flow of load current supplied by the battery. The rate of charge or current that will flow into the battery will depend on the difference between the battery voltage and the applied voltage (the voltage supplied by the solar PV system etc.).

Once the battery is fully charged, it is necessary to stop the charging otherwise it will damage the battery. Its here the charge controller comes into picture. A charge controller, usually used with the solar PV system, is thus essential to ensure that the battery is never overcharged.

Overall efficiency of charging and discharging of LA battery:
The temperature of operation and the rate of charge/discharge affect the performance of the battery. Because of the electrical resistance (internal resistance of the battery and the electrical path), some of the electrical power (I2R) and hence energy supposed to charge the battery is converted into heat. During the charging process, some of the hydrogen ions combine with free electrons and are converted into gaseous hydrogen. When hydrogen is lost during charging, energy is also lost. 

Thus, the charging efficiency is about 95%. 

Similarly, energy is also lost during discharging of battery and hence, 
the overall efficiency of charging and discharging of a lead-acid battery is nearly 90%.

Since the battery loss due to resistance is proportional to the square of the current, therefore 

high current charging or discharging will result in higher internal losses and reduced overall efficiency.

Effect of Temperature on Battery performance:

There is also an increase in the battery temperature during charging and discharging, particularly at high rates, due to higher I2R losses. If the battery remains too warm for a longer duration its life is reduced. Thus, charging and discharging rates should not exceed the specified values given by the manufacturer for a given type of battery. Increase in temperature accelerates the chemical reaction process in the battery. Thus, in high temperature the battery is liable to get self-discharged without any external load and hence has to be kept in a cool place with preferable temperature of 27 oC.

A fully charged 24 V battery will have a voltage of around 25.5 V. As the electrical load, connected to the battery through the inverter, draws the current, the level of charge in the battery is reduced and thus the voltage of the battery. The battery voltage will fall down quickly at first, but as the discharging continues, the rate of voltage drop slows down and will reach 24 V when the battery has drained to half of its rated charge capacity. As the battery approaches the fully discharged state, the voltage starts to fall at a greater pace again. When the electrical load is drawing a heavy current from the battery, the voltage will drop. The larger the battery, smaller is the voltage drop.

What is different in a Deep discharge flooded lead-acid battery?

Deep discharge flooded lead-acid battery, commonly used with solar PV system in India, uses antimony to strengthen the lead and can be used down to 20% of their initial capacity. The plates are thicker, with less area and hence are designed for sustained lower level currents. Flooded batteries need water topping at least twice a year. Although deep discharge batteries are capable of deep discharge, but their life depends on the depth of discharge incurred every time during the operation. Deep discharge i.e. 70 to 80% discharge of the rated charge leads to decrease in battery life.