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Monday, 7 December 2015

C-rating and Efficiency of a Battery

Batteries are the key elements used in Standalone Roof top solar PV systems and hence one should know a bit about these storage devices. Of the various types of batteries, based on the electrolyte material, the most commonly used battery type is the Lead-Acid type particularly in solar PV applications.

How batteries are rated?

Batteries are rated according to their:
1.   Voltage,
2.   Storage capacity, and
3.   Ability to deliver the stored energy over a given time period.

What is C-rating?

Energy storage capacity is given in Ampere-Hour (Ah) at some nominal voltage and at some specified discharge rate. The storage capacity is never fixed and depends mainly on how fast the energy is extracted from the battery. The manufacturers usually specify the ‘Ah’ capacity at a discharge rate that would drain the battery completely over a specified period of time at a specified temperature. The ability to deliver the stored energy over a given time period is called the ‘C’ rating. Batteries are available in market with different ‘C’ ratings such as C 5, C 10, C 20, and C 100. The batteries usually used for solar application in India are the C 10 type.

Fig.1: C-10 rating battery used in Solar PV application.

Let’s go through an example….

For example, a fully charged 12 V battery that is specified to have a 10 hour, 100 Ah capacity could deliver 10 A for 10 hours, after which the battery will get fully discharged. This ‘Ah’ specification is known as ‘C 10’ rate, where ‘C’ refers to the Ah capacity and the 10 is hours it would take to completely deplete.

Dependency of Storage Capacity.

As mentioned above the ‘Ah’ capacity or storage capacity of a battery is very much dependent on the discharge rate and the temperature. More rapid draining of a battery i.e. higher discharge rate results in lower ‘Ah’ capacity and vice-versa. In simple words, the above 100 Ah, C 10 battery would not last for 1 hour if you drain at a 100 A discharge rate and on the other hand it would last more than 100 hours if the rate is 1 A.

The battery capacity decreases significantly in cold conditions. There is an apparent improvement in battery capacity with the increase in mercury; but this does not mean that batteries are safe in hot climates. Rather the battery life is shortened by approximately 50% for every 10oC rise above the optimum operating temperature of the battery normally 27oC.   

Since the rated capacity of the battery is specified at a temperature and discharge rate (specified by the manufacturer), one has to adjust the battery capacity according to the prevailing temperature at the site and the discharge period. 

Does battery connection affect the ‘Ah’ capacity?

The ‘Ah’ capacity of the battery also depends on the connection of the battery. For example, two batteries connected in series will have the same current and hence the ‘Ah’ capacity remains the same. If the same batteries are connected in parallel, their current adds up and so is the ‘Ah’ capacity. But the energy stored in a battery bank, whether the batteries are connected in series or parallel, remains the same. Batteries when connected in series have higher voltage and lower current and hence the voltage drop and power loss are lesser. When batteries are connected in parallel, the weakest battery will bring down the voltage of the entire arrangement. Similarly, in series connected batteries, failure of one battery will completely shut down the system.

Why the manufacturer specifies both ‘Ah’ efficiency and ‘Wh’ efficiency?

Since the voltage varies throughout the discharge period, one cannot calculate the energy delivered by the battery during its discharge by simply multiplying 12 V x 10 A x 10 h = 1200 Wh. Therefore, battery storage capacity is mentioned in ‘Ah’ rather than ‘Wh’.

Manufacturer specifies the efficiency of their battery in terms of ‘Wh’ efficiency and ‘Ah’ efficiency. For example, the ‘Ah’ efficiency and ‘Wh’ efficiency of Luminous make Flooded Lead-Acid tall tubular batteries are greater than 90% and 80% respectively as claimed by the manufacturer.

So let’s have a look into these two efficiencies.

The amount of electrical energy stored in a battery is measured in ‘Wh’  

Energy efficiency in Wh 
= energy discharged in Wh / energy required in Wh to completely recharge

‘Ah’ efficiency 
= 'Ah' discharged / 'Ah' required for complete recharge

The energy or ‘Wh’ efficiency of a battery is always less than the ‘Ah’ efficiency because battery discharges at a lower voltage than they charge at.

A battery should never be discharged more than 80%, even under worst conditions. The more you extract from a battery every day, the more it will wear out. The wear out rate depends on the type of the battery and its cycle life. The life cycle, as mentioned in the Luminous battery catalogue, with 80% Depth of Discharge (DoD) is 1500 cycles, 50% DoD is 3000 cycles, and 20% DoD is 5000 cycles.   With this one can understand what the C rating, ‘Ah’ capacity and ‘Wh’ capacity is and how to interpret the nameplate rating of a battery.