In this post we are going to discuss the factors affecting battery capacity in the off-grid /stand-alone Solar PV systems.

FACT:

The Battery bank is typically the second most expensive component in most photovoltaic systems , next to the PV Array.For a life cycle of twenty to thirty years of PV systems, the replacement and maintenance costs for batteries may be the highest.

For the above reasons, PV plant designers should have good understanding of the factors affecting battery capacity.

Now get this :

Capacity is a measure of a battery’s ability to store or deliver electrical energy, commonly expressed in units of Ah(ampere-hour).

The ampere-hour(Ah) is the amount of charge in the battery that will allow one ampere of current to flow for one hour

For example, a battery which delivers 10 amps for 20 hours is said to have delivered 200 ampere-hours.

Before we go any further:

Battery energy storage capacity can also be expressed in kilowatt-hours(kWh), which can be approximated by multiplying the rated capacity in ampere-hours by the nominal battery voltage and dividing the product by 1000.

For example :

Nominal Battery Voltage : 12V

Battery Capacity : 100 Ah

Energy Storage Capacity  = (Battery voltage * Battery capacity)/1000,

= (12*100)/1000

= 1.2 kilowatt-hours.

Factors Affecting Battery Capacity:

Lets dive in:

The capacity of a battery depends on:

Design factors  :

  • Quantity of active material.
  • Number , design and physical dimensions of the plate.
  • Electrolyte Specific gravity

 

Operational factors affecting battery capacity :

  • Discharge rate
  • Depth of discharge.
  • Cut-off voltage
  • Temperature
  • Age
  • Charge cycle

 

Let’s dig a little deeper:

1. Effects of Temperature on  Capacity:

Battery manufacturers usually rate capacity at a temperature of 25 degree Celsius.

Cold temperature decreases the total capacity available from the battery. Conversely, a battery operated at temperatures greater than 25 degree Celsius will deliver more than the rated capacity.

So let’s take a closer look:

In cold temperatures during discharge,  the does not penetrate as deeply into the active material on the plates, and the cut-off voltage is reached sooner.

PV system designers must be aware of the effects of temperature on battery capacity.

If the battery size in a PV system is calculated based on the expected capacity at 25 degree Celsius, the battery may be too small to provide the necessary autonomy period during cold temperatures.

As a result, the battery could be severely discharged and thus making the system not provide the required power to the electrical loads.

Therefore additional battery capacity must be installed in off-grid/stand-alone PV power systems to compensate for the expected reduction in capacity at lower temperatures.

As we said, battery operated at temperature grater than 25 degree Celsius will deliver more than the rated capacity. However, under no circumstances battery should be heated or operated at elevated temperatures to increase the available capacity.

Most battery manufacturers recommend their batteries be operated in temperature ranges of between 20 degree and 30 degree Celsius.

We apply the above data during our battery sizing calculations to insure that even during the coldest times, the PV systems has the required capacity to supply the loads.

When we calculate the amount of capacity we need to give the days of autonomy desired, the calculated value must be increased by an appropriate factor if the battery is operated below 20 deg C.

2. Cut off Voltage Affects Battery Capacity:

The cut off voltage is the lowest voltage which a battery system is allowed to reach in operation, and defines the battery capacity at a Specific discharge rate.

The Battery manufacturers often rate capacity to a specific cut off, or end of discharge voltage at a defined discharge rate.

For Lead-acid batteries, the cut off voltage used to rate capacity is generally 1.75 volts per cell or 10.5 volts for a nominal 12 volt battery. The cut off voltage for nickel-cadmium cell is typically 1.0 volt.

Note: 

The cut off voltage defined by a battery manufacturer most often represents a fully discharged battery.

Bottom line :

Batteries used in PV system should never allowed to reach low of a cut off voltage, and should be genearlly limited to no more than 80% depth of Discharge as determined by the low voltage disconnect point of the battery charge controller.

3. Effects of Self Discharge rate on Capacity

In open-circuit mode without any load or charging, a battery undergoes a natural reduction in state of charge, due to internal mechanisms and losses within a battery.

Different battery types have different rates, the most  significant factor being the active materials and grid alloying elements used in the design.

Higher temperatures result in results in higher discharge rates, particularly for lead-antimony  deigns.

4. Discharge Rate Affect Capacity

When a battery is discharged, the lead sulphate and water reaction products are formed. During fast discharges , the reaction are confined to the layers of the active material that are in immediate contact with free electrolyte, limiting the cell capacity.

Due to insufficient time for the electrolyte to diffuse into the pores of the plates and the sulphate molecules, forming at the surface clogs the pores, preventing full use of all active material.This effect is amplified as the rate of discharges increases.

During Fast discharges , the final cut off voltage is reached sooner and less total capacity is usable. So battery  is not a fixed value, but depends on the rate of discharge.

Discharging a battery delivers more capacity from the battery, while discharging it quickly delivers less total capacity of the battery.

And get this :

The above effect is not permanent. If a battery is fully discharged at a fast rate, it only delivers a fraction of its rated capacity. If it is then fully recharged and it is discharged at a slower rate, then more capacitywill be available.However, a few cycles may need to be performed on the battery to achieve stable capacities at the new rate of discharges.

Related Articles :

References : GSES Standalone Phtovoltaic Systems

I would be happy to hear  more about other factors affecting battery capacity. Do share your thoughts what other factors you consider in designing battery size in PV Systems in the comment section below.

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