Energy storage

There are many different types of batteries on the market. This is the case because there is no single type of battery that is superior in all applications. Each type of battery has uses different combinations of materials that have different advantages and disadvantages. All battery designs have to balance the following characteristics:

• Cycle life (life span)
• Cost
• Specific energy (energy capacity relative to size)
• Specific power (power capacity relative to size)
• Safety
• Performance under variable conditions
• Maintenance

A battery may have a long life span and be able to handle large loads, but it will have large and heavy or cost too much. A battery may be able to supply lots of power and be realtively small in size, but it wil not have a long lifespan. A battery must therefore be chosen based upon the particular application. One type of battery will be appropriate for a portable radio and another type will be appropriate for stoarge in a PV system. Small-scale PV systems with batteries typically rely upon lead acid batteries as they are a good compromise between all of these different factors.

Every type of battery has specific conditions for use that must be must be followed. It is very important to read the user manual and specifications sheet for any time of battery to make sure that it will be appropriate for its intended use. Failure to choose the right battery or failure to use it properly can damage equipment or cause injury.

Characteristics of batteries

The three most important characteristics when choosing a battery are:

Voltage

Batteries are rated with a nominal voltage (Vn). This voltage is called a nominal voltage as the voltage of batteries constantly varies depending on if they are being charged/discharged, their State of charge and the Temperature.

Storage capacity

The energy storage capacity of a battery will be rated in Amp-hours (Ah). or Watt-hours (Wh). 1 Amp-hour is equal to 1 amp of current transferred during the course of an hour, which is the standard rating method used for lead acid batteries. In the PV sector it is becoming more common to rate batteries in Watt-hours because it is easier to understand the total storage capacity of an energy storage system. To be able to compare the storage capacity of a system to the potential loads it is necessary to convert from Amp-hours to Watt-hours. The formula is simple:

Storage capacity = Ah × Vn

• Example 1: You have a 12V 120Ah battery. What is the capacity of the battery?

Storage capacity = 12V × 120Ah

Storage capacity = 1440Wh

• Example 2: You have two batteries connected together that are each 12V and 60Ah. What is the capacity of the battery bank?

Storage capacity = 12V × 60Ah × 2 batteries

Storage cpacity = 1440Wh

Different sizes and voltage of batteries can be conneted together to achieve the same amount of storage capacity.

Cycle life

Cycle life is the number of charge and discharge cycles that an energy storage device can provide before performance decreases to an extent that it cannot perform as required. The cycle life, or longevity, of all batteries depends upon the following factors:

1. Depth of discharge (DoD)
2. Temperature
3. Proper charging
4. Maintenance

Using batteries

The life of a battery is greatly affecting by how it is charged, discharged, maintained and under what conditions it is used.

State of charge (SoC)

The state of charge (SoC) of a battery is the total amount of its storage capacity that remains available for use. It is important for system users to understand how much energy remains for use to avoid damaging the battery by withdrawing too much energy. For example, if 25% of the capacity of a battery has been used, then its state of charge will be 75%. A fully AGM battery will have a voltage of around 12.8V and an empty battery will have a voltage of around 10.5V. If a battery is being discharged, its voltage will drop and if it is being charged its voltage will increase. In a PV system, which is constantly being charged and discharged, this can make it difficult to get a proper state of charge measurement from voltage. Nonetheless, many small-scale battery-based systems do not rely on a shunt and users commonly use the voltage of the battery bank as their only guide. To get an accurate measurement of state of charge there are three options:

State of charge vs. depth of discharge for a Trojan AGM battery .

1. The ideal method to measure the state of charge of a battery is to use a device called a shunt. A shunt is a device that can measure the amount of current flowing in and out of battery to estimate its available capacity.
2. Disconnecting all of the loads and charging sources and then waiting three hours to let the voltage stabilize.
3. The state of charge of a Flooded lead acid battery can be taken using a specialized device called a hydrometer that takes a mesurement of the electrolyte soluton inside the battery. These devices are not available everywhere and if used improperly can be hazardous as the electrolyte solution inside a battery is highly acidic.

Depth of discharge (DoD)

Depth of discharge is the inverse of state of charge - it is the amount of the storage capacity that has been removed. Depth of discharge is important when designing PV systems as it determines the size of the energy storage system and how many cycles it will last for. Lead acid batteries are not tolerant of regular deep discharges - a typical lead acid battery should not be discharged more than 50% regularly as it will greatly shorten its cycle life. Lithium ion batteries are far more tolerance of regular deep cycling. The actual usable energy in battery bank is calculated as follow:

Usable energy = Storage capacity × depth of discharge

• Example 1: You have a battery bank with 1440Wh of capacity, but the system is only designed to be used to a depth of discharge of 40%. How much usable energy is there?

Usable energy = 1440Wh × .4

Usable energy = 576Wh

Proper charging

All battery types have a maximum charging current and maximum charging voltage based upon the battery's temperature. A battery must be paired with a Charge controller that is designed for use with that type of battery. Each individual battery make and model will have its own specific characteristics that will have to be programmed into the charge controller.

Temperature

Regardless of the battery type, heat greatly affects performance and shortens the cycle life of a battery. For lead acid batteries it is estimated that for every 10°C increase in temperature above 25°C shortens the batteries life by half. This means that a lead acid battery operating for one month at 35°C is equivalent is equivalent in terms of battery life to operating the battery for two months at in battery life to two months at 25°C.

Safety

The most common lead acid batteries available are 12 volts, but the nominal rating means that the battery will not always have a voltage of 12 volts – Amp-hours The amp-hour rating of a battery is the total rated capacity of amps that a battery is capable of supplying over time. Ah = amps x hours. Amp-hour ratings are only useful when comparing batteries that have the same nominal voltage.