Energy

Battery banks help ensure reliability of power supply for stand-alone renewable electricity generation systems, especially wind or photovoltaic systems which can have periods of low generation.

On this page:

• Battery storage systems
• Charging batteries
• Battery options
• Installation requirements
• Maintenance and disposal

Battery storage systems

Battery storage systems require:

• batteries
• a connection to DC electrical cables for input and output
• a regulator or controller on the input side to ensure the batteries are not overcharged
• an associated dummy load for redirecting excess power if the batteries are fully charged (this is usually an electrical element which can get very hot)
• an inverter.

The battery storage capacity should be able to maintain a charge of at least 50%. Rechargeable, deep cycle batteries, which are designed to be discharged to 50% or less without damage, will provide the optimum battery life. A battery bank will usually cost over $10,000. Batteries require replacement every 5–12 years, depending on the type and how they are used.

Charging batteries

When charging batteries, the process requires:

• manual or automatic monitoring of charge level
• protection against overcharging
• supplementary charging when battery charge is low or load is high, e.g. a generator starts automatically
• charge equalisation – often done by a diesel generator
• consideration of the average daily depth of discharge (DOD) – it is lower with a larger capacity battery bank, which gives longer battery life.

A supplementary battery charger may be required.

Battery options

Most renewable energy system storage batteries are:

• composed of a number of cells
• supplied as any of 12 V, 24 V, 48 V or 120 V
• built as a block of many individual 2 V cells – though occasionally from mono-blocks of 6 V or 12 V batteries.

Lead acid batteries are most commonly used in renewable electricity systems. They are wet batteries, containing sulphuric acid and can be valve-regulated (sealed or gel) batteries. These are very different from car or truck batteries.

Nickel-cadmium batteries are less common and much more expensive, but last longer.

Rechargeable lithium-ion battery systems are available in New Zealand from Panasonic, Tesla and other manufacturers. These may be wall mounted or sit on a floor depending on the model or system. Capacity of each unit ranges from around 4.5kWh to 13.5kWh.

An Australian battery system based on hydrogen is expected to become commercially available in 2021. Developed by researchers at the University of New South Wales, and being produced under the name LAVO, it uses compressed hydrogen as the main energy storage medium.

The inventors say the device (1.7 x 1.2 m, 196 kg) offers three times the amount of energy storage compared to other batteries of similar size. Efficiency is claimed at around 50%, meaning that half the solar energy that goes in will be stored, and the inventors say the unit is good for 20,000 cycles. The launch price will reportedly be around AU$35,000.

Installation requirements

• The battery room storage for lead acid batteries must be well ventilated to the outside (using either passive or mechanical ventilation) because batteries contain a flammable and corrosive mixture of oxygen and hydrogen during the last stage of charging. The amount of ventilation needed depends on the size of the battery pack.
• Do not locate electronic components above the batteries as they can corrode or cause an explosion.
• Mount batteries on stands or on thermal insulation if they are floor-mounted – they can react to cold surfaces such as cold floors, causing the electrolytes to stratify, which can reduce battery life and performance.
• Batteries must be able to be readily disconnected for maintenance, repair or replacement by including an isolating switch or fuse between the batteries and the electrical system.
• Locate batteries out of direct sunlight as sunlight can buckle the electrodes.
• Specify drip trays to capture any spilled acid.
• Provide space above the batteries for a hydrometer to check their charge levels.
• Install a cover or shroud over the batteries to prevent accidental shorting.
• Provide an adjacent space to store water and sodium bicarbonate in the event of an acid spill.

Maintenance and disposal

• Maintain battery life and performance by keeping the terminals clean and tight.
• Ensure the electrolyte is above minimum levels using only distilled water.
• Protect against acid burns (for wet lead-acid batteries) by wearing protective clothing and eye protection
• Dilute spilt acid with water and neutralise with sodium bicarbonate.

Disposal of batteries should be at recycling stations, not in landfills (because they contain lead and acid).

Updated: 09 February 2021