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 now available in New Zealand from Panasonic, Tesla and other manufacturers. These are typically mounted on a wall. The Panasonic system has 8 kWh capacity, the Tesla 6.4 kWh.

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.

A new Australian/New Zealand standard is being developed for battery systems used with small-scale renewable energy systems. It looks at risks that may be associated with battery systems and specifies installation methods that eliminate or reduce risk.

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: 11 July 2017}