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"AC Batteries"

Characteristics

Some specialist battery packs have built in electronics such as inverters which generate the AC power from the battery's DC output voltage to provide portable mains power remote from the utility supply grid or emergency mains back up power.

Battery characteristics / requirements

  • Short term power supply

Many emergency power systems are only required to provide power long enough to enable a graceful shut down of an application to prevent loss of data or loss of continued control of a critical industrial process or medical procedure. Where a vital process must be maintained it may only be necessary for the battery to power the critical application until an alternative energy supply (standby generator) can be brought on line. For such critical applications a dedicated Uninterruptible Power Supply (UPS) may be provided. This usually contains a battery with sufficient capacity to power only the critical application for the expected duration of any outage and a built in inverter to provide the AC power to the load. The rest of the establishment will have to live with the power outage.

These applications require deep discharge batteries but they are rarely called into use and are not subject to repetitive duty cycles.

  • Long term power supply

Deep discharge batteries are also required for load levelling applications to provide power on demand from intermittent supplies such as solar, wind and wave power applications. These applications require robust, long life batteries able to tolerate constant charge-discharge cycling.

For very high power applications such as emergency back ups for the national electrical power grid system or temporary storage of surplus low cost energy, very large capacity batteries are required. While enormous lead acid batteries with capacities up to 40 Megawatt hours or more have been constructed, several unusual systems for storing such large energies have also been implemented. See Alternative Energy Storage Methods.

In these high power applications the inverter will be a separate unit or the system may generate AC power directly as is the case with pumped storage systems.

Inverter characteristics / requirements

No loss of power to the load

High quality clean sinewave output.

Frequency stabilisation

High efficiency / low loss

Possible integration with battery float or trickle charger

Advantages

Fast acting - Much quicker than rotary or other standby AC generators.

Shortcomings

Limited capacity compared with motor driven rotary generators.

Applications

Uninterruptible power supplies. See UPS

Emergency power

Standby power sources

Load levelling

Costs

Very wide range of costs

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See also How to Specify Batteries - Timing Issues and Depth of Discharge

 

 

 

 

 

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