Neil Oliver, technical marketing manager at Accutronics, explains how rack mount industrial battery back-up systems for mains power can be modular
There are many applications that need to run 24/7 on mains power but need battery back-up if the mains fails. Traditional lead acid battery systems are both heavy and have low volumetric energy density. This is where lithium-ion battery technology can provide a solution.
It is common for these systems to be housed in 19” racks of the kind commonly found in IT environments.
In applications where a product must exist separately from mains electricity, rechargeable lithium-ion (Li-ion) is rapidly replacing virtually every battery chemistry, such as nickel cadmium (NiCad), nickel metal hydride (Ni-MH) and lead acid, as it has higher gravimetric and volumetric energy density, thus providing longer run-times and lower weight.
Unlike older lead acid battery technology, it can be used in any orientation and is easily recycled. Lithium-ion batteries contain no heavy metals and there are no restrictions on their disposal. As with many other energy storage products, the transportation of lithium-ion batteries is well regulated, and all lithium-ion batteries must pass electrical and mechanical testing to ensure they are safe to transport and correct packaging, labelling and documentation requirements must be met before these batteries can be shipped.
While this is easily manageable throughout the supply chain for lithium-ion batteries up to 100Wh, batteries that exceed 100Wh have additional restrictions placed upon them which mean they must be transported in compliance with the Class 9 Dangerous Goods Act - this can make their movement both difficult and expensive for all those involved in their shipping.
It is this >100Wh category that poses problems for OEMs when they require a reliable, high energy density portable source of power. In this instance, modularisation offers the high energy density and flexibility of smart lithium-ion battery technology without the limitations of dangerous goods shipping.
Modular back-up power
A system drawing its power from multiple sub-100Wh lithium-ion batteries controls both charge and discharge, and can be used to power any type of electronic device that is connected to it. Systems can be configured to provide between 126Wh and 1.42kWh either as an unregulated battery voltage of between 17.4V and 12.0V or as regulated voltage fixed anywhere between 5V and 24V.
Using multiple, hot swappable lithium-ion smart batteries offers a new way of storing electrical energy. A loading system can allow batteries to be easily removed for shipping without the need for tools. Once removed, the individual batteries are exempted from dangerous goods shipping regulations, which in turn makes them easier to transport by air, sea, road or rail.
A compact rechargeable lithium-ion battery can be specifically designed with energy density, functionality and safety in mind. Internal smart battery electronics allow a battery module to communicate with the system, ensuring safe and efficient operation is maintained at all times. During charge, the module communicates its charging requirements back to the modular controller so it is only fuelled when needed. During discharge however, the module provides information regarding its state so that this data can be summed with information from other batteries and provided to the host equipment upon request. The module slides into the modular system and can be held in position by twin spring latches.
Screw fixing can be achieved via mounting brackets, and special brackets for mounting in a 19” rack are available for units configured with eight batteries. The modular system offers multiple outputs and the main output connector can provide both unregulated and regulated outputs in one connector. The output of the highest voltage battery is available on the unregulated output - this is between 17.4V when fully charged and 12V when fully discharged. For devices that require a fixed output, this can be fixed at any potential between 5V and 24V. In addition, a regulated 5V 500mA output is available on the data connector - this can be used to allow software start-up of host equipment.
Internal charge circuitry
A modular system can be driven from an external 24V DC supply which is passed through to power host equipment with excess energy being used to charge the battery modules. Each battery module fitted into the system can communicate its charging requirements to the internal charge circuitry. The system provides charging current if available until each battery signals that it is full. This ensures safe and reliable operation and minimises energy consumption.
During discharge the system control module communicates with all batteries and manages their discharge into the host equipment. The fuel gauging electronics in each battery provide remaining capacity and run-time information which can be summed and provided to the host equipment via Inter-Integrated Circuit (I_C) bus, RS232 or USB. Additional information such as Time Remaining to Charge, Low Capacity Alarm, System Battery Capacity, Input Voltage and Output Voltage are also available over these interfaces. In addition, the system provides relay contacts that can also be used to signal system status if the host system cannot use an I_C, RS232 or USB interface. A modular power system can be configured for individual customers using standard parts assembled and programmed in line with customer demands.
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