Energy Storage

Overview
Energy storage can mean many different things to many different people. The phrase is probably most commonly associated with Hybrid Electric vehicles (HEVs) and Electric vehicles in general. That particular interpretation of energy storage concerns itself primarily with mobile storage technologies. Away from the automotive world the primary interest in energy storage comes under the loose heading of energy security. This can range from a desire to manage renewable energy fluctuations, to reduce oil imports, to manage grid systems peak demand, or to reducing power outages due to generator failure. There are very many different types of energy storage technologies in use or under development today such as fuel cells, flywheels, compressed air, etc. Here we focus on storing energy in banks of Li-ion cells and highlight those products which can deliver safe and efficient system operation over time and temperature whether the end application is grid support or the storage of wind/solar generated energy in an industrial or domestic setting.

Battery Monitoring
The Li-ion cells are generally grouped in modules or packs usually containing between 6 to 12 cells. These modules are then bolted together in series (a battery string) to produce the desired overall DC stack voltage. The stack voltage is generally chosen to ease the job of the DC/AC inverter. For example, to generate a 110V AC output, the typical stack voltage might be 170V DC. Assuming a Li-ion cell voltage of 3.6V this might lead to 48 cells in a series string, arranged in eight packs of 6 cells. To increase the energy storage capacity of the battery bank additional strings are added in parallel. It would not be unusual to have over 30 strings in parallel giving a total of 1,440 Li-ion cells in the array. Two strings are shown here.

Assigning dedicated monitoring components to each string eases individual string removal and permits tighter control over the Li-ion cells. One area which is critical to longevity of the cells is accurate current monitoring. Highly specialized cell monitor products such as the AD7280A and the AD8280 provide full monitoring and backup capabilities for each Li-ion cell in the string. Each products can monitor up to 6 Li-ion cells with the AD7280A containing a precision 12-bit ADC with a 1uS per-channel conversion time while the AD8280 is comparator-based and reports out-of-bound conditions for monitored cell voltages and temperatures. Both devices offer the capability of being daisy-chained together when monitoring a large number of serial cells without the need for isolation barriers between the individual devices.

The individual string currents are monitored by the ADuC7039. This device is a highly sophisticated system-on-a-chip with the capability to measure low-side current, stack voltage and single point temperature. This ARM-7 based device is also used to communicate with its associated series-connected string of Li-ion cell monitors, running cell balancing algorithms for that particular string and communicating with the master controller.

Control / Processing
In terms of control, ADI's latest low power fixed point Blackfin® DSP controller family is suitable for the control and communications functions required in this application. The BF50x family delivers 400MHz of processing performance in a dual multiply and accumulate architecture equivalent to 800 MegaMACS of performance.

Communications
The BF50x communicates with the various system blocks via ADI's proprietary iCoupler® isolators to allow the entire front end system to float and to electrically isolate the back-end from high voltages. When communicating at high data rates or over relatively long distances found within a typical battery stack, differential data transmission offers superior performance to single-ended transmission. Popular protocols for this communication task are RS-485 and CAN. Analog Devices offers a wide range of iCoupler®-based isolated RS-485/RS-422 transceivers to suit this application with the ADM248xE and ADM258xE families which include integrated isolated DC/DC converters. More recently, Analog Devices has released the first of a family of isolated CAN transceivers, the ADM3052/ADM3053, with and without an integrated isolated DC/DC converter. For proprietary protocols there are also numerous standard iCouplers available such as the ADuM140x family, which are quad channel isolators.

Measurement / Sensing
Typical dc current sensors used in battery stack applications include Hall Effect (HE) sensors and shunt resistors. Devices like the AD8212 are high side current monitors producing an analog output proportional to the current flow through a shunt while a device like the AD7400 is a sigma-delta modulator producing an output 1's density stream proportional to current flow, also through a shunt. This latter device incorporates its own isolation barrier inside the package to provide galvanic isolation between the sensed current and the digital output. Each sensor type has its own advantages and disadvantages but all require precision, low noise, low power op amps like ADI's OPX177 family to complete the signal conditioning demanded by the different current sensor signals.