Welcome to Engineering.com's series on the design of battery packs and battery management systems.
In this video, we're going to take a closer look at battery management systems in large scale applications, specifically, batteries integrated into the electrical grid and batteries designed for electro-motive applications.
Battery packs using Li-ion technology come in various sizes from gigantic to very tiny. At the huge end, we have batteries meant to provide power to the electrical grid. In a typical operating day, there will be times when the electrical grid is running at a peak and other times when it there's less demand for electricity. “Sustaining Grid Systems” use batteries to help meet the demand for electricity during peak times and then re-charge these batteries using the extra generating capacity available during off-peak times.
The battery packs used in a Sustained Grid System can be as large as 6 feet high, 8 feet wide and 4 feet deep with a voltage capacity of 600 Volts at 800 Amps and produce 250 kilowatts of power. Now that's a BIG battery!
If we take a step down in size, we have batteries designed for motive-power like this unit. This is a high-power battery pack which consists of 8 series-connected Lithium Polymer, high-performance, high-rate cells. This pack is designed to be a replacement for a 24-Volt lead-acid battery for an electromotive application like an electric motor for a vehicle so it's subject to high pulse currents and high continuous loading. It has a capacity of about 24 Volts, 120 Amps, 746 Watts, or approximately 1 horsepower.
The size and number of the cells in a given battery are chosen to deliver the power required by the application. In this battery, the cells are parallel plates as opposed to cylindrical batteries where positive and negative electrodes are rolled up together like a Swiss roll. In this case, they're in a flat format and the electrolyte is a gel encased in an aluminum pouch. The construction of this type of pack is designed for expansion as the cells heat up and the design of the case for this battery has to accommodate this expansion.
The power board also affects the dimensions of the battery. It requires a finite area to dissipate the heat from the current transfer in the MOSFETS in the protector circuit.
In terms of the operation of the battery management system, the components are the same as in the smaller circuit boards we've seen. We have the control combined protector and fuel gauge. Then we have the cell balancer, which is especially important in this case since the pack consists of 8 cells and the more cells you have connected the more important it is to keep them balanced.
On the other side of the pack we see the battery protection MOSFETS. We have a set of Charge path MOSFETS connected in parallel and set of Discharge MOSFETS connected in parallel, they essentially function like one complete charge MOSFET and one complete Discharge MOSFET and provide the protection function. So no matter how large your application is the battery management system provides the same basic functions; protection, fuel gauging and cell balancing.
For more information on the individual components that make up a battery management system and design challenges facing battery pack designers like space, heat dissipation and usability requirements check out the other videos in the series.