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 look at a battery management system designed for a medical application. Battery-powered medical devices have some unique requirements. Increasingly, medical devices are relying on battery power to increase patient mobility. Many of these devices have a requirement to provide 24 hours uninterrupted support. It's also crucial that these medical devices have accurate gauging for the amount of charge and run time they have left.
This battery management system comes from an OXYGENATOR, a machine that delivers oxygen to aid people with breathing difficulties. The machine is battery-driven and this is the control board for the machine, the functional elements are common to all battery management systems. Here is the protection circuit which drives the MOSFET switches [00:48] that protect the battery from overcharge or discharging below a safe level. In this case, the unit provides dual function of protection and fuel gauging.
On this board, we also have cell balancing capability which optimizes the life cycle of the battery by allowing cells to fully charge cycle but not allowing them to become stressed by overvoltage. The voltage supplied by the charger is distributed across the entire pack of cells so it's very important to ensure that the voltage is equally split amongst each cell otherwise one cell will get electrically stressed .
On this end of the device we find an industry-standard connector that connects to an output display so the battery's fuel capacity can be reported.
On this board we also have a charger which is fed by a DC source from the system. As the battery charges, it charges at a constant rate defined by the control chip here and the sense resistor which controls the current. As each cell reaches full charge, the voltage levels off and the current tapers off. When the current decays to one tenth of the charge current setting, then the charge is terminated.
As always, the charger is guarded by the protector which drives these MOSFETS, if the cell voltage increases above the maximum allowable voltage the protector will cut off the charger by opening the charge MOSFET.
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 and examples of large scale applications check out the other videos in the series.