This article was written and contributed by Ken Chisholm, CEO, Teviot Technology Inc., maker of electric batteries and battery management systems.
Lithium nickel manganese cobalt oxide (NMC) batteries offer many benefits, but also present safety concerns. In particular, under certain circumstances, NMC cell overvoltage could lead to spontaneous combustion. To combat this concern, early electronic protection circuits were brought to market by companies like Benchmarq, Seiko, Ricoh and others. Called primary protectors, these rather crude resettable circuits included discharge current tripping, but not in a stable process. Electronic protection circuits, which can be applied to any chemistry, are now integrated in full battery management systems (BMSs). This article will look at the evolution of these systems.
Electronic Protection Circuits
Primary protectors were just the beginning. Soon, companies such as Motorola elected to implement secondary protectors. These are one shot circuits which, when activated, physically open the power path. These one shot circuits were activated by any cell voltage above a set threshold higher than the threshold for the resettable primary protector. For example, the secondary protector voltage threshold could be set at 4.5V and the primary protector threshold could be 4.35V, thus allowing for no crossover due to tolerances. The most common implementation uses an in-line, thermally activated fuse driven by heaters from the activation circuit (sometimes known as chemical fuses). Both classes of protector have time hysteresis to avoid unnecessary activation due to noise/spikes.
Electronic protection circuitry can also incorporate cell balancing. This prevents cell overvoltage due to unbalanced cells, as the charge circuit works on a common pack voltage. A single lower cell capacity (its voltage rising quicker than the average) could activate the primary protector and cause premature charge termination.
Circuit diagram with electronic protection functions circled in red.
Fuel Gauging (Gas Gauging)
Battery fuel gauging is used to determine a battery’s state-of-charge (SOC). Early in battery technology, fuel gauging was desirable and very cost conscious. Soon, the benefits of being able to dynamically determine battery capacity created a market where Application-Specific Standard Product (ASSP) microprocessors could more economically fulfill a fuel gauge function. The growing demand resulted in a new specification, System Management Bus (SMBus or SMB), created by a committee including Intel and Microsoft. Its purpose was to enable hardware and firmware platforms from different manufacturers to work seamlessly. Part of the specification was a complete data protocol derived from I²C, which implements a two wire bus with a maximum clock rate of 100kHz.
Today, other protocols are gaining popularity, such as CAN (Controller Area Network) Bus. The age of connectivity is here, and data can be downloaded dynamically to a local host and onward to a cloud based data collection system. Increasing sophistication of ASSPs and SOC microprocessor-based algorithms figure in this trend. The benefits of data collection are many, such as scheduled servicing, determining battery usage for warranties, and so on.
Most fuel gauging circuits employ coulometric counting to determine dynamic capacity—in other words, keeping a tally of current in and current out. It’s an easy principle, but not easy to do accurately and consistently. The holy grail is to enable life usage forecasting. Much research is being undertaken to achieve increasingly sophisticated forecasting, which is especially important for large battery installations.
Often, battery management systems now integrate the primary protector as well as fuel gauging. They have become a true system application capable of interactive control with the host. BMS circuitry is typically contained on one PCB assembly, but can be separated into a power switching module and a signal PCB.
As lithium and other advanced battery technologies continue to gain market share in increasingly electrified and battery-powered industries, such as mobility, automotive, power tools, industrial equipment, and telecommunications, battery management systems will continue to advance our ability to accurately and efficiently charge, measure, and use these batteries.