So Why Have a Battery Simulator?
Many new products are incorporating Lithium-based batteries for the high performance and light weight characteristics they offer. In fact, many of the more sophisticated applications involve connecting a multitude of cells to achieve the desired pack working voltage, often hundreds of volts. Since Lithium cells are prone to ill effects if allowed to over charge or over discharge, these series packs incorporate monitoring systems that keep tabs on each cell potential to avoid such problems. The process of developing these multicell battery monitoring systems (BMS) requires a convenient means of stimulating the circuitry to test the effectiveness of the control and protection algorithms. Ideally, the stimulus would be actual cells, but then to vary the state-of-charge (SOC) to trigger different functional actions in the BMS becomes a slow and cumbersome matter.
Multiple lab power supplies are frequently used but this is a very expensive solution. So for simple functional tests, resistor strings are often just biased to provide a rudimentary cell simulation. The resistor strings have significant limitations since they present a fairly high source-resistance, and thus introduce system artifacts that are not representative of actual cells. Even with dedicated supplies though, if the system under test involves active cell balancing, then the supplies must accommodate virtual charging current (i.e. current reversal). The bottom line is that it is desirable to have a means of having a multiplicity of compact cell simulators to provide easy lab testing of the BMS functionality. Another useful aspect of having a battery simulator is that such an item is readily transportable by air freight for operations away from the lab, whereas an actual Lithium cell pack usually has to be shipped by surface vessel.
Choosing a Practical Circuit
The primary feature that we need is low source impedance and 2-quadrant operation (positive voltage but bidirectional current, so we can simulate both discharge and charge directions). We also