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Create an electronic battery simulator

Posted: 06 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Lithium? batteries? battery monitoring systems? BMS? battery simulator?

Many new products utilise 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 multi-cell 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 need to isolate the various cell simulators so that they may be wired in series like the actual pack. This latter requirement suggests the use of transformers and for compactness, a switching-type architecture. One particular switching topology offers both isolation and 2-quadrant operation, namely the synchronous flyback converter.

In a simple flyback converter used as a voltage booster, a low-side switch operates at a duty cycle that sets the output current on an output section as shown in figure 1. In this idealized form, the rectifier diode conducts during the time that the switch is off, and allows output current to flow in the inductor as magnetic energy is transferred to the output capacitor in a unidirectional fashion. When regulating, the switch experiences a flyback peak voltage dV above the 12V supply, where dV is on the order of the supply voltage in most designs.

Figure 1: Basic Flyback Circuit Generating Boost of dV Volts.

To make the converter isolated, we replace the inductor with a transformer as in figure 2 so that the output appears on the secondary side. While the output is now isolated, the magnetic energy transfer is just the same as an inductor. The transformer turns-ratio N is selected to optimise the operation with the specific input and output voltages desired. Here again, the switch experiences a flyback peak voltage dV above the 12V supply. Notice that this circuit cannot prevent the output voltage from being forced above the set-point by an external current (this only supports one quadrant of operation).

Figure 2: Basic Isolated Flyback Circuit Generating dV/N Volts.

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