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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?

A synchronous version is created when the rectifier is replaced by another switch as shown in figure 3. This both improves the efficiency, since the switch will dissipate less power than a forward-conducting diode, and creates a second quadrant of operation because now the circuit has symmetry. This circuit can accept reverse current in the secondary that induces primary winding fly-back current back into the main supply, thus the output will hold its set point even with a forced reversed output current. We need to be cognizant of the possibility that the source supply for the circuit could itself experience a reversed current if the simulated cell is being heavily 'charged' (current flowing into the positive output voltage).

Since the outputs are all isolated, the source power can be shared amongst any number of circuits so that a single bulk supply can conveniently provide power to an entire array. Such an array connection also consolidates the parasitic circuit losses so that it becomes unlikely that the source supply experiences a current reversal in normal usage (i.e. as long as net 'charging' power

Figure 3: Synchronous Isolated Flyback Circuit Supports Bi-Directional Current Flow.

Looking at the details
One particularly well suited IC for this converter function is the Linear Technology LT3837. The typical application for this circuit is to provide low, battery-like voltages at several amps from higher voltage bulk supply rails. The only difference for the cell simulator function is that we would like an adjustable output voltage. Since turnkey high-power bulk supplies are available at 12V, we can optimise the design to use this as a source. Given that the range of Lithium cell chemistries is from just under 2V to just over 4V, we can establish a corresponding tuning range that provides versatile usage and the ability to simulate a wide range of SOC states.

To provide voltage adjustment, the feedback network supports an op amp control signal such that zero volts represents about 4.2V output and 3V commands about 1.9V out. For good user control, each cell circuit is configured to have a 'vernier' fine tune, and then an array set is group controlled with a coarse and fine adjust (master adjustment signal MCTL can be connected to several converter sections). For the values shown, the output voltage group coarse is about 0.9V, the group fine is about 0.15V, and the cell verniers are about 0.1V, so collectively the maximum desired range is achieved (in order to provide vernier controls, ability to cross-control cells to the full limits was sacrificed). All the control circuitry is powered by 3.3V derived from the 12V bulk supply. For computerized voltage control, the op amp signals can be replaced with DACs such as the 16-channel LTC2668.

Q101 and T100 are the main flyback elements, with Q102 being the synchronous rectifier. For fast and isolated control of Q102, the gate is driven by T101 via current buffers Q103 and Q104. Feedback is scaled from an auxiliary winding in T100. A 10m? series resistor is included at the output so that current sense measurements are possible by taking Kelvin connections to a voltmeter (by use of signals I+ and I-). The total output impedance of the circuit is about 25m? and provides a solid 6A capability. Static losses are about 1 Watt per cell section, so with an array of 24 cells, the likelihood of a 12V supply reversal is minimal and the power level scales well for use with an off-the-shelf 12V/300W supply like the TDK-Lambda SWS300-12.

Conclusion
Building a battery simulator is a practical solution to providing a high density and easily transported BMS development tool. A 24-cell simulator can be packaged in a 2RU rack-mountable chassis complete with a 12V bulk supply, and provide precisely adjustable voltages in the 1.9V to 4.2V range with 6A capability.

About the author
Jon Munson contributed this article.


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