Battery stack voltage monitor development (Reference 1) is aided by a floating, variable potential battery simulator. This capability permits monitor accuracy verification over a wide range of battery voltage. The floating battery simulator is substituted for a cell in the stack and any desired voltage directly dialed out.
Figure 1’s circuit is simply a battery powered follower (A1) with current boosted (A2) output. The LT1021 reference and high resolution potentiometric divider specified permits accurate output setting within 1 mV. The composite amplifier unloads the divider and drives a 680 µF capacitor to approximate a battery. Diodes preclude reverse biasing the output capacitor during supply sequencing and the 1µF -150k combination provides stable loop compensation.
 |
|
| Figure 1. | Battery simulator has floating output settable within 1 mV. A1 unloads Kelvin-Varley divider; A2 buffers capacitive load. |
Figure 2 depicts loop response to an input step; no overshoot or untoward dynamics occur despite A2’s huge capacitive load. The battery monitor determines battery voltage by injecting current into the battery and measuring resultant clamp voltage (again, see Reference 1). Figure 3 shows battery simulator response (trace B) to trace A’s monitor current pulse into the output. Closed loop control and the 680 µF capacitor limit simulator output excursion within 30 µV. This error is so small that noise averaging techniques and a high gain oscilloscope preamplifier are required to resolve it.
 |
|
| Figure 2. | 150k-1µF compensation network provides clean response despite 680 µF output capacitor. |
 |
|
| Figure 3. | Battery simulator output (trace B) responds to trace A’s monitor current pulse. Closed loop control and 680 µF capacitor maintain simulator output within 30 µV. Noise averaged, 50 µV/ division sensitivity is required to resolve response. |
Reference
- Williams, Jim, and Thoren, Mark, “Developments in battery stack voltage measurement,” Application Note 112, Linear Technology Corporation, March 2007.
