Measure Multiple Temperatures in Battery-Management Systems, and Save Power Too

A similar version of this article was published in Electronic Design on October 6, 2011.

High-voltage, multicell, series-stacked batteries are found in electric vehicles, hybrid electric vehicles, electric bikes, power tools, and many other devices. Because of their high energy density, lithium-ion batteries are popular for these applications. It is critically important that these high-energy battery stacks have a good battery-management system for monitoring the many cell voltages and cell temperatures. Without that monitoring, thermal runaway can lead to a battery explosion.

Current data-acquisition ICs for battery packs measure multiple cell voltages (typically 12), but they only scan and measure two temperatures at most. This design idea presents a low-power circuit that measures the temperature of up to 12 thermistors. It powers the multiplexers and puts the muxes into shutdown to save power when not measuring temperatures.

The low-power circuit of Figure 1 scans and measures the temperature of all cells in the battery. The two MAX382 multiplexers switch 12 thermistors to the two auxiliary inputs of a data-acquisition IC (like the MAX11068 or MAX17830), two thermistors at a time in six pairs. The data-acquisition IC provides bias for the thermistors, while powering the multiplexers and controlling their switching and enable/disable functions.

The thermistors are biased from the data-acquisition IC's thermal-supply output (THRM). This configuration saves power because an internal switch disables THRM when the auxiliary inputs are disabled for scanning. Note that those auxiliary inputs should be disabled (not scanned) when measurement of the external temperature-sensing device is not required. Connecting THRM to the multiplexer-enable inputs saves additional power by placing the multiplexers in shutdown mode when temperature measurements are not needed. When the auxiliary inputs are not being scanned, the two multiplexers draw only 0.56µA from V_AA. THRM enables the multiplexers only for the brief interval when the auxiliary inputs are being scanned (i.e., when the temperature measurement is needed). The data-acquisition IC's GPIO ports switch the auxiliary inputs among the 12 thermistors.

A scope shot of the THRM, AUXIN1, and AUXIN2 waveforms (Figure 2) shows that THRM is enabled only for the maximum acquisition time (~700µs). This maximum time is used here only for illustration. The actual settling time for acquisition is software programmable and should be determined so that the capacitors at the AUXIN_ have enough time to settle.

Using the circuit of Figure 1 and the pseudo-code (Table 2), the output of the ADC is read at different temperatures. Table 1 compares the output of the data-acquisition IC with and without the multiplexer, and shows the error in percentage. Error (%) = [(ADC output with mux) - (ADC output without mux)]/4096 × 100 (where 4096 is a full-scale ADC value in decimal). The on-resistance of the multiplexer contributes to the error. To minimize this on-resistance, a thermistor with a relatively higher resistance (at higher temperatures) is used (Murata thermistor 100kΩ, NXFT15WF104FA2B050).