Low-Power Circuit Measures Multiple Temperatures In A Battery Stack

Sept. 16, 2011
Description of a way to monitor voltage and temperature in high-voltage, multi-cell, series-stacked batteries

Fig 1. Two multiplexers (U2 and U3) enable this data-acquisition IC (U1) to monitor up to 12 battery temperatures in a configuration that saves power when it isn’t actively making measurements.

Fig 2. To conserve power, the THRM, AUXIN1, and AUXIN2 signals are active only when the system initiates a scan of the input channels.

High-voltage, multi-cell, series-stacked batteries are the power source for electric vehicles, hybrid electric vehicles, electric bikes, power tools, and many other devices. Lithium-ion (Li-ion) batteries, because of their high energy density, are a popular choice for these stacks

However, it’s critically important for these high-energy battery stacks to have a good battery-management system. If the many cell voltages and cell temperatures aren’t monitored, thermal runaway can occur and lead to a battery explosion.

Data-acquisition ICs for battery stacks can measure multiple (typically 12) cell voltages, but at most these ICs only scan and measure two temperatures. By using multiplexers, though, you can form a low-power circuit that measures the temperature of up to 12 thermistors (Fig. 1).

Two multiplexers (U2-U3) can switch 12 thermistors (two thermistors at a time in six pairs) to the two auxiliary inputs of the data-acquisition IC (U1). A 100-pF capacitor in parallel with each thermistor will help reduce noise.

The data-acquisition IC both powers and controls the multiplexers. The VAA pin provides the power, while the general-purpose I/O (GPIO) ports control multiplexer switching among the thermistor pairs.

The thermistors receive their bias signal from U1’s thermal-supply output (THRM). Because an internal switch disables THRM when auxiliary input scanning is disabled, this arrangement provides an opportunity to save power.

Simply disable those auxiliary inputs (i.e., don’t scan) when measurement of the external temperature-sensing device is not required, such as during the interval between scans or when there is no significant load on the stack.

Connecting THRM to the multiplexer-enable inputs saves additional power by placing the multiplexers in shutdown mode when temperature isn’t being measured. When the auxiliary inputs are not being scanned and THRM goes low, the two multiplexers draw only 0.56 µA from VAA.

A trace of the THRM, AUXIN1, and AUXIN2 waveforms shows that THRM is enabled only for a short time during the measurement process (Fig. 2). The trace shows the signal’s duration as ~700 µs, which is the data-acquisition IC’s maximum data acquisition time, and is only an illustration.

The actual enable interval is software programmable and should be set so the capacitors at the auxiliary inputs have enough time to settle before initiating the measurement. The pseudo-code of Table 1 shows the programming sequence needed to initialize and read the stack temperatures.

The presence of the multiplexer in the battery-temperature monitoring circuit does contribute some error to the measurement because the multiplexer’s on resistance is in series with the thermistor. To minimize the effect of this on resistance, the circuit uses a thermistor with a relatively high resistance (at higher temperatures): Murata’s 100-kΩ NXFT15WF104FA2B050 thermistor. Table 2 compares the data-acquisition readings with and without the multiplexer and shows the error in percentage using:

Error (%) = \\[(ADC output with mux) – (ADC output without mulitplexer)\\]/4096 × 100

where 4096 is a full-scale ADC value in decimal.


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