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Measuring Cell Temperature: Watch Where You Put that Sensor

Nov. 28, 2018
Accuracy in cell testing can be affected by temperature, thus the need for temp-measurement sensors. And the positioning of that sensor is a sometimes overlooked, but essential, factor.

When testing cells, temperature can influence many of the cell properties. Therefore, when connecting a cell to a piece of cell test equipment, along with connecting voltage sense leads and current-carrying leads, you may need to attach a temperature-measurement sensor. Temperature-measurement sensors are typically thermocouples, RTDs, or thermistors.

If the parameter being measured is very sensitive to temperature, placement of the temperature sensor is important. If the cells have been given time to reach thermal equilibrium with their environment, then sensing the nearby air is satisfactory and exact placement of the temperature sensor isn’t critical.

To ensure accurate temperature measurement, a test engineer will often place the temperature sensor directly on the cell. For measurements of cell self-discharge current, which can be highly influenced by cell temperature, the test engineer may even choose to put the temperature sensor directly on the cell’s contact or electrode.

Temp Sensor Positioning

Thermally, it’s best to put the temperature sensor in the position where it will get the most meaningful measurement of the cell temperature. Is there an electrical impact of where the temperature sensor is placed? Can harm be done simply by placing a temperature sensor directly on the cell?

For starters, cells are electrically isolated, aka floating, by their nature. This can simplify test setup and design of cell test equipment. Since the cell is floating, that means the test equipment doesn’t need to be floating off ground or isolated from cell channel to cell channel.

When creating a high-channel-count cell tester, the equipment vendor may choose to design the cell test equipment to have the cell measurement channels share a common ground and ensure channels aren’t isolated from each other. This can save on system complexity, yielding a system that’s smaller, more efficient with power consumption, and ultimately lower in cost. Since the cells are naturally floating, the cell test equipment doesn’t need to be, too.

A common configuration in a multi-channel cell tester is to tie the negative sides of all cells together; this common negative could even be at earth ground. To the cell that’s naturally floating, none of this will matter.

1. A typical thermistor.

Now the test engineer comes along and wants to measure temperature. Let’s say the engineer is using a thermistor (Fig. 1). If you look carefully at this thermistor, you can see the thermistor body and the two leads soldered to the ends of the thermistor. Wires are soldered to the leads and those wires run to the data-acquisition instrument that will measure the thermistor and convert that measurement to a temperature reading.

Electrically Speaking…

In addition to positioning the thermistor to be in the right place to measure the right temperature, placement of the thermistor must be considered from an electrical perspective. If the thermistor has exposed leads, care must be taken in terms of where you contact the cell. If you put the thermistor on the body of the cell, and the cell body is insulated with plastic, or coating, or label, then there’s no issue electrically. The thermistor leads can’t make electrical contact with the cell.

But some cells have the entire body as the negative electrode. Or, less commonly, the entire cell body could be the positive electrode. In either case, it means when you put the thermistor in contact with the cell, you’re shorting one of the thermistor leads to one of the cell electrodes. Of course, if you put the thermistor directly on the cell contact (or electrode or tab or button or terminal), then you most certainly are shorting the thermistor to the cell contact.

2. A typical thermistor measurement circuit.

Is shorting the thermistor to a cell contact harmful? Let’s look at how a thermistor is measured (Fig. 2). The thermistor resistance varies with temperature. A thermistor-measurement circuit uses a voltage divider to measure. The hazard could come if the thermistor’s measurement inputs are part of the same electronics that are used for charging, discharging, and measuring the cell (Fig. 3).

3. Hazardous condition created when the thermistor shorts out the cell (top). Incorrect temperature measurement created when thermistor shorts to ground (bottom).

As mentioned earlier, the cell-measurement channels could have a common ground that could be earth ground. Furthermore, the thermistor’s measurement circuit may also share a common ground with the cell channel electronics. In this case, the thermistor’s exposed leads could short out the cell (worst case, Fig. 3, top) or short out the thermistor (Fig. 3, bottom), which is non-hazardous but certainly undesirable as it will generate measurement errors.

Unfortunately, many cell test equipment manufacturers don’t provide schematic diagrams that will tell you whether the thermistor’s measurement inputs are floating or share ground with the cells. So, for safety’s sake, it’s best to assume the thermistor measurements are non-isolated. This means that when placing the thermistor on the cell, you should ensure that the thermistor, or any temperature sensor type for that matter, is insulated by tape or some other insulator when touching any exposed metal surface on the cell. If the cells are encased in plastic or have a plastic label where the temperature sensor is being placed, then no special precaution is necessary.

Bob Zollo is Solution Architect for Battery Testing, Electronic Industrial Solutions Group, at Keysight Technologies Inc.

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