NiMH Battery Temperature Test Method

The National Semiconductor LM35 Centigrade temperature sensor is used to measure the NiMH battery temperature while charging.

The LM35 is an integrated circuit sensor that can be used to measure temperature with an electrical output proportional to the temperature (in oC).

The LM35 - An Integrated Circuit Temperature Sensor

• You can measure temperature more accurately than a using a thermistor.
• The sensor circuitry is sealed and not subject to oxidation, etc.
• The LM35 generates a higher output voltage than thermocouples and may not
  require that the output voltage be amplified.

What Does an LM35 Do? How does it work?

• It has an output voltage that is proportional to the Celsius temperature.
• The scale factor is .01V/oC
• The LM35 does not require any external calibration or trimming and maintains an
  accuracy of +/-0.4 oC at room temperature and +/- 0.8 oC over a range of 0 oC to
  +100 oC.
• Another important characteristic of the LM35DZ is that it draws only 60 micro
  amps from its supply and possesses a low self-heating capability.
• The sensor self-heating causes less than 0.1 oC temperature rise in still air.

What Can You Expect When You Use An LM35?

• The output voltage is converted to temperature by a simple conversion factor.
• The sensor has a sensitivity of 10mV / oC.
  
• Use a conversion factor that is the reciprocal, that is 100V/ oC.
• The general equation used to convert output voltage to temperature is:
• Temperature ( oC) = Vout * (100 oC/V)
• So if Vout is 1V , then, Temperature = 100 oC
• The output voltage varies linearly with temperature.

The Basic Calibration Test

We started our test at a room temperature of 23 degrees Celsius which means the voltage output of the sensor should be 230mV. We then added hot water and put the sensor in the hot water protected by a thin layer of plastic wrap.

We took a few measurements as the water cooled down.

Room Temperature 23 degrees Celsius = 230mV. The sensor measured 239mV.

Point 1 : Digital Thermometer 41.8 degrees Celsius = 418mV. The sensor measured 407.7mV.

Point 2 : Digital Thermometer 41.2 degrees Celsius = 412mV. The sensor measured 403mV.

Point 3 : Digital Thermometer 38.2 degrees Celsius = 382mV. The sensor measured 373.5mV.

Conclusion

The LM35 Temperature sensor provides an accurate temperature reading with an approximate deviation of 1 degree Celsius.

The purpose of our temperature tests is to find out whether or not the charger is causing the battery to over heat and to get an idea of the temperature characteristics during the charging cycle.

The measurements taken with this sensor are very reliable. When you see a graph in a charger review, remember to divide the mV by 10.

Example.

The graph above shows a scale of 0 to 1V. The starting point is 0.230V ( or roughly on this graph 0.250V ).

The Math

0.23V = 230mV

230Mv / 10mV per degree Celsius = 23 degree Celsius.

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