battery pack thermal protection

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A custom battery pack should have thermal protection to prevent cell damage or rupture during charging or discharging. The standard is to use an NTC thermistor 10k 3950. That is, R25°C=10kΩ, and B=3950. B is the sensitivity index constant. Note that 25°C=298.15°K. To calculate temperature from resistance, T = B / (ln(R/r)).

An NTC thermistor can also be used for surge protection. It gives a soft start when equipment is first powered up. When powering up the NTC is cold and resistance is high thus limiting current. As the current flows the NTC thermistor will get warmer. This causes resistance to drop which removes the current limiting factor from the circuit.

Measure RT

Put the NTC thermistor in series with a fixed value resistor to create a voltage divider. Vcc will typically be 5V or 3.3V for a microcontroller. In this example I use a 10kΩ resistor. Measure the voltage drop across the NTC thermistor, Vdrop = RT / (RT + 10000) * Vcc. So this gives us, RT = 10000 / (Vcc / Vdrop - 1).

PTC Fuse

A PTC resistor may also be used as a thermal fuse to temporarily cut power to or from the cell.

Table of Resistance versus Temperature for M 3950

RT(Ω} T(°C) RT/R25 TF(%) α(%/°C)

995600.0  -55  99.56    23.2  -7.71
689500.0  -50  68.95    20.1  -7.42
483800.0  -45  48.38    17.3  -7.15
343700.0  -40  34.37    14.8  -6.89
247100.0  -35  24.71    12.5  -6.64
179600.0  -30  17.96    10.6  -6.41
132000.0  -25  13.20     8.8  -6.18
 98030.0  -20  9.803     7.3  -5.97
 73510.0  -15  7.351     5.9  -5.77
 55850.0  -10  5.585     4.7  -5.57
 42510.0   -5  4.251     3.7  -5.39
 32750.0    0  3.275     2.8  -5.21
 25440.0    5  2.544     2.0  -5.04
 19920.0   10  1.992     1.4  -4.88
 15720.0   15  1.572     0.8  -4.73
 12490.0   20  1.249     0.4  -4.58
 10000.0   25  1.0000    0.0  -4.44
  8057.0   30  0.8057    0.4  -4.30
  6534.0   35  0.6534    0.8  -4.17
  5331.0   40  0.5331    1.2  -4.05
  4376.0   45  0.4376    1.7  -3.93
  3612.0   50  0.3612    2.2  -3.81
  2998.0   55  0.2998    2.8  -3.71
  2501.0   60  0.2501    3.4  -3.60
  2097.0   65  0.2097    4.0  -3.50
  1767.0   70  0.1767    4.6  -3.40
  1496.0   75  0.1496    5.3  -3.31
  1272.0   80  0.1272    6.0  -3.22
  1087.0   85  0.1087    6.7  -3.13
   932.1   90  0.09321   7.4  -3.05
   802.7   95  0.08027   8.1  -2.97
   693.9  100  0.06939   8.8  -2.89
   602.0  105  0.06020   9.5  -2.82
   524.3  110  0.05243  10.2  -2.75
   458.1  115  0.04581  11.0  -2.68
   401.7  120  0.04017  11.7  -2.61
   353.3  125  0.03533  12.5  -2.55
   311.7  130  0.03117  13.2  -2.48
   275.9  135  0.02759  14.0  -2.42
   244.9  140  0.02449  14.7  -2.37
   218.0  145  0.02180  15.5  -2.31
   194.5  150  0.01945  16.2  -2.26