46
For a single voltage of 3.3V, the OCV can represent either 70% SOC or 30% SOC depending on whether the cells
were just discharged or charged. Because some of the sections of the curve are very flat; < 1 mV per % SOC, even
a small 1 mV error in the voltage reading can result in an error of several percent.
Temperature also affects the OCV values of the cell, but its effect depends on the SOC of the cell. Above 30% SOC,
the effect of temperature is positive on the OCV and below 30%, its effect is negative. Figure 23 shows the highly
non-linear relationship between SOC and the effects of temperature on OCV. The rate of change for each point is
linear between -30 and +35 °C. So for example, at 50% SOC, the OCV will vary with temperature, from -30 to
+35 °C, at a positive linear rate of 0.13 mV/ °C
Figure 23 – AMP20 cell temperature effects on OCV with respect to (wrt) SOC
Coulomb Counting SOC (iSOC)
Another method of fuel gauging uses only current and time. Based on a known starting SOC point, the BMS
calculates the present SOC by integrating the measured current going into and out of the battery. This method is as
accurate and resolved as the current and time measurements are. The problem with this algorithm is that the
starting SOC is not always known. In addition, because the algorithm integrates the current signal, very small
current levels, noise, inaccuracy and small offsets can gradually increase the error over time.
Combination of vSOC and iSOC
The problems with both vSOC and iSOC can be somewhat mitigated by using a combination of the two algorithms.
For example, one can determine the vSOC fairly accurately at times when the actual SOC is either near the top of
charge of bottom of charge. At these times, vSOC can be weighted higher than iSOC. During other times, when the
actual SOC is in the middle range, the iSOC can be used to measure the reported SOC. The estimated OCV is based
-
0.0004
-
0.0003
-
0.0002
-
0.0001
0
0.0001
0.0002
0
0.2
0.4
0.6
0.8
1
d(OCV)/dT (V/'C)
SOC
d(OCV)/dT fit for 20 Ah cells
AMP20 data
fit
AMP20 cell ID SGHGMB001
d(OCV)/dT fit
