bq34Z100 - SOC for 40Ah, 400A Peak Current, Battery Pack?

Hi Allen,

First question is whether you will be gauging these 400 A peaks, or are they overload situations? If you want to accurately gauge during such peaks, the sense resistor must be small enough to keep the voltage drop down under 125 mV. So, you might use a 300 micro-ohm sense resistor. If you then tell the gauge that the sense resistor is 30 milli-ohms you have effectively scaled the gauge by a factor of 100:1, and 400A would be reported as 4A. You wouldn't be able to simply scale by 10:1 because the 400 A would be reported as 40A, which is too high for the number system in the gauge. The maximum current report is 32.7A.

Using 100:1 scaling will of course present you with an unwanted side effect. Small currents will be measured as noise. So, the best scaling factor depends on your load profile. If you want accurate measurement at 400A, with best results at lower currents, you should probably tell the gauge that the 300 micro-ohm sense resistor is 15 milli-ohms. Thats 50:1 scaling and 400 A would be reported as 8 A. Your 40Ah battery would have to be represented in the gauge as 40/50 = 0.8Ah. In fact, all of your mA, mAh, W, and Wh parameters would need to be scaled 50:1.

However, if the 400 A peaks are not something you need to accurately measure, you can treat them as an overload condition and use a more typical and easier to use scaling of 10:1 with a sense resistor that is somewhat larger. For example, you could use a 400 micro-ohm sense resistor and tell the gauge that it is 4 milli-ohms. In that case, 320 A would be accurately measured as 32 A, but 40 A will no longer be in a linear measurement region. This would simplify the application and provide better dynamic range.

Rgds,

Doug