Hello, I am currently working on an exciting project involving the design of an intelligent battery sensor for automotive starting batteries. The sensor is performing well, but I have encountered a minor issue. I am aiming to create a sensor that can be used for both motorcycles and passenger cars (PKWs), and I have selected the SH6918F500AHEP shunt resistor with the following specifications:

- Resistance: 50uOhm
- Tolerance: 1%
- Temperature Coefficient: 175ppm (-60°C - 175°C) / 100ppm (20°C - 60°C)
- Power Rating: 25W

My SDADC (Successive-Approximation Analog-to-Digital Converter) has a voltage range from -2.5V to 2.5V, and I want the shunt resistor to handle only 25W to avoid excessive length.

To provide some context, a typical 12V 70Ah lead-acid battery can deliver around 700A of cold cranking amps (CCA). For the sensor, I need a charging current of approximately 17.5A (25% of the total capacity). Here's my calculation:

- Umax (Voltage drop at max current) = 700A * 50uOhm = 35mV
- Umin (Voltage drop at charging current) = -17.5A * 50uOhm = -0.875mV

To ensure that these voltage levels are within the SDADC's range, I've chosen an amplification factor of 25:

- Uadc,max (Maximum amplified voltage) = G * Umax + 2.5V = 3.375V
- Uadc,min (Minimum amplified voltage) = G * Umin + 2.5V = 2.478V

Based on these calculations, it seems that the selected shunt resistor and amplification should work within the SDADC's range. However, if you have any doubts or if there is another approach that might yield better results, I would appreciate your input.

Please let me know if you have any suggestions or if you'd like me to provide more information about the project.

 

Maybe I'm missing something due to both lack of knowledge of the subject and lack of information (schematic?), but if your SDADC is good for -2.5V to 2.5V Uadc,max at 3.375V looks higher than 2.5V.

I have worked with current sensing amplifiers on a previous project so I think I understand where this is going, but never with an IBS. My experience was not as precise / accurate as a battery monitor would need to be. A bit of math on a curve solved my issues.

 

This is the schematic. I'm using 2 amplifiers to better handle disturbances. However, the issue is that I want to measure 35 mV with high accuracy. What could be the alternative?

 

I’d recommend low side sensing with an AD8418. (It will also do high-side on 12V). You can set it to output half Vcc at zero current so that it can read both positive and negative current.

 

I am currently utilizing the EBW8518PNT-L250FB resistor for low-side sensing, which boasts a resistance of 250µΩ with a 1% tolerance and a temperature coefficient of 100ppm/°C. It is capable of handling up to 36W of power. However, I am considering a switch to a shunt resistor with the following specifications:

**Shunt Resistor:**
- Model: SH6918F500AHEP
- Resistance: 50uohm
- Tolerance: 1%
- Temperature Coefficient (TCC): 175ppm (-60°C to 175°C) / 100ppm (20°C to 60°C)
- Maximum Power Rating: 25W

For amplification, I am using the INA190 amplifier, and my schematic resembles the following:

**Amplifier: INA190**
- Wide common-mode voltage: -0.2V to 40V
- Input Offset Voltage: 15
- Input Offset Drift: 0.13
- Common-Mode Rejection Ratio (CMRR): 150
- Power Supply Rejection Ratio (PSRR): 120

My question is, what modifications should I make to my schematic to accommodate the SH6918F500AHEP resistor?