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Original question:

ADS1115: Measuring Lithium ion Battery Voltage

ADS1115: Question about Differential Mode for Lithium batteries

Paolo Gaggero
Prodigy 60 points
Part Number: ADS1115

Tool/software:

Hi everyone. I find myself facing a voltage measurement circuit of 4 lithium batteries in series (16.80V total).

Initially I opted for a Single Mode measurement, therefore measuring each battery from the positive to the GND (0V) of the last battery and to read each single value I did

16.8V ----- | +  B4  - | ----- | +  B3  - | ----- | +  B2  - | ----- | +  B1  - | ----- GND


B4-B3 = 16.80V - 12.60V = 4.20V

B3-B2 = 12.60V -  8.40 V = 4.20V

B2-B1 = 8.40 V - 4.20 V = 4.20V

B1-B0 = 4.20 V - 0 V = 4.20V

(The numbers listed are obviously only indicative since in reality fluctuations or other causes the values ​​to differ slightly)

This result is fine but it is a bit summary especially because I calculated the values ​​by putting gain GAIN_TWO (for an input range of +/-2.048V) and then imposing voltage dividers.
So a little bit of tolerance on the values ​​of the resistors, a little bit of the error that increases as the value differs as for example on the value read on B4-B3 and on B3-B2 (being those with higher voltage) I believe that in the end the readings are not as precise as I would expect.

So I ask you:

1) do you think it makes sense to carry out a differential reading on each battery?

2) In my case obviously I would need 2 modules (as I would have to measure B4/B3, B3/B2 and with this I would end a module and then the second module to measure B2/B1 and B1/B0), the measurement I would obtain would be greater ?

3) By maintaining a Gain_TWO I would still need a divider but does a divider for reading voltages of max 4.20V certainly generate less error than a divider for example for reading voltages of 16.80 or 12.60V?

Thanks in advance

  • 0
    TI__Expert 8130 points

    Hi Paolo,

    I saw your post and I'll get back to you by tomorrow.

    Best Regards,

    Angel

  • 0
    TI__Expert 8130 points

    Hello Paolo,

    There is a problem with the approach described here to measure the voltage of the batteries.

    While it is true that the differential voltage between the 16.8V and 12.6V is 4.2V, the absolute voltage at these points with respect to GND for the ADC is still 16.8V and 12.6V.

    This would be above the recommended operating conditions of the device and could damage the part.

    Paolo Gaggero said:
    a little bit of the error that increases as the value differs as for example on the value read on B4-B3 and on B3-B2 (being those with higher voltage) I believe that in the end the readings are not as precise as I would expect

    This is likely the reason why your readings have errors when compared to the B1-B0 case for example (4.2V-GND) where the voltage inputs are within the recommended operating conditions of the device. 

    You could use a resistor divider network to bring down the voltages you want to measure to be within the recommended analog input voltage and that would be a valid solution. 

    The downside to this approach would be that you would be consuming current from your batteries when implementing the resistor divider network to lower the voltages to acceptable input levels. 

    This is not a big problem if the resistor divider network is only connected to the batteries for a short period of time to test the voltages, but if intended to monitor at all times, it could drain the batteries quickly. 

    You could just monitor the batteries individually rather than in a series configuration and this would eliminate the problem of having to measure voltages higher than the recommended inputs for the ADC.

    Is it simply to monitor the voltage and condition of the batteries, or is there a reason that they must be in series?

    Best Regards,

    Angel

  • 0 in reply to Angel Rodriguez de San Miguel Bermudez
    TI__Expert 8130 points

    Hi Paolo,

    Another, but rather expensive solution, would be to galvanically isolate each ADC from each other and then reference the ADC ground to the battery cell negative pole of each cell it is measuring.

    I would also recommend taking a look at our battery management products as you are likely to find a product that suits your battery monitoring needs there:

    Battery management ICs | TI.com

    Battery management ICs product selection | TI.com

    Best Regards,

    Angel