Measuring the voltage of series connected batteries

[Dreamer14388]

Hi, I am using the ADC of PIC18 to measure the voltages of the battery bank in which the batteries are connected in series. I have to measure the voltages of each battery in the bank. I am using differential op-amp with voltage divider to bring the voltage into the range of the ADC. Now as the batteries are connected in series so when I connect it with the circuit it is measured as zero. while not connected in series the measurement is fine. I need soluton for this problem any help will be greately appreciated. what should I do to measure the battery voltages while connected in series. Thanks

[crossrulz]

Do you have a wiring diagram or circuit to show your setup?

What is the voltage of the batteries?  What is your supply voltage for the Op-Amps?  How many batteries?

What I'm guessing is that the common-mode voltage of the battery is higher than the supply voltage and therefore both sides of the differential op-amp are railing and therefore no difference.

[johnsold]

I agree with crossrulz. The problem is likely in the way you have connected everything.

If you have a battery pack for an electric car or an uninterruptible power supply where the total battery voltage reaches hundreds of volts, you have a significant challenge to make the measurements.

Please tell us the number of cells, the maximum battery voltage, and the accuracy and resolution required.

Lynn

[Dreamer14388]

There are 4 12V Batteries connected in series and I am taking the + and - of each battery to my differential op-amp. The last battery connected in the series connection is measured fine ,but when I connect the others the result is zero. I think due to the series connection the difference at the op-amp input is zero thats why I get zero at the output. A sample pic of the connections is attached.Please suggest solution. 

[crossrulz]

What are your supply voltages for the op amps?

[Dreamer14388]

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@crossrulz wrote:

What are your supply voltages for the op amps?

Its 5V on the positive and Ground on the negetive.

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[johnsold]

Then you are guaranteed to have five dead op amps! You cannot connect 48 V, 36 V, 24 V, or 12 V to an amplifier powered from 5 V.

Maybe your amplifiers can survive, depending on the voltage dividers.  Please show us the complete circuit with the dividers and the power connections.

Lynn

[Dreamer14388]

I have voltage dividers to take care of that....

[johnsold]

I still suspect that you have a common mode voltage problem or something not connected correctly.  Without a complete schematic, it is useless to spend any more time guessing.

Lynn

[Henrik_Volkers]

My 2c:

build for every input (also for the GND)  1:200 voltage divider with 2x 270k and one 2.7k  , add 1nF cap parallel to the 2.7k (unless you want really dynamik measurements)

choose a intrumental diff amp with gain 40 and make shure you choose a type that support inputrange to GND.

you can add a 4.096V Ref to the ADC ...

the 0.5M input resistor together with the input diodes of the IA should work as a input protection...