Input isolation necessary?

Hello there,

I'm seeking some advice on the following concept:

I'm designing a VRLA battery monitoring system that also requires measurement of some temperatures and some other 2-20ma current loop sensors. There are 420 batteries in a string, and there will be two physically separate data acquisition modules used to monitor a battery string. Because of the high common-mode voltage of the series connected battery cells, my measurement front-end will probably use difference amplifiers, and I'll probably partition the cells into groups of 32 or 64 (32 x 2.75 volts = 88 volts common-mode) per data acquistion card. This battery string is used to drive a large PWM electric motor, so EMI and transients are prevalent.

My primary question is, should I optically (galvanically) isolate the inputs using linear optocouplers, using the difference amp in front of the optocoupler in the servo-feedback circuit to linearize the optocoupler? Or do you think this isolation is overkill?

On another note, I would like to employ digital-to-analog converter chips that have a built-in I2C interface for the acquiring the data on the (maybe) isolated side (eg. TI DAC5574). Any comments/advice here?

Final note, I am thinking of using PC/104 CPU boards, and QNX to run the show. Any comments/advice here?

Thank you, one and all!


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I designed a thing like this once (but never built it.) I was planning on using a bunch of solid-state relays, one per cell. Imagine one floating voltmeter and two input rails, one rail for odd relays and one for even. For the first cell, turn on SSR1-->railA and SSR2-->railB, and measure A:B. Then turn on SSR2 and SSR3, again into rails A:B, and that measures the next cell, with polarity swapped of course. Some series resistors or fuses for safety, maybe.



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John Larkin

You can power your instruments from the battery that they are measuring and send the serial data through a simple opto coupler this will also help you with the EMI issues. I2C is hard to isolate but rs232 or similar is easy. If you use a current loop for your rs232, +20mA for mark -20ma for space, you get a very noise immune link.

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