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Old 11-12-2008, 03:25 PM   #11 (permalink)
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Uhm, no... because it was JUST A SUGGESTION. The reasons were given in my reply. As well, for the reasons given in my reply, i can't select a part number because i don't know the parameters of his project. I don't understand why you're being so persistant about this?

I suppose that "better" is highly dependant. For some people, a geo metro is better than a Enzo Farrari... but the farrari is definately more complicated, costly and difficult to build. I think in terms of accuracy and reliability. You will get better accuracy if you do not leave it to resistive dividers and instead use ADC's specifically designed for the intended voltage range.

If it is a quick and dirty hack job you are looking for, then by all means use an analog multiplexer and resistive dividers... whoever is paying for it may want better.

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Old 11-12-2008, 03:35 PM   #12 (permalink)
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anyway, getting back to constructive and non presumptious things:

There may be another option that builds on Matts suggestion, that is both easy to hook up and not terribly expensive, and that is to utilize atmels I2C (or 2 wire) protocol, and make a bunch of attiny devices on that network that monitor the batteries (perhaps one attiny can monitor a pair of batteries or something). So each battery (or pair of batteries) gets a chip and get strung into the i2c network.

The floating grounds might be a problem though, if it were easy to make the attinys autorange in relation to ground, or if Matt has a chip suggesion for a floating ground adc that might prove helpful too.
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Old 11-12-2008, 03:52 PM   #13 (permalink)
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er... be very careful about using I2c over range. It really is a bus better (there's that word again) used between chips on a board rather than wired between boards. RS232, CAN, even TTL serial or parallel are better options for that. The reason being the resistance and capacitive limits of I2C are very small.

Finding a good I2C chip is a great idea because you don't need many ports on the processor (it is a 2-wire bus that allows multiple components to connect together and to two pins on the cpu). I still would recommend running a wiring harness to each of the nodes of the battery net and keeping your ADC's all on one board.

Digi-Key Part Search

The link above will get you to the parametric search on the digikey website under the ADC section. You select your bus (I2C, serial, whatever) then select your voltage range, temperature range, your chip package (dirt-sized SMT or big honkin DIP packages) and all the other necesities. It will allow you to find multi-channel adc's and (literally) all the other options out there.

Another caveat with I2C components... some of them (including the I2C ADC that is on a board that I am currently working with) have a fixed adress, so you can't put multiples of the same chip on the same bus or they won't know which one is being adressed. If you can't find a way around this, you will have to use an analog multiplexer to feed into a single adc.

ps - what's an attiny device?
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Old 11-12-2008, 11:51 PM   #14 (permalink)
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Anton, I may have mistakenly assumed that you were looking to build the whole thing from scratch.

If that's not the case, you may want to look at this:

PakTrakr Multi Battery Monitor Battery Pack Monitor

Can be configured to monitor up to 40 batts.

Quote:
Q: Is there a serial interface for logging data to a PC?

A: Yes, the PakTrakr sends out a CSV data stream every second,
containing all the battery data. By attaching a PC to the PakTrakr
via the optional serial interface, the data may be easily read into a
spreadsheet for analysis.
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Old 11-13-2008, 01:42 AM   #15 (permalink)
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matt, attiny is just a little microcontroller, like a pic, but if you know an adc that handles floating ground that would be useful, I don't know how to search for it.

Here's a page that provides some more ideas, a basic stamp based battery individual battery monitor. He has plain optos on each battery that draw a continuous 25ma (maybe) so not 100% ideal or easy to disengage in a sleep mode, but still plenty to think about: BATTERY MONITOR





I also have to think that too many chips and a network will add to the "leakage" power consumption just as a voltage divider would. I wonder if you can make an auto-ranger out of a 4051?
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Old 11-14-2008, 04:14 PM   #16 (permalink)
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What about measure the voltage of each node with respect to ground? It will take some processing cycles to convert them into voltage per battery, but it should turn out cheaper. Use some MOSFET type optoisolators to switch the data acquisition device from node to node. Add some series resistors to prevent accidental short circuits.

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