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Author Topic: Battery Pack, individual cell voltage measurement  (Read 3907 times)
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op-amp that has a very high common mode voltage rejection rating

I thought about something like that, but it requires much more circuitry [45 opAmps,
plus etc] than the other approaches. But would be very good for a few sample points
only.

Or just one with a 2 pole 45 throw switch ahead of the opamp. This could just be 45 DPST relays that you select one at a time to route the selected cells terminals to the single opamp.

Lefty
 
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45 DPST relays

On the other thread you mentioned old style CMOS chips. There was also a time
between vacuum tubes and transistors that they used relays for computer logic gates.
I imagine your 45 relay design would sound something like one of those computers,
:-). The reptiles might go crazy, LOL.
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Check out the linked pdf doc.
Wow, +-250V CMRR. That's impressive.

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separate Arduino processors, powered at different points in the battery chain, and each reading 6 A/D channels,
I also was thinking about this overnight. I thought that my above design could be modified to handle either 6 batteries (although 6x15v is getting too high for good resolution) or maybe "12 volts"  where that is made up of various combinations, 6x2v, 3x4v, 2x6v or 1x12v.

You would then hang boards at various location on the battery string as required. In the case of this thread's application that would mean 3 batteries per board or 15 boards, maybe as few as 8 if you up the range per board.

Another option it to incorporate a special monitor chip like the 12-channel MAX11068, they can only handle 5v per cell which is no good for me but would work here. Also there's the LTC6802.

______
Rob
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The off the shelf battery management system that is failing, is it a balancer type system or just a battery monitor.
A balancer type does more than just measure individual cell voltage.
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Or just one with a 2 pole 45 throw switch ahead of the opamp.
Can you get those at Radio Shack?
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A balancer type does more than just measure individual cell voltage.
Good point mauried.

@phoyt
How much to these gadgets cost?

______
Rob
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That's what I'm talking about @oric_dan.  You only need 4 banks, and 4 Megas for 0.04v resolution. 
45x2 tiny resistors.  No other hardware.
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relays for computer logic gates.
We did an entire runway lighting control system with relays once, 1000s of them in 19" racks, all plug-in modules with NAND, AND, OR, flip flop etc functions and all at 48 volts.

It was destined for an Asian country (Malaysia I think) and the design brief was that it had to be simple technology that the "natives" could fix.

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and 4 Megas
You didn't mention 4 megas originally which partly explains why I didn't get it but you still have to isolate the megas from each other. This means powering each mega off it's bank, then having an isolated comms link. Is that what you have in mind?

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You got it Rob!
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I'm not the fastest horse in the race, but I get there eventually smiley

I've modified my design to allow monitoring of 6 batteries and 2 temperatures. If phoyt or anyone is interested I'll post the new design. This is very similar to what you're proposing but without the extra crap that comes with a Mega and that isn't needed. However with mine you still need a single Arduino (just a small one will do) to talk to a PC, display the results etc.

______
Rob
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We did an entire runway lighting control system with relays once, 1000s of them in 19" racks, all plug-in modules with NAND, AND, OR, flip flop etc functions and all at 48 volts.

It was destined for an Asian country (Malaysia I think) and the design brief was that it had to be simple technology that the "natives" could fix.

And the byline to this is that they could hear it in Indonesia.
[check your map if you don't know geography].
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Would it be possible to use zener diodes to drop the voltages down into the range that the Arduino can measure?
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Nope, leastways I can't think of a way to use zeners.

If you just want to drop the voltage down the easiest way is to use 45 voltage dividers, all set to provide the best range possible for the voltage at that point in the battery string.

Truth is that's probably the simplest method but it's not scalable, picking all the resister would be a pain and this approach needs 46 wires. Also it means some very high voltages are on your PCB, something I would not be happy with.

_______
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With my idea, there is no reason to use different values for each resistor.  Only 2 values are needed.  Sure with different values you could get more precision with the first few cells, but the one at the end would only get 0.04v precision.  So why not just use the same resistors for all, giving them all the same precision instead of increasing the precision of the first cell to 5/1024v.  Maybe I haven't explained it properly.  It seems like Rob and Dan got it.
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Peter is on to something.  If you use Zeners to drop the 30-40v wires toward the end in my plan down to 10v, you would get more precision, if you needed it.  Everything would have to be calibrated manually in software with a DMM.  With this method, and 45 inputs, you could possible do it with only 1 Mega?
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