How to read 60 voltage inputs ??

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Really I am just asking for ideas as to the best way to read 60 different voltage inputs ranging from 3v - 4.2v in the smallest form factor and lowest parts count?

Background Information
This is for my ebike. I have 10 battery packs and each one has 6 cells. Right now when traveling I am just reading the overall voltage of the batteries. This is not an optimal system of measurement. If one cell goes too low or too high there is the potential for thermal runaway with the lipo batteries I am using. This has never happened and the cells have always been very well balanced but I would like this information for piece of mind.

When I arrive home I can easily check each cell with the battery charger but this takes time and I have to remove and insert the charger 10 times to read and balance all 60 cells.

Solution so far
so far all I can figure is 10 atmega8 chips using every adc port and i2c protocol to relay that information back to an lcd. Anyone have one with fewer parts. All ideas are most welcome!

Lachlan

What we need to learn,
we learn by doing.

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You need some kind of multiplexer for example CD4051 so you can multiplex input signals to one input of µc.

Alexander

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I believe the 6 cells are in series in each pack.

Do you have enough resolution that you have one ground, and read the various tap voltages with separate resistive dividers and ADC inputs, (3.5V, 7V, 10.5V, 14V...)?

Does this give you adequate resolution for measuring each cell's voltage?

And, if this is the setup then the readings and output are all referenced to ground, and you are not using any floating differential inputs?

JC

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Hi Lachlan,

Have you looked at these offerings?

http://parametric.linear.com/Mul...

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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lmc222 wrote:
Really I am just asking for ideas as to the best way to read 60 different voltage inputs ranging from 3v - 4.2v in the smallest form factor and lowest parts count?

Background Information
This is for my ebike. I have 10 battery packs and each one has 6 cells. ....
Solution so far
so far all I can figure is 10 atmega8 chips using every adc port and i2c protocol to relay that information back to an lcd. Anyone have one with fewer parts. All ideas are most welcome!

Those 6 cells will be in series, and that complicates things.
The chip count is mainly going to be set by the '10 packs', as you do not want to run cell-tap wires all over the place.

Atmel do have battery micros, but they do not mention 6 cells.
http://www.atmel.com/products/mi...

So you may be better with the smallest uC with 12b ADC, to allow you to tap-divide those cells in series, and still have good enough precision.

Maybe something like ATXMEGA8E5, from Atmel, or cheaper and smaller and wider Vcc range, maybe the Zilog Z51F0410HCX, or even cheaper, the Silabs C8051F86x series, which has a 125'c spec as a bonus.

Almost any code size will do, you are really just buying a configurable ADC

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Just thinking aloud.
Extending your idea a little further
How about a ATtiny13 slave micro connected across each cell which measures its own Vcc using the internal 1.1V reference.
Each micro has a unique address. Using a suitable bus/protocol, a master wakes up all the micros master, which then do a conversion and then are polled for the battery value. When there is no bus activity the MCU's go into a low power mode until next wake up from master.
Sixty ATtiny13's will cost about $30...cheap!

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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The problem is not uC address, it is that cells are stacked so there is no common ground. With 6 cells the top of string is ~24V above ground.
You could use isolators of some type and a micro-per cell, but isolators are not cheap.
Resistors to level shift, are cheap,and give about 9.4 bits of ADC equiv across the top most battery, with a 12b ADC on the micro.

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Ah yes, need to think that out a little further! :oops:

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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Opto isolation?

Ross McKenzie ValuSoft Melbourne Australia

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Rugged circuits has an example of stackable cell monitor on their site.

Imagecraft compiler user

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lmc222 wrote:
Solution so far
so far all I can figure is 10 atmega8 chips using every adc port and i2c protocol to relay that information back to an lcd. Anyone have one with fewer parts. All ideas are most welcome!
I assume that the ebike runs off one pack at a time?

How can you get a lower parts count than one mcu? Granted, you'd need 5 divider taps for a total of 10 resistors, plus a bypass cap.

You could include an LCD (or other display interface of some kind) in each pack, or have a separate 'cradle' for the display. This would let you pop each battery into the cradle in turn to get a report. While this is more work than just pressing a button on the pack to get a report from an integrated display, it does lower your parts count.

You could of course also move the mcus out of the packs and use only one in the test cradle, but you'd need connectors with 7-pins instead of only 3-pins (or 2-pins).

Also, while permanent taps will result in some leakage current, you can probably get away with resistors on the order of megaohms, for leakage currents measured in microamps. The high impedance will affect the bandwidth of the ADC, but you're basically measuring a static voltage anyway.

JJ

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

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bobgardner wrote:
Rugged circuits has an example of stackable cell monitor on their site.

Specifically at http://ruggedcircuits.com/html/c...

However be aware that
- the batteries have been incorrectly drawn (all reversed)...
- this circuit can only handle up to 3 batteries in series because of the op-amps max supply voltage.

Ross McKenzie ValuSoft Melbourne Australia

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Seems like a lot of parts ?

I would favour a simple divider with a higher precision 12b ADC, and still get > 9 bits of precision, but if that is not enough, and the app really needs to measure cells to 12bit precision, then a Current Sense amp gives a good way to voltage shift from higher levels.

eg the SOT23-5 Si8540* has external gain set resistors, and can sense/shift up to 36V levels.

*Typo fixed

Last Edited: Mon. Feb 10, 2014 - 05:56 AM
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Who-me wrote:
eg the SOT23-5 Si8450 has external gain set resistors, and can sense/shift up to 36V levels.
Are you sure that is the correct part number? Link?

Thanks,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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valusoft wrote:
Who-me wrote:
eg the SOT23-5 Si8450 has external gain set resistors, and can sense/shift up to 36V levels.
Are you sure that is the correct part number? Link?

Oops, transposed some digits - try SI8540-B-FW.
I'll correct it above.

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A sincere thankyou to all those who took the time to respond.

I found a very interesting paper on this exact problem and it may be helpful to anyone who is facing a similar problem. His solution is similar to what has been outlined here.
http://www.diva-portal.org/smash/get/diva2:348810/FULLTEXT01.pdf

Lachlan

What we need to learn,
we learn by doing.