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Balancing battery packs with a stamp — Parallax Forums

Balancing battery packs with a stamp

petripetri Posts: 5
edited 2008-02-29 12:41 in BASIC Stamp
·Hi all, 1st post here. I know a little about electronics and I'm looking to learn more in the near future. This is an offshoot of another hobby, radio controlled airplanes and helicopters. I've come across the need for a Cell balancer that will balance 14 cells in a battery pack. They are A123 M1 cells with a max voltage of 3.6V. From the little bit of reading I've done I see I'm going to need an ADC to read voltage and communicate it to the basic stamp2. I was looking at a MAX1258, has 16 channels and runs on 5V. The balancer would need to read voltage of each cell and then store it as a variable. Program logic would then need to Discharge any cell over 3.6V thru a transistor and resistor at hopefully 2A. After all cells were equal to or less than 3.6V it would then need to read voltage of each cell and discharge each cell that is higher than the lowest cell to bring the pack into balance. Anyone have any thoughts on this whole idea or maybe working on something similar? Thanks Rob D.

Comments

  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2008-02-28 21:46
    petri
    You might want to look at this as well


    http://www.intersil.com/data/an/an1333.pdf

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    ··Thanks for any·idea.gif·that you may have and all of your time finding them

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-02-28 21:52
    I assume these cells are in series to form a 50V battery pack. Since no more than one of the cells can share a common ground with the Stamp at one time, you will need a couple 15:1 analog multiplexers to connect each one in turn: one for ground, and one for an analog input. For discharging, the analog multiplexer won't be able to handle the current. For that, you may need a bank of opto-isolated MOSFET switches — one for each cell. (There has to be a cheaper, lighter way to do the discharge, but it's not coming to me offhand.)

    -Phil
  • petripetri Posts: 5
    edited 2008-02-28 22:12
    Thanks for some ideas guys. Phil, why couldn't the cells share a common ground as long as you used something like a voltage divider circuit for each cell above 3.6 volts? Do you think it would be possible to do it "on the cheap" and use RC time circuits, w ith pots to calibrate, to measure the voltage of each cell? I guess I'll try to tackle the input side of this first and then worry about the discharge end of it.... I'll think I'm going to try to build a 2 cell balancer on my homework board and see what I can learn from that to start with. Thanks again for the ideas....
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-02-28 22:28
    With a voltage divider, you'd lose about four bits of precision, you'd have to do some calculations to get the individual cell voltages, and the whole thing would have to be calibrated; but otherwise — sure — it could work. You'd still need a multiplexer to pick which tap to measure, either externally, or by using a 14-input ADC. My preference would still be to use two multiplexers and measure the cells individually.

    -Phil
  • petripetri Posts: 5
    edited 2008-02-28 23:55
    If I could hold .005V accuracy I'd be happy. I didn't even know what a multiplexer was until I just looked it up on google [noparse]:)[/noparse] Looks like it would make thing a lot more simple but complicated at the same time. I was going to just use a seperate pin on the stamp for each cell. I should be able to do an 8 cell pack (8 voltage measurement inputs and 8 discharge outputs to mosfets) with a 16 pin Stamp? Could I just gang up two stamps and have stamps compare lowest voltages between each to drive the logic? How does the MUX and Stamp communicate? I'm guessing there's a clock-timing issue to deal with. which is way over my head at the moment, but I'm willing to learn! [noparse]:)[/noparse]
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-02-29 00:55
    The Stamp doesn't have analog inputs, and I'm not sure you'd get sufficient accuracy using RCTIME to measure the voltages. Better, would be to use an analog-to-digital converter. To get 5mV precision from an 8-cell pack (30V), you'd need a 13-bit ADC (e.g. Microchip MCP3301) and one 8:1 multiplexer (e.g. Intersil DG408). This setup will require seven pins on your Stamp: three for the ADC, and four for the multiplexer. The DG408 multiplexer can run on up to 34V and would be have to be powered directly from the battery pack. The ADC will run from the Stamp's Vdd.

    The output from the multiplexer will feed your voltage divider, thence to the ADC. Since the multiplexer has an enable input, you can turn it off, thus disconnecting the voltage divider from all of the cells to save current drain.

    This may well be the simplest practical setup. With two multiplexers, you would need only a 9-bit ADC, but sequencing the multiplexers to avoid over-voltage and negative-voltage situations could get a little tricky.

    'Hope this helps...

    -Phil
  • petripetri Posts: 5
    edited 2008-02-29 03:53
    It does help, it makes me realize I have a lot more to learn before attempting this [noparse]:)[/noparse]
    http://www.astroflight.com/store/store-type-tem.html?item=products:af-106-123&sid=0001o71rHpVhXhEXWY5f8L9
    I have two of these now and you can chain them together but they only have about 130ma current dissipation per cell. I was thinking I'd just upscale the whole thing for the higher current and aditional cells. It uses a PIC chip to run the whole thing. I sent the manufacturer an email requesting what I'm looking for, maybe I'm not alone...... I'm not sure how astro is doing it, but it looks easy when done with SMT parts [noparse]:)[/noparse] I'll have to reverse engineer it and see if I can learn something... Thanks again for the help!
  • petripetri Posts: 5
    edited 2008-02-29 04:08
    I think this is a big part of it! http://www.chipcatalog.com/Microchip/PIC16C770.htm
    on-chip A-D converters 6 channels and 12 bits.....
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2008-02-29 12:41
    petri

    Thank You for posting this Link

    www.astroflight.com/store/store-type-tem.html?item=products:af-106-123&sid=0001o71rHpVhXhEXWY5f8L9
    They have some cool stuff


    I'll have to reverse engineer it and see if I can learn something...

    Please let me know what you come with if you do not mind

    Thanks

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    ··Thanks for any·idea.gif·that you may have and all of your time finding them

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    Sam

    Post Edited (sam_sam_sam) : 2/29/2008 5:14:32 PM GMT
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