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li-poly battery protection for high cell count packs

Discussion in 'Electronic Design' started by Michael, Sep 14, 2007.

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  1. Michael

    Michael Guest

    Hi - in the near future I will need to implement a circuit that
    monitors and protects a lithium polymer battery pack that has about 12
    cells in series in it. There are alot of special purpose chips
    designed to protect lithium polymer battery packs consisting of 2-4
    cells - but I've never seen anything for this many cells. Does anybody
    know of such a beast?

    If not - how best would I go about making this? Size is a huge factor,
    cost is not nearly as much of one.

    Thanks!

    -Michael
     
  2. linnix

    linnix Guest

    You can do it with a uC and 12 channels A2D. The important thing is
    to monitor shorts in any one cell and disconnect charging. Otherwise,
    you have a good detonator.
     
  3. D from BC

    D from BC Guest

    I'm no expert on Lith batts..
    But how about a temperature sensor on each battery.
    12 thermistors + 12 comparators + 12 input OR gate + shut down
    electronics. Bulky but dodges uC's.

    (I hope I don't get too beat up on SED for that suggestion. :| )


    D from BC
     
  4. There is an application note on the Maxim site, for using their single
    cell monitor IC, to monitor multiple cells. The same techniques can be
    applied using one of the four cell monitor IC's, to handle larger packs.
    Some applications using packs with large numbers of cells like this,
    actually split the pack, for charging, so having the pack in (for
    instance) three 'four cell' modules, and charging these with their own
    monitor IC. This is also useful, since it allows charging off lower
    voltages.
    Most people doing this 'in circuit', will almost certainly use a cheap
    micro, to do the whole job.

    Best Wishes
     
  5. budgie

    budgie Guest

    you probably should ;-)

    The protection modules guard against a number of conditions. Temperature can be
    one, but is not always included. The most important functions of these modules
    are undervoltage, overvoltage, excessive discharge current. Often excess charge
    current sensing is also included.
     
  6. D from BC

    D from BC Guest

    Oh crap...
    Ok..how about
    12 differential amplifiers for V monitoring on each battery.
    +
    12 I sense resistors..
    +
    12 differential amplifiers for I monitoring on each battery.
    +
    A 24 to 1 analog demux to an ADC luckily all in a uC.

    But I think this can be reduced..

    (2) 12 to 1 analog demux's for differential input A
    (2) 12 to 1 analog demux's for differential input B
    (1) differential amplifier used for voltage monitoring.

    Current sensing will need only 1 12 to 1 analog demux.
    (One of the voltage sensing channels will be used as a common. )
    So..Only one differential amplifier for current monitoring.

    It starts off with something like that....lots of channels anyways...


    D from BC
     
  7. Gary Tait

    Gary Tait Guest

    You don't need differential anything, just a number of single ended A/D
    inputs connected to each node, calculating the voltage across any two
    nodes in software. Since the batteries are in series, there is only one
    current path to read, per bank.

    If the packs are externally charged, it could be rigged to charge in
    series paralell, and discharge in series.
     
  8. D from BC

    D from BC Guest

    :p ... Ok...I'll try again ..How about?

    Let's say each battery is individually charged so that each cell
    current can be monitored.

    +--repeat to cell 12
    |
    +-cell3-+-Rsense-
    |
    +-cell2-+-Rsense-+
    | | |
    | Vcheck Charge Isense
    |
    ---cell1-+-Rsense-+
    | | |
    Com Vcheck Charge Isense

    ewwww.... This looks aweful...
    Glad I don't have this problem... :p


    D from BC
     
  9. budgie

    budgie Guest

    (snip)
    in which case a readily available 4-cell chip would provide the basis for the
    monitoring side of protection.

    One complication is the readily available protection modules normally interpose
    a series "switch" as their method of protection/intervention. For S/P charging
    that would be fine, but for series discharge you would (probably) wind up with
    N/4 series switches, all of which present a finite impedance, and which is sort
    of suboptimal. Generally these switches can't be isolated from their driver
    which ties them back to their group of cells, and the drivers can't readily
    interface with a common switch.
     
  10. Michael

    Michael Guest

    I don't think this is true. If I used 1% precision resistors for my
    voltage dividier, my measured voltage would be accurate to +-500mV or
    so, assuming a perfect ADC with infinite resolution.

    -Michael
     
  11. Michael

    Michael Guest

    I can't seem to find this app note. Can you point me to it? Thanks,

    -Michael
     
  12. Michael

    Michael Guest

    How readily available are these chips? I had thought that I had seen
    them before - but now I'm going through all the usual suspect's
    websites and I'm turning up essentially nothing. The only chip I've
    found that looked reasonable was the MAX1666: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2022
    - but that part appears to hold true to the Maxim standard - it's not
    available - anywhere.

    -Michael
     
  13. budgie

    budgie Guest

    Ironic, that old maxim ;-)

    As an aside, we used a Maxim controller chip in one lithium charger design, and
    had the devil's own job getting supplies. Wound up using an offshore supplier
    who could source them well ahead of the next Maxim scheduled run. It appears
    that maxim's policy is to design/develop/prototype achip and then announce it.
    When/if enough orders are to hand, a run will commence. until then, it is
    really vapourware.

    On protection modules, we looked at TI/ Benchmarq but wound up using
    preassembled modules (Powerlogics PLM4005) from a Korean fab, the Powerlogics
    PLM-4005. (www.powerlogics.co.kr) which used a MM1294A from Mitsumi and a
    Vishay S14431DY power MOSFET as the series switch.

    From my notes and their doco at the time:

    "The PLM-4005 is designed to open circuit the pack terminals in the event of:
    · undervoltage (on discharge) - <2.35 +/- 0.1V [hysteresis 0.7V]
    · overvoltage (on charge) - >= 4.35+/- 0.05V [hysteresis 0.2V]
    · excess pack current (charge or discharge) - > 5 +/- 0.5A.
    · voltage difference between cells exceeding (sorry, indecipherable)
    These parameters are not user adjustable.

    The protective action of the module is non-destructive. The connection between
    cells and external pack terminals is re-established when the condition causing
    the intervention is no longer "out-of-bounds". To achieve this, the module
    continues to monitor the external load condition and cell voltage while the
    "open circuit" condition exists."

    Another lead was the PS401 from Powersmart.

    One version we considered but never actually evaluated (if the difference makes
    sense) was from International Components Corporation. Another used the Mitsumi
    MM1414 chip, might pay to look at their range if you are planninga possible
    build of your own module.

    I hope those give you some leads.
     
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