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Battery and Charger Strategy - any ideas?

Discussion in 'Electronic Design' started by [email protected], Apr 17, 2007.

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  1. Guest

    Our product is an automotive battery tester which recharges its
    internal 6V SLA battery from the same clips which perform the battery
    test. Owners tend to clip the tester on a spare battery when driving
    between breakdown service calls, and it takes a "drink". Internally,
    we have temp-compensated voltage charging, so the SLA is charged
    quickly and prolonged charging only floats the battery nicely. If the
    SLA runs flat, you can get testing again after a few minutes clipped
    to a battery.

    Now we have to replace the SLA with Li-Poly or NiMH or other non-lead
    technology, but it gets tricky.

    NiMH batteries tend to be used in cycle use, where you fast charge up
    to a voltage inflexion or delta temperature. Trouble is, we can't
    initialise a fast charge every time the tester connects to a battery
    because 50 fast charge cycles a day will destroy the battery!

    GP Batteries rate NiMH cells for 0.1C trickle charge over long times,
    so one option is to charge at 0.1C whenever the clips are connected.
    Unfortunately, this means a 16 hour charge time from flat, whereas the
    SLA got 90% charged in an hour or so.

    Li-Ion or Li-Poly also tend to be used in cycle mode. They do voltage
    charge, like the SLA. But they don't like to be at full voltage
    indefinitely - you float for 3 hours after charge, then stop. Trouble
    is, when the clips are connected, you don't know if you are continuing
    a previous long charge, or if the battery is ready for the 3 hours.
    In addition, keeping a Li-Poly at full voltage tends to reduce its
    capacity, and mechanics tend to keep the tester on charge. I wonder
    if we can reduce charge voltage slightly and put up with 85% of
    capacity, to lessen the capacity deterioration effect when constantly
    under charge.

    I can think of clever microprocessor schemes, such as coulomb
    counting, but we'll need field trials and the project is "interesting"
    - ie. will have unexpected, time consuming and costly lessons to teach
    us. Also spark plug pulses will reset the processor at times. The
    coulomb counting IC typical circuits seem to show horrendous
    quantities of parts.

    I'm not keen on Li-Ion or Li-Poly, because of the brutal handling by
    mechanics. Testers reach 60C sitting under the windscreen, get driven
    over, dropped into the fan, zapped by spark plugs, connected to 240V
    mains, and we can't tolerate a single battery combustion incident.

    Please feel free to ask questions, make suggestions, or solve the
    problem!

    thanks,
    Roger
     
  2. Eeyore

    Eeyore Guest

    Why not incorporate a microcontroller which looks at the battery's charging
    'history' ? It also ought to be possible to get a reasonably accurate figure for
    its state of charge. Combine these data and you can establish the best charging
    regime at any point in time.

    Graham
     
  3. Eeyore

    Eeyore Guest

    Not if it's properly designed.

    Graham
     
  4. MooseFET

    MooseFET Guest

    On Apr 16, 9:37 pm, wrote:
    [...]
    I assume that the device has a micro in it etc. that remembers things
    and uses the battery as a power source to hold that memory up.

    I don't see any problem with having the micro also remembering the
    time of the last charging of the battery and other stuff like that.
    You can gate the charger onto the fast charge mode only if the
    estimated state of the battery is nearly discharged.

    You can charge at a very low rate almost all of the time. I don't
    think having two levels of charging makes the circuit much more
    complex.

    I'm thinking of a circuit like this:

    Two bits from the micro run to level shifters that make 0 to +12V gate
    drive signals of them.

    Two N - MOSFETs with source resistors that will bias on a couple NPNs
    if the current tries to go too high. Note: I assuming the positive end
    is connected directly and the neg. is the one switched. Turn stuff
    around if you can't do this.

    The NPNs pull down the gate drive of the MOSFETs to regulate current.

    When the external battery is disconnected, the body diodes of the
    MOSFETs conduct allowing the battery to take over.

    An ADC channel in the micro circuit will monitor the battery voltage
    so the micro can decide.
     
  5. john jardine

    john jardine Guest

    [...]
    [...]


    What's the "have to" business?. Is it specific to your industry, or are we
    all going to be buggered by new regulations?.
    Personally I wouldn't choose to specify anything other than sealed lead
    acid. It's Victorian technology but a damned sight more
    reliable/predictable/cheaper than the newer fluff.
     
  6. I would not say this is always the case. Qutie a lot of work regarding
    NiCd behavior was done for space missions, for example, and those
    applications did not generally involve full cycles or fast charges.

    I am aware of three types of charge that NiCd and NiMH can commonly
    accept:

    * Fast charge, 1C or more, with appropriate dV/dt or dT/dt termination

    * C/10 charge, for several hours after it's full but not forever

    * C/300 charge forever

    You can get a fairly good impression of the state of these cells from
    the voltage, perhaps temperature compensated - certainly better than you
    can get from a lead acid using just the voltage. It seems like the best
    approach for you is to set a charge cutoff below which you will perform
    a fast charge and above which you will perform a slow charge.

    I would recommend NiCd, which is more robust in most ways than NiMH, and
    is probably the most robust and least expensive technology remaining
    once you eliminate lead acid for whatever reason is forcing you to do
    that.

    If you can accept less than a full charge, you can charge to a
    (temperature-compensated) voltage level; you can't get to 100% using
    this technique like you can with lead-acid, though, because the voltage
    starts going back down for reasons my chemistry is too weak to properly
    understand.

    Forget about lithium; the safety issues will double your engineering
    work in exchange in an application where none of the advantages matter
    much. If you have to go with them, look at A123's cells.
     
  7. Guest

    That's what I called "coulomb counting". Laptop battery monitors do
    this. Its the development time, parts count that put me off. Its
    still an option, perhaps in simplified form.
     
  8. Guest

    NiMH are more like NiCd than before, so much of your NiCd ideas now
    apply to NiMH. GP Batteries NiHM specs permit C/10 for 12 months, C/
    20 forever. The NiCd space application angle is a good idea - I look
    up info.
    Lead and Cadmium are out, this is for EU.
    Thanks. I think something similar could work with NiMH. Switches to
    C/10 when voltage comes up above a set point, below that current
    increases to C/3 etc. Temperature compensated.
    Your advice much appreciated.
     
  9. Rich Grise

    Rich Grise Guest

    Don't car batteries have some kind of exemption? Couldn't you get a
    similar exemption because you're in the car battery maintenance business?

    As far as a replacement - I'd go with Ni-Cd - you've got good cold
    cranking amps, and they're easy to keep charged. I don't trust any o'
    them newfangled batt'ry tecknowledgies, y'know! ;-)

    Or have they banned cadmium too?

    I guess we'll see the asymptote when they finally ban carbon itself. >:->

    Good Luck!
    Rich
     
  10. Guest

    RoHS lead and cadmium are out. We had a good run with SLA.
     
  11. Guest

    Wrap the spark plug leads around almost any appliance and it will
    crash or break. With thousands of units in the field, processor
    crashes are my assumption.Yes, a watchdog is required. Yes, you can
    shield stuff at a price.

    The problem is that I replace a switchmode chip and a few parts with a
    fairly sophisticated solution. Time and money. Plus politics.
     
  12. Eeyore

    Eeyore Guest

    It's not actually very difficult. Microcontrolelrs typically have A-D converters on
    board now. Add a serial EEprom for data backup and you're away.

    Graham
     
  13. Eeyore

    Eeyore Guest

    A watchdog is simply good practice !

    Shielding after the event is the wrong way to do it. You make the equipment
    tolerant of the environment *by design*.

    I don't follow you.

    Graham
     
  14. Eeyore

    Eeyore Guest

    Not in batteries they're not. Whatever gave you that idea ?

    Graham
     
  15. Eeyore

    Eeyore Guest

    You apparently misunderstand the relevant EU legislation.

    Graham
     
  16. Guest

    My terminology was wrong. RoHS does not apply to batteries, rather the
    EU Battery Directive:

    http://eur-lex.europa.eu/LexUriServ/site/en/oj/2006/l_266/l_26620060926en00010014.pdf

    Our batteries are sealed, user replaceable, hardly "industrial", which
    means cadmium is out as I read the above doc. In any case, we are not
    going to leave lead to go to cadmium!

    However, you may be right about lead - maybe we have a few more years
    left with lead - my boss and EU distributor are calling the shots on
    this, so I'll check the story with them.

    Roger
     
  17. Guest

    On Apr 18, 1:20 pm, Eeyore <>
    wrote:

    This is get it right first time. We have found that a new battery
    solution requires 2 years of field use before the real story comes in,
    and it can mean 100% product replacement. Temperature, battery life
    under wierd charging patterns, battery explosions, user expectations.
    If I can leave out a processor, I will. The main processor is not
    available for the job.

    This is a cost controlled PCB in a plastic case.

    I'm removing risk and cost factors before I start. I not going to be
    clever. This isn't a $70K project, its a $2K project. That is why I
    appreciate all the insights I can get from you people - and the
    challenges to my assumptions.

    Roger
     
  18. Guest

    I don't claim to understand it!

    Our EU distributor is pushing this. The new EU Battery Directive
    provides for equipment manufacturers to "participate" in the cost of
    battery recycling, as each member state sees fit. Over the whole of
    the EU, this looks like a wide open exposure to the distributor - he
    gets nervous.
     
  19. For $2k you can hardly afford to make a few prototypes, much less pay
    for any engineering time. I respect your willingness to try, but I
    would consider recognizing up front that a project with your stated
    reliability requirements is simply not going to happen on a budget like
    that, and not waste your time trying until adequate resources are
    available. Of course, if the company politics dictate that the only way
    you will ever get the resources is to start and then go overbudget...

    Your best bet is probably to find a firm which has already developed the
    technology and license it from them.

    I saw elsewhere that you are primarily concerned with
    recycling/disposal; my other comments regarding cadmium mostly apply to
    NiMH as well.
     
  20. ehsjr

    ehsjr Guest

    Time for re-design that does not require any nternal battery.
    Run your circuit from the battery under test. Charge up a
    supercap from that battery if it needs memory between events.
    You'll probably need to re-design for low current. If an
    automotive battery can't provide enough power to drive your
    circuit, it's shot anyway - no need to test further.

    Ed
     
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