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What do dead Nicads do ?

Discussion in 'Electronic Basics' started by LED Man, Feb 27, 2005.

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  1. LED Man

    LED Man Guest

    I have a solar powered torch, battery 2.4v 280mah nicad, hard pcb wired.

    I am thinking of wiring up a second nicad battery 750 mah in parallel.

    My question, when one nicad, probably the 280 mah dies, what happens ?

    Does it go open circuit, closed circuit, how does it affect charging and
    useage of the higher capacity nicad

    Any experience of same ??

    TIA
     
  2. Don Bruder

    Don Bruder Guest

    Depends on failure mode - Did it die of pure old age? It probably went
    short, then. Did it die of abuse (rough handling/vibration?) Likely it
    went open, but it's impossible (without having it in hand, anyway) to
    say it didn't short.
    Gonna depend on how it fails. If open, probably very little, if any,
    effect. If short, it'll probably drain the other one quickly, and/or
    prevent charging at all.
    No actual experience this way, but common sense says it isn't likely to
    be a good idea. I'd pick one or the other, and use that one exclusively.
    No point in messing around, I'd say...
     
  3. CWatters

    CWatters Guest

    It's not generally a good idea to connect two voltage sources in parallel.
    If the voltage of one is slightly different to the other (due to charge
    state) what limits the current.....Hint I=(V1-V2)/R but R is very small if
    it's wire.
    What do you mean by "dies"? flat? old? faulty?

    Most likely the larger battery will supply all the current when the smaller
    on is empty but batteries can be unpredictable. Mostly it works if you
    charge them both before connecting them but watch out for melted wires and
    fire if one is empty!
    With two voltage sources in parallel the results are unpredictable. Old
    batteries tend to have a slightly higher voltage on charge so most of the
    current may go into the larger external battery leaving the internal one
    uncharged/less charged. If the larger battery is a different make it might
    have a higher on charge voltage and all the current may go into the smaller
    internal battery. Personally I would remove or disconnect the smaller
    internal battery.

    Caution: I've see batteries explode due to missuse. It's can be dangerous.
     
  4. LED Man

    LED Man Guest


    Ta
     
  5. PeteS

    PeteS Guest

    As other have noted, depends on dies. If we are speaking simply of
    discharging, then the (fairly) simple answer is that as the smaller
    capacity battery gets more discharged, it's terminal impedance
    increases, and the other battery supplies more of the current. Note
    that NiCads in this state have rather horrendous leakage
    characteristics, so the smaller battery may be taking charge from the
    other battery too. In effect, the larger battery is charging the small
    one, to a certain extent. Parallel batteries should always be of the
    same type (type, construction and capacity).

    Other things to watch out for are the exact type of battery. There are
    different types of NiCad (optimised for a particular issue) and it is
    very unwise to mix them. High temperature types, for example, can't
    supply very much discharge current (<C/10 for some of them). The list
    is rather lengthy.

    Charging is a real issue - for ordinary (see note about different
    types) NiCads, standard charge is C/10 for 14 hours (where C is the
    capacity) so the 280mAh battery would need 28mA, but the larger one
    75mA. If they were in parallel, it's very difficult to know the
    effective terminal impedance of each (meaning you can't really properly
    regulate the charge).
    Some NiCads can never be fast charged (normally C/3 for 4.5hours)
    although some can be boost charged (C rate for about 2 hours), but
    again, you can't mix capacities in a charger without the distinct
    possibility of killing the battery(ies), with a possibility of making
    them come apart at the seams quite spectacularly.

    Cheers

    PeteS
     
  6. LED Man

    LED Man Guest

    Ta
     
  7. Doug McLaren

    Doug McLaren Guest

    | | > I have a solar powered torch, battery 2.4v 280mah nicad, hard pcb wired.
    | >
    | > I am thinking of wiring up a second nicad battery 750 mah in parallel.
    |
    | It's not generally a good idea to connect two voltage sources in parallel.
    | If the voltage of one is slightly different to the other (due to charge
    | state) what limits the current.....Hint I=(V1-V2)/R but R is very small if
    | it's wire.

    It's not so bad with two rechargable batteries (as long as it's the
    same chemistry of cells, and each battery has the same number of
    cells) in parallel. At least not with regard to discharging ...

    | > My question, when one nicad, probably the 280 mah dies, what happens ?
    |
    | What do you mean by "dies"? flat? old? faulty?

    For the record, most NiCd and NiMH cells seem to die open rather than
    short.

    | Most likely the larger battery will supply all the current when the
    | smaller on is empty but batteries can be unpredictable.

    This isn't so unpredictable -- it's generally the way it works.

    | Mostly it works if you charge them both before connecting them but
    | watch out for melted wires and fire if one is empty!

    Nah. If one is empty, the current coming from the full NiCd/NiMH cell
    will bring it's voltage up to match the full one very quickly, and
    then the current will stop, even though one battery is almost
    completely empty and the other is almost completely full.

    (Corilary: You can't tell how full a NiCd or NiMH cell is just based
    on it's voltage. Adding a small load helps somewhat, but even so, you
    really can't tell just based on voltage.)

    | > how does it affect charging
    |
    | With two voltage sources in parallel the results are unpredictable.

    Charging will be totally screwed up. What will generally happen is
    hinted at what I mentioned earlier -- one cell will get most of the
    charge and the other won't get charged much at all.

    It probably won't melt down, unless you charge at a really high rate
    expecting equal amounts to go into each battery, but you're likely to
    really only charge one battery.

    | Old batteries tend to have a slightly higher voltage on charge so
    | most of the current may go into the larger external battery leaving
    | the internal one uncharged/less charged. If the larger battery is a
    | different make it might have a higher on charge voltage and all the
    | current may go into the smaller internal battery. Personally I would
    | remove or disconnect the smaller internal battery.

    That's probably best.

    Note that this applies to NiCd and NiMH cells. I do lots of R/C plane
    stuff, and putting NiCd and NiMH packs in parallel works fine for use,
    but you need to remove them and charge seperately.

    For LiPo and Pb cells, you *can* put them in parallel and you can even
    charge them like that and it works fine, but you want to make sure
    that the voltages are similar before you connect them (otherwise it'll
    melt down like Cwatters said. It generally won't melt down with NiCd
    or NiMH, but will with LiPo and Pb batteries.) But once connected,
    you can just treat them like a larger cell.

    | Caution: I've see batteries explode due to missuse. It's can be dangerous.

    Yup. Especially the Lipo/LiIon cells.

    In any event, I agree with Cwatters -- remove the original battery,
    and just use one.
     
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