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OK to leave Li-ion battery in charger with no power connected?

Discussion in 'Electrical Engineering' started by Paul, Oct 6, 2009.

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  1. Paul

    Paul Guest

    After I have charged my spare lithum-ion camera battery I like to
    leave it in the charger with the mains disconnected.

    (It's a handy place to keep the battery.)

    In that sort of setup, would the disconnected charger still provide a
    circuit which slowly discharges the battery?
  2. Rich.

    Rich. Guest

    Most any manufacture will state in the instruction manual to NOT leave
    batteries in a charger. Leaving them in will do as you suspect, slowly drain
  3. Sjouke Burry

    Sjouke Burry Guest

  4. Modern battery chargers do NOT EVER overcharge. They have monitoring
    circuitry. It is YOU that is doing the assuming.

    So, one should not leave them in a NON-energized charger as that WILL
    deplete them. Leaving them in an energized charger will do no harm, as
    the charger tapers to nil once the charge condition is met, and resumes
    higher charging voltages when the criteria for charging is met. Whenever
    a charged battery goes in, nothing happens (charge wise). When a
    non-charged battery goes in, the charge cycle begins. There are
    typically two monitors in a modern charger, and two pairs of cells or two
    single 9 volt batteries can be charged at the same time, while each pair
    is separately monitored. Once the charge voltage is reached, that charge
    circuit goes into an idle watchdog mode.

    So the battery in an energized charger will remain "topped off". The
    non-energized charger will deplete a battery.
  5. James Sweet

    James Sweet Guest

    It depends on the design of the charger, but in most cases there is no
    harm in leaving the battery in place. Lithium battery chargers have
    fairly complex control circuitry that will isolate the cell(s) from
    anything else.

  6. It is not plugged in, so THAT AC input circuit is not complete.

    The circuit that battery is attached to may well be "complete enough"
    to trickle drain the battery.

    You need to continue your education. Hopefully, in a field other than
  7. Vince Ent

    Vince Ent Guest

    Could the charger have some sort of isolating device such as a
    capacitor in the charging circuit and thereby prevent discharge
    of the li-ion battery when the charger was not energized?
  8. James Sweet

    James Sweet Guest

    Batteries are DC, capacitors block DC, so that won't work. Really cheap
    chargers for less fragile NiCD, NiMH, and lead acid batteries use a
    diode to provide both rectification and isolation. In either case, a
    transformer provides isolation between the AC line and the battery.

    Generally Li-ion chargers use a mosfet to control current to the cell,
    and in multiple cell packs there are multiple mosfets to allow
    individual control of each cell. Li-ion batteries are notoriously fussy
    and require careful monitoring in both charge and discharge to keep the
    voltage, current, and temperature within a relatively narrow window. At
    any rate, when the charger is unplugged, the mosfets will be off, and
    present essentially an open circuit. Some Li-ion cells even have their
    own charge/protection circuitry built into each cell. These can be
    charged using a simple current limited power source and by nature are
    connected to their charger all the time.

    For those curious about the chargers, check out the datasheets for some
    of the common charge controller IC's out there. Texas Instruments,
    Intersil, and Dallas/Maxim among others have a number of products in
    that category.
  9. Probably an effect which is no more than the effective
    self-discharge of the lithum-ion battery pack, and so nothing
    to worry about providing you don't leave there for months
    with no use.

    If you leave the charger plugged in and it's a good quality
    charger, it may periodically do a top-up charge to counteract
    this. OTOH, it might not be a micropower standby product, and
    there's always a minisule extra fire risk (probably significantly
    less than from the lithum-ion battery itself though).
  10. A capacitively coupled DC charger? Sounds a bit more than a little

  11. Nonsense. It would have to be greater than no connection at all.

    Shelf life discharge is pretty slow. Slower even than a few microamps
    of discharge would be.
  12. James Sweet

    James Sweet Guest

    Lithium batteries have negligible self discharge. Unfortunately they do
    wear out just sitting there whether used or not, more quickly than NiCD
    and NiMH cells do.
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