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Max Charging time ?

Discussion in 'Electronic Basics' started by Charger, Jul 7, 2005.

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  1. Charger

    Charger Guest

    6v 4ah sealed lead acid battery (Light)

    Supplied charger PSU 12v 500ma

    No suggestion in instructions as to max charge time

    Your thoughts ??

    Ta

    (Tesco 2,000,000 candle torch / Superdrug 1,000,000 candle torch)
     
  2. Bob Monsen

    Bob Monsen Guest

    I just charged one of these. The charger supplied with mine is only
    300mA, so I used my bench supply to charge it at 450mA till it got to
    6.9V. It took about 15 hours from a dead flat battery to full charge.

    The 12V supply is a problem, since if you leave it on, it'll cause the
    battery to die due to overcharging. How hard could it be to design a
    simple CCCV supply, and build it into the thing, so you could just leave
    it on the charger? Oh, I guess they are are also selling batteries,
    aren't they? ;)

    --
    Regards,
    Bob Monsen

    If a little knowledge is dangerous, where is the man who has
    so much as to be out of danger?
    Thomas Henry Huxley, 1877
     
  3. Charger

    Charger Guest

    Opened it up, (the yellow Tesco version) there is an in-line 10R 5W
    resistor.

    Rather than try to calculate, I unsoldered a lead, put charger in
    circuit, measured 250 ma.

    So I presume charge time is around 8 hours, plus a bit extra.
     
  4. Bob Monsen

    Bob Monsen Guest

    I pulled the battery out of the flashlight before I charged it, but
    there may be a resistor in mine as well. Sadly, that doesn't help much
    with the overvoltage thing, because the current drops, making the
    resistor useless to drop voltage.
    Your battery is 4AH. Thus, at 250mA, minimum time is at least 16 hours.
    However, you generally get less than 100% charging efficiency, so it'll
    be longer than that. This presumes a completely flat battery, which
    isn't usually the case unless you've let it set for a while, or drained it.

    A constant current constant voltage source can be built using a 3 NPN
    transistors, a few resistors, and a TL431 shunt regulator. If you are
    interested, I'll post it. I use it to charge li-ion batteries, which are
    picky about final voltage. CCCV is the same strategy suggested by the
    manufacturers for sealed lead acid batteries, except that the actual
    voltages aren't so critical, and they suggest a final 'float' voltage
    that is lower than the constant charging voltage.

    --
    Regards,
    Bob Monsen

    If a little knowledge is dangerous, where is the man who has
    so much as to be out of danger?
    Thomas Henry Huxley, 1877
     
  5. Charger

    Charger Guest

    Maths 5/10 for me !!!
     
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