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picking switch-mode battery chargers

Discussion in 'Electronic Design' started by ssylee, Dec 31, 2007.

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  1. ssylee

    ssylee Guest

    I'm planning to buy chargers for two types of sealed lead acid
    batteries: one for a 12V, 4Ah battery, another one for a 12V, 1.5Ah
    battery. I'm planning to buy from Mouser Electronics. From their
    catalog shown in , I'm not
    sure about the criteria in which I should be choosing between each
    charger. Does anyone know how I should be picking the chargers?
  2. krw

    krw Guest

    Dead link.

    One SLAC charger should be able to charge either type. Lead Acid
    batteries should be charged with a temperature compensated constant
    voltage source. It should sense the terminal voltage and switch
    to a float charge at something like 95% charge. SLACs should last
    ten years if treated properly. Don't skimp on the charger.
  3. gearhead

    gearhead Guest

    Manufacturers of small SLA's recommend limiting the charging current
    to 30% of the battery's capacity. So for the 1.5 Ah battery, a .5 amp
    charger would do. For the 4 Ah battery, get a 1 or 1.5 amp charger.
    You don't have to adhere to that 30% figure rigidly; it won't kill the
    battery to go a little over, and going under just means it will take a
    little longer to charge.
  4. ssylee

    ssylee Guest

    Thanks for the tips. According to the catelogue choices, I personally
    think there's no point of me of buying a 0.5 A and 0.8 A charger
    separately. I'm planning to use a .5 A charger for both, although it
    will take the 4Ah battery about 8 hours to charge.
  5. Bob

    Bob Guest

    No, lead acid battery charging is about 66% efficient, it will
    take about 12 hours.

    The charger range you gave a link to is a boost+float type acording
    to the brief description so it should get the battery up to close to
    a full charge fairly quickly.
    On float charge the current does not stay at the maximum capacity
    of the charger then suddenly drop to zero when the battery is full,
    the current drops slowly over a few hours.

    I had one of those huge torches ("flashlight" to merkins) that
    have a 100watt bulb and a small lead acid battery inside.
    The unregulated wall wart charger produces a rarther
    high voltage with little load. The torch was left on charge
    for weeks. No wonder the battery failed after a few months.

  6. neon


    Oct 21, 2006
    Why buy two chargers when one will do both jobs just buy a 6 amps charger add a diode in series with the battery. it is absolutely not true that batteries needs voltage to charge as long as the voltage is above floating voltage. it needs amps as specified too much for too long they get hot adding to short time failure. it is ridiculous to control the voltage what for my 12v battery is stady at 14,7v my other car is 13,7vLEAD ACID BATTERY LOOSE CHARGE every hour on the hour a floater it just sense a voltage set and recharge at some min current. that sucker will never charge a discharged battery in a week. it is just there asa peaker for lead acid losses.
    Last edited: Jan 1, 2008
  7. ssylee

    ssylee Guest

    Just got the battery charger. I've managed to charge my 12V 4Ah
    battery in half an hour with the 500 mA charger, which I expected to
    take longer. I got a suggestion that it may be charged beforehand and
    to get the runtime, I would have to put a load on it. I removed this
    battery from an alarm system that is saying I have low battery. Does
    it mean the battery would have to be replaced as well?
  8. ehsjr

    ehsjr Guest

    Your gel cell was either almost fully charged, or it
    is sulfated and needs to be replaced, assuming your
    new charger is working properly. If you are interested
    in digging in to learn a little more about your battery
    read on. If not, the most likely problem is the gel
    cell - just replace it.

    Here's a circuit you can use to discharge a lead acid
    battery to a specific voltage with a known load, to get
    a good runtime test.

    +----------------------------------o o---+
    | | | |
    | S1 |>| |
    | __ |
    +--o o---+--------+------------+---|<---+
    | | | | |
    | [1K] [1K] | |
    |+ | | /e o
    B | +---[1K]---| 2N5401 To Load & clock relay
    A P | a \c o
    T 1K O<----[TL431] | |
    T T | +----+ |
    | | | | | |
    | | | [D1] [Rly] |
    | | | | | |

    Use a heavy duty automotive relay with an 88 ohm
    coil. The circuit, with no load connected, will
    draw about 140 mA from a 12 volt battery, starting
    when S1 is pressed and continuing until the battery
    discharges to the voltage set by the 1K pot, at which
    point it will turn off and draw absolutely no current
    from the battery.

    A second relay can be connected to the load terminals,
    and that relay's contacts can be used to turn a clock
    on and off. That way, you can automatically time how
    long it takes to discharge the battery. You can (and
    should) add a load resistance at the load terminals
    to place the load you want on the battery.

    Set the 1K pot to turn the circuit off when the
    voltage drops to 11.5 volts. That protects the gel
    cell from being discharged too far. Add a load
    resistor and second relay such that the total draw
    is around 700 mA. Connect the contacts of the second
    relay to turn a clock on and off. Set the clock to
    12:00 and press S1. Go about your business.
    When you come back to the circuit, the clock will
    display the elapsed time, and will stop running when
    the circuit shuts off. You can add an LED and
    resistor to the circuit to indicate when it is running
    if you like.

  9. neon


    Oct 21, 2006
    fool.lET ME TELL YOU THIS if you are measuring voltage across the battery you are wasting time. my previous entry mentioned current device that what a battery is a low inpedance current supply. a bad battery 12v can show 18volts and more and all the voltage disapear as soon as you put a load to it. for a simple 1.5v A battery to see if it is good apply a load of .040 amps then measure the voltage for a 9v apply a 25ma load then measure the voltage otherwise you are waisting time and effoerts. for 12v hi capacity that load depends on the amps/hour attention of what you read it can be inportant. for 1/2 hour charge you probably got yourself a 15 minutes usefull battery you are charging @6 watts rate get the battery hot then next month throw it away. amature.
    Last edited: Jan 5, 2008
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