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Battery charger for sealed lead acid

Discussion in 'Hobby Electronics' started by Arpit, Oct 8, 2003.

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  1. Arpit

    Arpit Guest

    Hi, ive had a powertech 12 volt sealed lead acid battery charger. It
    puts out 350 mA. Its a plugpack, and I bought it from Jaycar. It has a
    single LED, which flashes slowly (stays on) when charging, and flashes
    fast when trickle charging, apparently. I use it for charging a 7 amp
    hour battery on my bike for the headlight. THe headlight is a 55 watt
    number, so drains it pretty quickly. I looked at the data sheet for
    the battery, and it says the standby cahrge voltage is 13.5-13.7
    volts, and the cycle charge is 14.5-15 volts or thereabouts. I beleive
    I should be charging at the cycle charge rate, since im not leaving
    the charger on for months without using the battery- at most a week.
    How do I find out what voltage the charger puts out? A trickle
    charging battery measures 13.5 volts, open circuit reads 0.5 volts
    (it wants a battery), and connected to a capacitor (15000ufd), the
    voltage varies from13.5-15 in a fairly regular pattern. Theres a
    variable resistor on the circuit board (I cracked it open with my
    vice), as well as 4 transistors, 6 diodes, 2 electro caps, and a few
    resisors. ANy help/info/advice would be much appreciated as to what
    charger ive got, how its charging, what voltage its charging at, and
    whether I should get a new one. Thanks
  2. From what I've read, for maximum battery (cycle) life, you should have a
    maximum discharge rate of C/10. so your &amp hour is safe to drive a 8.4
    Watt (12V) light.

    Some people argue that C/5 is acceptable to modern batteries, so that
    give you 16.8 watts (Say 20Watt). This is what my wife bicycle lighting
    is (20W front + 6 Watt rear) on a 7amp hour battery. since she uses it
    for about 1 hour per week, I think it is a reasonable cost effective
    arrangement. Lasted over a year so far. Charger is one of the Powertechs
    that you mention.

    I have massively more lights, including a 50watt + 20watt, but I run the
    18 Amphour, which only gets hit for 30 minutes per day (5 days per
    week). The Powertech charger works okay. Again for a years use.

    My 2c is that you would only need another charger if you are not getting
    sufficent recharge between use. Although, with what you are doing, I
    would also log starting voltages so you can start to see when the
    battery is starting to fail.
  3. Russ

    Russ Guest

    I'm pretty sure it is correct to measure the voltage across the battery with
    the charger connected, so the 13.5v trickle charge looks to be correct.
    However, 55 watts at 12 volts is about 4.6 amps, which is pretty hefty for a
    gel cell - any reduction in life is probably going to be due to that moreso
    than charging factors.

    Since 7Ah cells are pretty cheap these days, I wouldn't worry prematurely
    since you can get a replacement for about $17-$20.

    I'd be more worried about motorists getting blinded by your 55w lamp -
    particularly if it's a 38 degree halogen.

  4. Arpit

    Arpit Guest

    Yeah, I was more thinking about time between recharges as opposed to
    whole life...
    Suffice to say Im visisble :p
  5. Arpit

    Arpit Guest


    What trends should I look out for? bearing in mind the battery sits at
    13.5 volts atm,
  6. With the lead acid batteries, they recommend taking them off charge,
    leaving them stand for an hour, then reading the voltage across the
    terminals. Compare to recommendations.

    I've done similar with SLA's with unknown history that have come into my
    hands. I give them a few charge/discharege cycles and measure each time.
    If they don't keep the charge, then they are cactus. I think it is <
    10.5V, then rubbish. Also easy to spot the ones getting dodgy.

    So, if you are charging them up overnight, disconnect and commute to
    work, measure the voltage on arrival at work and just log it. If you
    graph it, then over time, you be able to spot when it is going off.
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