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VRLA versus flooded lead acid, absorption stage

Discussion in 'Electronic Basics' started by kell, Sep 11, 2005.

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  1. kell

    kell Guest

    I'm designing a battery charger to charge motorcycle batteries at 14.5
    or 15 volts during the absorption stage, when the battery draws one or
    two amps. Then, as a drop in the current drawn by the battery
    indicates to the charger that the battery has taken full charge, the
    charger will lower its output voltage to about 13.5 volts to float the
    battery.
    I wanted to build the charger for batteries ranging from 5 AH to 12AH,
    about the size of batteries I can use in my bike. (I can use a battery
    as small as 5 AH because the bike kick starts.)
    I tested a couple of lead acid batteries on constant voltage charging
    to see if the charging current drops off during the absorption stage,
    as I have read it does. I used a 7815 voltage regulator. I bulk
    charged the batteries, then put them on the regulator. I tested a VRLA
    first.
    With 15 volts at the battery terminals, a 7 AH valve regulated lead
    acid battery had a charging current that declined from more than an amp
    initially all the way down to 70mA before I disconnected it.
    Next I tried a 12 AH flooded battery. I equalized this battery about a
    week ago, so it had a pretty full charge when I started the test. At
    15 volts, this battery settled down and drew just over half an amp all
    afternoon, and didn't decline any farther than that.
    So at full SOC and after the current stabilizes, the flooded battery
    draws more than 7 times the current the VRLA does at the same voltage,
    although the flooded battery has less than twice the capacity the VRLA
    has.
    Either I need to build a complicated charger with a switch to select
    between two charging protocols, or I have to build a charger that
    functions much less than optimally on VRLA's in order that the float
    function kick in when I have a flooded battery connected to it.
    I just wanted to ask if flooded batteries always draw so much current
    even after they level off. I don't have any more batteries to test.
     
  2. Ban

    Ban Guest

    You can charge both battery types with the same method, but you have to
    strictly observe the maximum charging voltages.

    max. charging voltage(T=25°) should not be more than 14.4V in the absorption
    stage, even if the valve battery can use a higher voltage. The current
    should also be limited to <c/5, which means 1A for the 5Ah battery. A good
    adjustable regulator like LM317 can be used, which then can be switched to a
    lower voltage (13.8V) for trickle charge.
    You need a current monitor to do the switching, any high-side current
    monitor can be used (ZXCT1009, AD8210). The switchover should be at <C/10,
    meaning 500mA for the 5Ah battery, no need to change this for the other
    sizes.

    What you did(overvoltage 15V) will destroy the batteries fast and will
    deteriorate the acid level through gassing. You need to study the data
    sheets a bit deeper and get a better grip of electronic circuits or else
    just look for suitable circuits on the net.
     
  3. ehsjr

    ehsjr Guest

    You need to re-think the design. Use voltage level to
    change the charge mode, not current sensing. An op-amp
    comparator is a very easy, cheap and accurate circuit
    to detect a specific voltage and change output state.

    There are many different charger configurations available.
    The intent of this post is to focus on the op-amp
    comparator, but I also included one possible charge regulator
    for completeness. It is the simplest charge regulator
    configuration you can build, consisting of just two parts:
    an LM350 and a resistor. Since you want a step in the
    charging algorithm (rightly), one extra resistor is included,
    and it is selected by the comparator circuit. Finally, one
    diode was added across the LM350 to protect it in the event
    of a power failure while charging.

    Here's a comparator using 1/4 of an LM324 that drives
    a relay. You adjust the pot (100K) to transfer the relay
    at the exact voltage you select. The comparator circuit
    is connected to the battery terminals. The relay contacts
    are connected to the regulator, shown in the second diagram
    below. You don't need to use relay output - there are
    many variations. I chose the relay output for simplicity.


    + -----+---+---------+-------------+----+
    | | | | | POT = 100K
    | | +----- | ----+ [D1] [RY] D1 = 1N400x
    | | | | | | | RY = 12v relay
    | | | |\ | [R3]10M +----+
    | P | | \4 | |
    10K [R1] O--+--3|+ \ | e/
    | T | >1---+----[R2]---| PNP
    | | | / 1K c\
    +-- | ----2|-/11 |
    | | |/ | |
    6v [Zd] | LM324 | |
    | | | |
    Gnd ---+---+---------+------------------+


    Here's a circuit to regulate the charge *current*
    in the steps you select. I'll assume 2 amps for the
    first step - you can select whatever you want for
    the float current. This circuit connects to the
    charging source and to the battery.

    1N400x
    +--------|<----------+
    | |
    | ------- |
    + ---+---in| LM350 |out---+
    ------- |
    Adj [R4] .6 5W
    | |
    | +---o o
    | | \ RY-1
    | [R5] o
    | | |
    +----------+----+
    |
    [BATTERY]
    |
    Gnd -----------+

    The LM350 is configured as a constant current regulator.
    R4 - .6 ohms - programs it to produce 2.08 amp when the
    relay is de-energized and R5 is shorted out. When the
    voltage rises to the point set by the 100K pot, the relay
    energizes and R5 is placed in series with R4. You determine
    the float current (I) you want, then figure the value of R5
    by R5 = (1.25/I) - R4 and pick the closest standard value
    resistor. For example, a 5 ohm resistor for R5 would limit
    current to ~223 mA; 10 ohms would limit it to ~118 mA.
    Wattage is computed by (I^2*R) - add a safety margin of
    at least 50%

    Ed
     
  4. mike

    mike Guest

    I like a good technical discussion as much as anybody. But consider this.
    The first time you press the starter button, that tiny motorcycle
    battery is gonna
    have it's guts wrenched out. Then it's gonna have the crap charged out
    of it when you drive off. All the care and precision you put into
    charging it won't make much difference ten miles down the road.
    Even if the only bike you have doesn't electric start, the problem
    can be much the same. Motorcycles are NOT easy on batteries.

    I put hundreds of hours and hundreds of dollars into building computer
    controlled charger/discharger/graphing hardware and software. I've
    watched nicad and NIMH and LiIon batteries charge and discharge for
    hours on end. I know a LOT about how they (mis)behave. But at the end
    of the day, about all I accomplished was to busy myself for hundreds of
    hours. Turns out that the outcome is much the same over a wide range
    of charge parameters.
    For the investment, I could have replaced all my batteries several times
    over.

    Sometimes, all you need is any old charger and an egg timer to tell you
    when to shut it off.

    mike
     
  5. Ban

    Ban Guest

    The question is if the bike has a proper regulator or not. Of course the
    cheapest have just a diode in the alternator and overcharge the battery and
    thus reduce its lifetime considerably, but it doesn't need to be like this.
    If your battery is *not* overcharged, because the bike has an electronic
    regulator, the battery will last even a couple of years and thus you might
    save a significant sum in replacement parts.
    Another question is how much will a simple 2 phase charger cost more than
    just a transformer with rectifier? LM317 costs 0.6$, the current monitor
    with comparator will be less than 2.50, and the additional bigger
    transformer and caps will be maybe 3 bucks. So the whole "fancy" technology
    costs less than 10$ additionally with a piece of veroboard to solder the
    parts. How much comes a new battery? 15-25 bucks?
    So is it worth the effort? The decision would be clear to me. When the bike
    stands in the garage most of the time you will have to add the comfort of
    always having a full battery without leaking sulphuric acid etc.
     
  6. john jardine

    john jardine Guest

    Nice one Mike!.

    In a simialr vein, have just been tasked to *de*-design a sealed lead acid
    charger. The existing '4 stage' TI (UC????)chip based switcher design is
    just too complicated/expensive/unreliable for use in the outback. Customer
    needs reliability/cost rather than squeezing in last few % of possible
    charge. Sometimes fancy technology is its own worst enemy.
    regards
    john
     
  7. ehsjr

    ehsjr Guest

    Exactly. Use a basic, simple, current requlator. Two
    stinking parts. Shut the charger off when done. If the egg
    timer won't work ("But I might not be there"), "done" is
    *easily* sensed with a comparator. Case closed. But you
    can't sell that idea to those who want to get the last ounce
    of perfection out of their chargers.

    Ed
     
  8. kell

    kell Guest

    cool off, brother.
    Same goes for the rest of you aggressive yoodles.
     
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