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24 volt battery charger

Discussion in 'Electronic Design' started by Jim Crow, Aug 14, 2006.

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  1. Jim Crow

    Jim Crow Guest

    I am trying to design a 24 volt central battery emergency light system
    for a college project and finding it harder than I thought. Can anybody
    help me with information and diagrams for the charger system and a low
    volt cut out, or reccommend some good sites to visit. Thanks.
  2. This isn't the best way to do a college project, is it? If you ask about
    a specific problem, you will get help here, not spoonfed the answers but
    help to work them out for yourself, which gets you credit. No credit for

    What sort of battery is it? If it's deep-cycle lead-acid, the
    manufacturers' web sites and literature give you a lot of information
    about care and feeding. You can often ask their Technical Support.
  3. Jim Crow

    Jim Crow Guest

    I am not asking for the answer to be SPOONFED, I am after general
    information to be able to start my project. I have checked
    manufacture`s sites and many other sites, but they all seem to be
    related to 12 volt car batteries and chargers and many are far too
    complicated for my needs.
  4. National Semiconductor has an application note
    for the LM317 linear regulator with a neat battery charger example in
    it. Once you understand that circuit, it is easy to modify to 24 V,
    higher currents, design away the LM324 for entirely discrete design or
    whatever your home work involves.
  5. Good, but you query gave me that impression.

    I asked you to tell us what sort of battery you are using. If you do
    THAT, we can direct you to sites with information, and even add to it.
    Without knowing the sort of battery, we can't help.
  6. Jim Crow

    Jim Crow Guest

    I have looked at some battery manufacturer sites and have think I
    should use 2 x 12 volt 7Ah Yuasa batteries which are deep cycle AGM
    VRLA type.

    How do I know what current to start charging these at, is it 1/10th of
    the output ?
  7. AJ

    AJ Guest


    I have used the UC3906 for charging and the MAX8212 for low voltage cut off
    in the past for a lead acid battery charging application. Checking the
    datasheets on these products would possibly give you some good clues on how
    things can be done..................... Assuming that you are using lead
    acid (I haven't checked the voltage rating on these parts).


  8. I can't find a 7 Ah deep cycle VRLA battery on the Yuasa site. What is
    the Yuasa part number? 7 Ah seems to be a bit low on capacity. How many
    lights are you running, for how long and what current do they draw?

    You won't get 7 A for 1 hour, or even 3.5 A for 30 minutes; that's a
    20-hour rating (350 mA for 20 hours, which is just about four
    There is some flexibility. If we can identify the exact product you
    need, the data on charging is available from Yuasa web site.
  9. Jim Crow

    Jim Crow Guest

    The Yuasa battery I saw was a NP7-12 which it says is for strand by
    operation, it is to feed about 20 x 24 volt 8 watt fluorescent slave
    units for a minumum of 1 hour. I have had a look at the datasheet for
    this battery but do not fully understand all details it specifies.
  10. Jim Crow

    Jim Crow Guest

    Thnaks for that, I have got that downloaded now and have a basic
    diagram to start my project with.
  11. Jim Crow

    Jim Crow Guest

    Many thanks AJ. The UC3906 appears ok but the MAX8212 has a max input
    voltage of 16.5 volts and I am trying to charge a 24 volt lead acid
  12. Each slave unit takes 0.33 A, so 20 take 6.7 A. The NP7-12 will only
    give 4.2 Ah at the 1 h rate. You need a bigger battery.

    Go to:

    and download the NP series data. That will tell you a great deal of what
    you need to know, including what charging currents and voltage limits
    you should use.
  13. Tam/WB2TT

    Tam/WB2TT Guest

    I haven't looked at the 8212 data sheet, but conceptually, you would put a
    2:1 divider between the battery and the device.

  14. AJ

    AJ Guest

    I thought the MAX8212 may be too low but I thought some of the circuit
    examples might be handy and perhaps you could pick another part or make you
    own. The following link shows and example of one used as a low battery

    The UC3906 might not be the best choice for your application but explains
    how a SLA is best charged. The info from that and the battery datasheet
    which John Woodgate provided a link to in a previous msg should give you
    plenty of info to get started.


  15. If you don't find a specific part to do the job of low voltage cutout ,
    maybe roll your own in the way of a lm3915 bar/dot driver. Basically a
    voltage comparator with 10 outputs and by setting the divider up to cover
    the voltage range of the battery - and by using an output to drive a
    transistor to switch off a latching relay to cutout the battery. Then you
    could have the latching relay energize when the mains come back up. Jtt
  16. Guest

    For emergency applications, a float charge on SLA would do the trick.
    What you want in the design of the charger is idiot-proofing. For
    instance, the battery eventually will get replaced. Is the new battery
    in backwards? Is the new battery too low in voltage to be deemed worthy
    of charging? Short circuit protection? Current limiting? A good design
    is one that won't cause a return or design out.

    Most SLA designs disconnect the battery at around 10V to prevent it's
    destruction. In emergency lighting, they may not do this, since , well,
    it's an emergency. ;-)
  17. They should, but also have enough battery capacity so that even in the
    worst case, the voltage never drops to 1 V per cell.

    The worst case is pretty easy to determine; it often isn't. The worst
    case is the maximum time it would take to evacuate the building. If it's
    empty, emergency lighting is not required. Some idiots specify that the
    system capacity must be enough for 12 or 24 hours!
  18. budgie

    budgie Guest

    It's worse than that. The fluorescent tubes are 8W rating, so the inverter draw
    will be somewhat higher.
  19. Guest

    I left out reverse power protection. This is where the battery is fully
    charged, but the charger itself isn't plugged in. It could be as simple
    as a diode, though in a high power charger you might do something more
    efficient. For a float charge, the diode is fine, but the voltage sense
    has to be after the diode to compensate for the drop. You do need to
    insure the sense circuit has protection. A reverse battery condition
    with the charger off is also needed.

    Some designs take care of reverse power. This would be where some moron
    plugs in a battery that is higher voltage than the charger.

    A robust charger takes lots of planning. Just ask Dell or Apple. ;-)

    Note that I know (I think) all the pitfalls of a charger, and when I
    needed one for home use, I just bought one off the shelf. ;-) Full uP
    control and most of the protection I mentioned. Some manufacturers
    (boards and chip level) put in protection modes not on the datasheet.
    There are reasons not to mention the extra protection modes, but that
    would be a paragraph or two more.
  20. Ah: emergency lights are SUPPOSED to be rated on consumption, not the
    lamp rating. Of course, they may not be. If so, 8.5 W is probably about
    right; the inverter is quite efficient.
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