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Anone Know How to Design a Battery Back-Up???

Discussion in 'Electronic Basics' started by PoiYui, Jan 1, 2005.

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  1. PoiYui

    PoiYui Guest

    I'd like to create a battery-back up for a 12V power supply. I don't,
    however, know how to wire the back up so that the battery doesn't draw
    down 'til the primary power cuts out. If you have experience with this,
    I'd really apppreciate any ideas or instructions.

  2. If the supply is 12v then it wouldnt be much use for charging the battery so
    I guess one method would be to have a relay that is held closed when the
    power supply is operating, should it drop then the 'back' contacts would
    connect the battery (or a powermosfet). It would be better to have the
    circuit normally operate on 14v if possible then you could use that to keep
    the backup battery charged and permanently in circuit for a seamless
    Perhaps a little more information is required to give an answer here.
  3. You can use a 12V relay with both normaly open and normaly closed contacts.
    Just wire the coil to the primary voltage source. So, when the voltage goes away,
    the relay switchs from the "NO" to the "NC" contacts.

    I like'm when there simple!
  4. Terry

    Terry Guest

    Basically a relay which is normally kept operated by the AC supply. When the
    AC supply fails relay releases and its contacts supply the 12 volts.
    Considerations are that the relay must be suitable for continuous use, its
    contacts must be able to switch and carry the 12 volt current load (also
    check to see if the 12 volts includes any inductive/capacitive reactance,
    because this increases the chances of excessive sparking and burning of the
    contacts) and also that if you get power hits/surges and/or brief
    interruptions the relay will tend to release and then re-operate quickly
    each time which may be undesirable? So then you might want to look into time
    delay relay circuits that will 'hold on' for a few seconds?

    Words of caution about keeping batteries on what I will call 'Continuous
    You can either ruin a battery by continuously overcharging or keep it in
    such a state of undercharge that in either case that when required it is not
    capable of performing!

    The voltage of say, a typical (six cell lead acid) car battery is
    'nominally' 12 volts.
    Once charged such a battery, at normal temperatures, will maintain 12 volts
    (that's 2.0 volts per cell) until the point at which the battery is almost
    fully discharged; at which time the voltage, depending on the load drawn
    from it, will quickly 'tail off'. (In other words she's empty!)

    Using voltage to try and tell the state of charge of a battery is not a good
    indicator. If some time after the battery has come off charge the voltage is
    12 volts that may indicate it is somewhere between fully charged and
    approaching discharge.

    The 'charging voltage' of a '12 volt' battery is about 14 volts. (that's
    about 2.3 to 2.4 volts per each of the six cells).
    Once the battery is fully charged pumping any further input into it becomes
    heat and can, if prolonged, damage the battery; I have seen where a battery
    has been just about 'boiled dry' and was ruined.

    Also a fully charged battery left standing will gradually self discharge,
    due to 'local chemical action' and after a few months can become useless.
    People sometimes store away their recreation batteries in a discharged state
    and next season find they are useless!

    Once a battery is fully charged and 'standing by' ready for use many
    administrations carefully 'float' their batteries, depending very slightly
    upon the type of lead acid cells, at between 2.15 and 2.20 volts, per cell.
    A typical figure for the Bell Telephone System, for over 50 years, for
    example, was 2.17 volts per cell; chosen to offset local chemical action
    without overcharging. In those large installations battery acid specific
    gravity was normally checked on a regular basis, due to the large investment
    and the critical nature of the use. e.g. 911, Ambulance services, Fire Dept.
    phone systems etc. However these days car batteries are now basically sealed
    and rarely tested that way.

    For our typical car type battery that's 6 x 2.17 = 13.02 or approximately 13
    volts . 'Floating'.
    That one or two volts may not seem like much of a difference; much of our
    equipment is quite capable of working fine over quite a range of "12 volt
    voltages". However 14/12 = 1.17 (i.e. 17% over voltage!).

    I don't think our tube type 117 line voltage radios would take kindly to say
    117 x 1.17 = 136.5 volts!
    But when it comes to the long term care, feeding and reasonable life (also
    costs of replacement!) of a rechargeable chemical lead acid storage battery,
    those voltages are critical to the battery being ready when it is finally

    The history is that at one time large battery installations were used on the
    charge-discharge method; so there were often two batteries while one was
    being used and discharged down to say 30% to 40% of capacity, the other was
    being recharged by an engine generator, windmill or whatever. later circuits
    were designed to be able to charge the battery and also use it at the same
    time without damaging the equipment using it with the too high charging

    These days we have much better voltage regulators and equipments that
    regulate their own voltages internally using zener diodes and the like.

    As a suggestion; just don't hook up a battery charger to the battery and
    leave on day and night. Either use a more sophisticated charger, such as
    those incorporated with the internal battery into a UPS (Uninterrupted Power
    Supply used for computers), or have a manual regime of 'topping up' the
    battery on say a weekly basis for an hour or two.

    We have a UPS which we run in conjunction with some old truck batteries.
    However after some years the batteries, which had many road miles on them
    before they were given to us, have now died due to age. Also I'm not sure
    that the UPS charger has the capability of fully recharging them following a
    prolonged power failure and or the UPS has been switched off for a while.

    Hope suggestions help?
  5. Byron A Jeff

    Byron A Jeff Guest

    -I'd like to create a battery-back up for a 12V power supply. I don't,
    -however, know how to wire the back up so that the battery doesn't draw
    -down 'til the primary power cuts out. If you have experience with this,
    -I'd really apppreciate any ideas or instructions.

    If you design the way you propose, there will always be a glitch between the
    detection of loss of AC power and the insertion of the battery into the

    Many UPS systems use a rechargable battery system that drives the circuit and
    uses the AC power to charge the battery. So when AC power goes out there is
    a glitchless transition to battery power because the circuit was always driven
    by battery power.

    I went through this with my sunrise/sunset outdoor light controller. I ended
    up driving the circuit with a 12V 7Ahr lead acid gel cell and charging the
    gel cell from the AC power. It's completely glitchless. When power goes out
    the controller just keeps on trucking.

    But the easiest way to accomplish what you want is to use steering diodes.
    You use a diode for the AC line and a diode for the battery. As long as the
    AC line voltage is higher than the battery voltage, then the battery will
    not have any power drawn against it.

    Here's a web page that describes the process:

    Hope this helps,

  6. <Snip>
    I can't seem to be able to remember ever using a relay with AC power source
    driving the coil. It's always been a switched DC source. So, in my example.
    You're powering the coil from the output from the main DC supply.

    The orginal poster wanted the power to switch in when:This implies a delay in supplying power or a drop to zero volts. Or, I would of
    simply sugested using two power diodes.

    The rest of your remarks (or possible problems) are completely dependent upon
    the load (and that wasn't given) and or the relay you choose to use to get
    around them.
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