12V Sealed Lead-Acid Charger for Battery Backup

Discussion in 'The Projects Forum' started by falblambiz, Jul 1, 2010.

  1. falblambiz

    Thread Starter New Member

    Oct 31, 2007
    Hi, I need to integrate a battery backup to a previous electronics project. I am an electronics engineering student and my professor assigned to me the sealed lead-acid battery type to do implement the backup.

    Searching this forum I found about the existence of a site called "Battery University"- which is great by the way - and on this link
    http://www.batteryuniversity.com/partone-13.htm they say that to improve the recharge times the right thing to do is to use a multi-stage charge algorithm involving a constant current (bulk) stage, a constant voltage stage and a float charge stage. The last state is important in this particular application because a requirement of the project is to automatically avoid overcharging.

    So, my questions are:
    1- Is this multi-stage circuit too complex to implement? can you give any example circuits or guide me to the proper information?
    2- Can I alternatively use a simpler approach (constant current only or constant voltage only)? being aware that it will represent a tradeoff in battery life and charging periods?

    Battery Specs:
    -4.5Ah for 20 hours
    -Sealed Lead-Acid

    I appreciate your help and advices. :)
  2. SgtWookie


    Jul 17, 2007
    First, locate a standard SLA battery that will be suitable for use. Then find several manufacturers for that battery size, and carefully read the maintenance sections in the datasheets; charging current, float voltage, maintenance charge, temperature coefficients, etc.

    In addition to the "bulk" charge, "absorption" charge and "float" stages, many SLAs require an equalization charge, which is a deliberate overcharge for a short period of time. The equalization charge stirs the electrolyte and removes sulphation from the plates. An equalization charge must not be applied to gel-cell batteries, as permanent bubbles will form in the gel which will reduce battery capacity.

    Lead-acid batteries have a negative temperature coefficient, usually specified at around -3mV*cell/1°C, with the standard voltages given when the battery core temp is 25°C.
    If you have a 12v SLA battery, it will have six cells, so the temp coefficient will be -18mV/1°C. A good maintenance system will compensate for battery core temperature. You can monitor the core temp by measuring the temp of the positive battery terminal.

    Take a look at this PDF document: http://focus.ti.com/lit/an/slua058/slua058.pdf
    The application note describes an efficient mains-powered lead-acid battery charger that incorporates bulk, absorption, float, and overcharge phases. If you added temperature compensation to it, you would have a very complete and efficient system that could maximize battery life and performance, while minimizing operating costs (efficient energy use).

    At the other end of the scale is the PB137 from ST Microelectronics.
    It is about as simple as you can get. It provides for a limited bulk charge and a float charge; no frills at all. It is also terribly inefficient, and will require a very large heat sink to dissipate the heat. While cheap and simple to build, it will waste a great deal of power, and operating costs will be significantly higher than the previous suggested circuit.

    Using a simple constant current charger will result in overcharging the battery, and very short service life.
    Using a constant voltage charger that is not current limited will cause the charger and/or battery to overheat, leading to a short service life.
    Last edited: Jul 1, 2010
  3. falblambiz

    Thread Starter New Member

    Oct 31, 2007
    Thanks a lot for your response, it has been of great educational value.

    The application note that you posted has helped me in my investigation for my report on the implementation of a professional project but it is pretty much advanced for the reach of this course. Thanks again.

    Now, unfortunately I haven't still found the manufactuer's datasheet for the battery that I am using and on the battery itself has this specs printed:

    -Hi Power Battery Co, Ltd. (which is obviously the brand)
    -6FM4.5 (12V4.5AH/20HR) (I don't understand the "6FM", but I get the "12V4.5AH/20HR")

    Now it presents a table which I don't understand clearly:

    Constant Voltage Charge (25°C)

    Type Voltage Regulation Initial Current
    Standby Use 13.50-13.80V Less that 1.35A
    Cycle Use 14.40-15.00V Less than 1.35A

    Does this mean that it can only be charged with a constant voltage current limited method?
    On the other hand, if I do not manage to get the datasheet, the constant voltage current limited method would be the safest (but slowest) method?
  4. SgtWookie


    Jul 17, 2007
    That is for temperature compensation. For a 6-cell 12v battery, your temperature coefficient is -18mV/1°C away from 25°C. So, if the battery core temperature (measured using the battery positive terminal) is higher than 25°C, you subtract 18mV per 1°C to obtain the proper charge voltage. If the core temp is lower than 25°C, you add 18mV per degree. It does not sound terribly significant, but it adds up quickly.

    If you elect to not use temperature compensation, you need to be aware that the further away from 25°C your battery is, the worse the life expectancy will be due to over/under charging.

    The "Standby Use" is the "float charge" voltage.
    The "Cycle Use" is the charge rate to use if the battery has been appreciably discharged, and/or for occasional "equalization" charge. You charge at a constant current <= 1.35A until the 14.4 to 15v threshold has been reached, and then you revert to the 13.5v to 13.8v "float" charge rate.