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12V power supply and 12V battery charger

Discussion in 'Power Electronics' started by Chassis, May 21, 2015.

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  1. Chassis


    Apr 21, 2015
    I am trieng to figure out the difference between a 12V power supply and a 12V lead acid battery charger. Can a power supply be used as a car Battery charger aswell or can somone please explain to me the difference.
  2. signalman72


    Jan 26, 2014
    A "power supply", such as a bench supply, is likely regulated where a charger is probably not. In other words, one will put out a cleaner source than the other.

    I have charged batteries from a power supply.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    And in addition to signalman's good comments. A 12v battery charger will put out a higher voltage approx 14 volts.
    you annot properly charge a 12v battery from the same voltave supply. This is applicable to almost e ery batterg and type
  4. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    A 12V power supply is typically well-filtered after rectification of the line voltage, providing a steady DC output that may be voltage or current regulated as well.

    A 12V lead acid battery charger is typically not filtered, or minimally filtered, after rectification of the line voltage because the lead-acid battery doesn't require it to accept a charge.

    There are other differences: the battery charger usually has a means to automatically reduce the charging current to just a trickle as the lead-acid battery becomes fully charged. This is done to prevent the formation of hydrogen gas that results from over-charging. There are battery management integrated circuits available for incorporation into lead-acid battery chargers, as well as for other battery chemistries that require specific "charging profiles" for safe charging and maximum battery life.

    It is possible to use an ordinary "12V power supply" as a car battery charger if care is taken to prevent over-charging, and if the output can provide the approximately 13.7V that a fully charged car battery exhibits as its unloaded (open circuit) terminal voltage. I have often charged sealed lead-acid battery packs (the kind used for emergency exit lighting) with a bench power supply set for 14 V output and operated in current mode with about 100 to 200 mA constant-current output. If the compliance voltage is set right, the "constant current" output tapers off to nothing as the battery approaches full charge. I don't recommend this procedure because it is possible to over-charge the battery if it isn't done right.

    Here at home I have a massive power transformer connected to a large full-wave bridge rectifier that I use to charge "dead" car batteries. It is easily capable of providing 100 A or more of charging or engine cranking current. Originally it was connected to the power line through a variable AC transformer (Variac) and the DC output current was monitored with a 50-0-50 A bi-directional meter that I happened to have on hand. The meter was by-passed if the supply was used for engine cranking. Since I built this rig thirty-something years ago, the Variac and meter have been removed because I deemed them unnecessary for battery charging. Instead I just hook it up to the car battery with heavy "jumper cables" and monitor the battery terminal voltage while charging, paying close attention to the charge state to avoid overcharging. I do this by disconnecting the positive lead from time-to-time to measure the open-circuit terminal voltage while observing how fast it decays.

    The only sure way to determine the charge state of a lead-acid battery is to measure the specific gravity of the sulfuric acid electrolyte in each cell, correcting the reading for ambient temperature. There are tables published to assist in determining what the measurements mean. Most car batteries today are built to prevent such measurements, presumably to ensure the safety of the consumer who can be seriously injured if the exposed cells are subject to over-charging or the electrolyte comes in contact with their skin or clothing. It is also important to allow the battery to "rest" before making the specific gravity measurements to allow the electrolyte acid-water mixture to become uniform in density.

    Some thirty years ago my brother (who knows nothing about electricity or lead-acid batteries) was giving me an after-hours weekend tour of his new place of employment, where there were several battery-operated fork-lifts connected to wall outlets for overnight charging. One of the fork-lifts wouldn't operate, so Brother removed the cell caps on one of these industrial-type batteries, presumably to see if there was any liquid inside. It was semi-dark inside the building, so he pulls out his Bic lighter to "inspect" the battery. Before I could warn him not to "flick his Bic" he had already done so, and the battery immediately exploded as the hydrogen gas that had evolved in the confined space ignited. Fortunately no one was injured, but my brother had some explaining to do the next working day.
    Arouse1973 likes this.
  5. Kiwi


    Jan 28, 2013
    Speaking from experience, I bet his ears were ringing for quite a while too!:)
  6. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    And so were mine! But at least I didn't have to change my underwear.:D Not so sure about my brother.
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