12vdc (falling) to 12vdc (regulated)

Discussion in 'General Electronics Chat' started by hdpete, Nov 11, 2011.

  1. hdpete

    Thread Starter New Member

    Nov 11, 2011

    I have a small electrical device which draws 12 watts at 12vdc and needs to be powered from a 12v battery. Currently, it has a mains inverter from the battery to provide a stable 230vac which is then dropped using the device's power supply back to 12vdc. The device is a portable scanner and so will be very specific as to the input voltage.

    There must be a more efficient way of powering this device direct from the battery, but when the battery voltage is dropping (to a minimum of 10.5v), the device still retains as close to 12v as is possible.

    Can anyone point me in the right direction of some kind of 12v to 12v DC convertor which can provide a regulated supply on a falling input?

    Many thanks for any advice
  2. SgtWookie


    Jul 17, 2007
    Hello Pete, and welcome to the Forums.

    You didn't put your location in your profile; and I can't quite guess where you are - I suspect the USA due to the voltage you mentioned, but not sure.

    Knowing your location is important, as otherwise we don't know what suppliers you might have access to.

    Click on the "User CP" link near the top of the page just below the thread title, then "Edit Your Details" on the next page, then under "Location" put your Country and state or province (you don't have to get overly specific if you don't wish to; just Country is OK) and then click the "Save Changes" button on the bottom of the page.

    Running even 12v deep-cycle batteries below ~70% of full charge will reduce their life span drastically. If you only discharge them to ~80% of full charge, you will obtain a maximum service life from them. If you drain them as low as 50%, you decrease their service life by 2/3. Draining them to 10.5v is actually below what is considered 100% discharged, and your battery/batteries will have a very short life.

    I suggest that you don't discharge the battery below 12v to begin with. If you need a longer run-time, then I suggest that you purchase a battery with a greater AH capacity, or multiple batteries and wire them in parallel.

    Whatever route you choose, you will still need a DC-DC converter that is capable of buck-boost operation at 12V 15W/1.25A output minimum; that way it will be operating at 80% or less capacity. You don't want to be operating a supply at 100% capacity if you can avoid it.

    There are a couple of ways to go; buy something off-the-shelf that is very close to your specifications (which will be the quickest way to get something going, and will likely be the most reliable and least expensive in the long run) or try to build something yourself.

    It will need to be a switching supply for the sake of efficiency. If you are not familiar with electronics; at least an intermediate-level hobbyist, I suggest that switching supplies are a bit too much to take on - the learning curve will be just too steep, and you won't have the test equipment available for troubleshooting (at the very least, you would need a multimeter and an oscilloscope).

    Here is one unit that meets the minimum specifications:
    12V, 15W, 1.25A, input 9v-36v, under $42.
  3. crutschow


    Mar 14, 2008
    You could used a non-inverting buck-boost switching converter such as Sepic or Ćuk designs which can operate with an input voltage above or below the output voltage.

    Edit: SW beat me to the punch with a more complete explanation.:)
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    True if it's a lead acid battery but he didn't specify that it was. Could be a Lithium Fe or some other 12V chemistry available. 12V @ 1A could be fed from a lot of different batteries, depending on required operating life for the application.
    Last edited: Nov 11, 2011
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    I might try a "high efficiency" linear design. You could build a battery using NI-MH cells and use 11 cells so the nominal voltage is 13.75V. Then build a low dropout linear regulator which could provide a regulated 12V output voltage down to an input as low as about 12.5V. At 1A, power loss in the regulator would be less than 2W and the design would be WAY simpler than an inverter.