I need to run a device on batteries (7.5v 500mA)

Discussion in 'General Electronics Chat' started by sydcomebak, Mar 10, 2015.

  1. sydcomebak

    Thread Starter Member

    Dec 13, 2011
    69
    5
    It's a baby monitor. This is an emergency construction and I'll be heading to Radio Shack for parts ASAP. What kind of batteries should I look for? D cells? I need it to run for maybe 24hrs.

    Thanks in advance!
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    That may be your best choice, at 500 ma it won't last long though..
     
  3. sydcomebak

    Thread Starter Member

    Dec 13, 2011
    69
    5
    Best guess? I'm pretty sure Dcells are 1.5v each, right?
     
  4. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    I assume 7.5V 500mA is the rating of the mains PSU? You need to know what the actual current consumption of the device is.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Cant you run it on AC line voltage? If so, head off to the local thrift shop and find a wall-wart that puts out ~6-9V dc....
     
  6. sydcomebak

    Thread Starter Member

    Dec 13, 2011
    69
    5
    Sorry, line power is not an option.

    The wall brick says 7.5v 500mA, so that's all I have to work with.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    5 D-cells ought to run it for 24h.
     
  8. sydcomebak

    Thread Starter Member

    Dec 13, 2011
    69
    5
    That's what I wanted to know! Thanks! A relative shipped a baby blanket to my old address and my wife and I suspect that the new house owners opened the box and kept it. My friend and former neighbor is allowing me to set the camera up on his WiFi and I'm going to box it up with my name and old address. I'll keep an eye on it for 24hrs and after that, I'll just drop by to pick up any mail they might have.
     
  9. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    Connect 6x D cells as 2 series groups of 3. Connect the (now 7.5V) batteries in parallel.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    ???

    Three 1.5V batteries in series makes 4.5V. Two of those in parallel still makes 4.5V
     
  11. sydcomebak

    Thread Starter Member

    Dec 13, 2011
    69
    5
    Isn't that only 4.5v? Forgive me. I may be mistaken.
     
  12. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    Sorry, I can't add up today..... please ignore my previous post..... 5 D cells is correct:confused:
     
  13. Roderick Young

    Member

    Feb 22, 2015
    408
    168
    5 D cells will probably work, but that's based on the circuit actually needing far less than the 500 mA that the power adapter is rated for. My experiments here suggested that the best NiMH D cell has only 7.5 AH of real capacity, and that a quality Alkaline would have maybe 5 AH. Lithium-Iron might get you to almost 11 AH. If you divide out the times at 500 mA, that would be 15, 10, and 22 hours, respectively.

    But again, odds are that the circuit doesn't draw nearly that much, and moreover, it might still function when the voltage sags to 5 volts or less, just with reduced power.

    I would also suggest that in the event that this is actually an issue of child safety, it would be better to just get a whole new monitor than to mess around with jerry rigging something.
     
    Last edited: Mar 10, 2015
  14. kdcouture200

    Member

    May 22, 2010
    17
    2
    Would it not be cheaper to buy one if its an emergancy?
     
Loading...