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Battery power to replace wall power supply?

Discussion in 'General Electronics' started by Robert Oschler, Feb 4, 2005.

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  1. Hello all,

    I have a video transmitter and camera that is just the right size for
    putting on a mobile robot. The problem is that it takes power from a
    typical wall wart power supply. I want to "cut the cord" and go mobile, and
    to do that I need battery based power that will output power in the
    electrical format that the transmitter requires, and out through to
    compatible jack. The wall wart power supply specs are:

    Input: 120V, 60 Hz, 18W
    Output: 12VDC, 400mA max
    The tip of the plug is positive (+)

    Where would I look (URL's, web sites, vendors, etc.) to find a battery based
    solution that is as close to my desired goal as possible (minimum amount of
    work to complete the project)?

  2. Robert Baer

    Robert Baer Guest

    *ALL* un-regulated wall wart supplies output a voltage that is greater
    than the "spec" when at full rated load.
    You could safely use a 12V sealed lead acid battery as the power
    source. The actual drain of the transmitter probably varies, and may be
    in the 100mA region.
    The wall wart is *not* safe to use as a charger for that battery!
  3. Robert,

    Thanks. I'll try to find one that isn't so heavy. Most of the lead acid
    batteries I looked for were about 4 pounds, which is pretty heavy.

    Just as a curiousity, even though it's manganese dioxide and not lead, would
    two batteries like these wired in series work?:

    They only weigh 7.5 grams a piece, but they drop off from 12V pretty quick.

    Also, is there any kind of electronics that I need to add between the
    battery output and the jack to smooth or regulate the output?

    Robert Oschler
  4. Those batteries are only 40mah, your camera would suck them down in a minute
    or two.
    I would suggest AA or maybe AAA NMH (nickle metal hydride) batteries in a
    ten cell series arrangement (which gives you 12v).
    The NMH batteries have a higher milliamp hour rating than the small 1300mah
    lead acid gel cells nowadays.
  5. Earl,

    Ok, thanks. Do I need to put a smoothing or regulating circuit between the
    batteries output and the output jack?
  6. The other replies you have received assume that the wall wart is providing a
    voltage which is regulated further in your device. This may not necessarily
    be the case and, if it is not, you may damage your camera.

  7. If you need the assembly small, unless you want it operate for very
    short period of time, the only way to go is with a high-efficient
    type. There is an article on Matt Blaze's page (about chargers, but it
    also covers batteries):
    (high efficient would be NiMH for rechargables, these are the
    cost-effective ones, not because they are cheap - the good quality
    ones are not - but because they can be reused). You will need 8-9
    batteries serialized for this (the voltage of NiMH is slightly less
    than 1.5V) and if the TX is not of high quality, the transmission
    frequency may drift through operation (I have this problem with some
    of the mini-TX cameras that were sold in Toronto over the X-mas - they
    wer only $40 - TX/RX and worked on 9V batteries but could not
    stabilize the transmission in time).


    Silviu Trofimov
  8. SteveB

    SteveB Guest

    No smoothing or regulation required, but you need a fuse close to the
    battery ideally, as 12v of fully charged NiMHs won't appreciate it if your
    cable shorts. One other problem with using 10 cells in series is that when
    one cell runs down flat before the others, your equipment may well carry on
    working OK, but that cell will be damaged by reversed current flow. The
    simple answer is to top-up charge before there's any chance of cells going
    flat, and to measure the voltage of individual cells and throw out any that
    are consistently below-par. Another way, the method that digital cameras
    use, is to automatically shut down if the battery voltage falls below a
    certain level, but this gets more difficult when you have a lot of cells in
    series. Approx 11v shut off would be about right. Lead acid may be easier.
  9. SteveB

    SteveB Guest

    Just to add, your equipment may well have the shut-off circuitry built in to
    it anyway.
  10. Robert Baer

    Robert Baer Guest

    There are many small 12V sealed lead-acid batteries: PowerSonic
    PS-1208 3.78L x 0.98W x 2.42H @0.8lb is their smallest (0.8AH at 20 hr
    rate); Panasonic LCR121R3PU 3.82L x 1.87W x 1.97H @1.30lb is their
    smallest (1.3AH at 20 hr rate); CSB GH1313 3.82L x 0.98W x 2.05H @1.28lb
    is their smallest (1.3AH at 20 hr rate).
    Sizes and capacities increase from there.
    Concerning those Energizer cells, i see that the 20K load spec (40K
    for 2 in series) would give (from their curve) a real service of 60-70
    hours *IF* your unit had such a low drain.
    And i do not think that your unit draws 500microamps!

    Here is the spec sheet on the smallest of those i mentioned:
    From that data sheet, the 5 hour rate (130mA to end at 10.2V) is about
    what i would guess as to the actual characteristics you would see if
    used with what you have (based on crude guess as to actual drain of that
    You should measure the actual current drain under typical operating
    conditions, and work from there.
    Lithium non-rechargeable cells give higher power density, but you
    *pay* for that.
    Some comparisons that may be of use, for D-cell size:
    Eveready Dry cell 8000mAH, Eveready Alkaline 18000mAH, Sanyo NiCd
    4400mAH, Energizer NiMH 2200mAH, Tadiran Lithium (3.6V) 16500mAH,
    Tadrian Lithium high capacity (3.6V) 19000mAH.
    That clearly shows that non-rechargeable cells perform better in this
    class range, and the vaunted "lithium advantage" is bad on the wallet -
    use alkalines instead, unless space/weight is a premium.
    Where a lead-acid cell lies, it is hard to tell. Using P.216 of the
    Radio Shack Enercell Battery Guidebook (62-1304 2nd Ed circa 1990), it
    would seem that a D-cell size (2.0V) would yield about 2000mAH.

    Hope this is of some help.
  11. Robert Baer

    Robert Baer Guest

    A "nasty" way to reduce reverse voltage damage is to put a schottky
    diode across each cell; gives worst case reverse bias of about 300mV.
    Natch, any useage below 80 percent of rating for a period of time
    (lower voltages, less time) will tend to decrease lifetime (like
    radiation; dose rate).
  12. Robert Baer

    Robert Baer Guest

    Almost a false assumption on your part.
    Almost all wall warts are *unregulated* and all of those have an
    excessively high output voltage which increases as the loading decreases
    from rated value.
    Output voltage could be 14 to 16 or more volts in his application.
  13. What's "almost a false assumption"? It's no assumption that almost all of
    the other replies assume that the wall wart he has is unregulated. It's
    also not an assumption that there are regulated wall warts. I have several.
    I also have a regulated switching wall wart. It's irresponsible of you to
    give someone the advice to replace their power supply without even asking
    him whether it's regulated or not. But, it's not your camera, right?

  14. Thanks for the detailed reply Robert. Are those PowerSonic batteries
  15. Robert Baer

    Robert Baer Guest

    By definition, lead acid batteries are rechargeable.
    Care and feeding of rechargeables:
    1) Avoid deep discharge if at all possible - try to go no lower than 80
    percent of rated value (which is 9.6V for a 12V battery). Recharge ASAP.
    For many rechargeable chemistries, going below 90 percent of rating
    (10.8V for a 12V battery) should be avoided for long periods of time.
    In your application, allowing a discharge to this level can give
    reasonable lifetime of use for the smaller batteries that i mentioned.
    If the battery is re-charged within a reasonable time and stored charged
    before re-use, the overall useability will not deteriorate much
    (guesstimate of 1-3 years for reliable use).
    2) Store charged - try for "trickle" charging during storage (or do so
    on occasion at minimum).
    Trickle charging for lead acid batteries can be based on a fixed
    voltage or a fixed current (no more than 20C wher C is the amp-hour
    A lead acid battery that has not been abused, and stored properly can
    last 2-5 years as a reliable power source for emergency lighting, alarm
    system power backup, electronic UPS systems and the like.

    You gave no info concerning overall weight limits, space limits or
    needed use time from one charge.
    Maybe use of Eveready alkaline (non rechargeable) AA or C cells will
    give a desirable useage time, in a fairly light and small package.
    Again, measure the typical load current for starters.
  16. Robert Baer

    Robert Baer Guest

    Well, excuse me!
    I know damn well that there are regulated wall warts - both switchers
    and linears; hence the care in the wording.
    It is obvious that the camera has its own regulator inside; camera
    batteries range from 3V to 6V.
    Thus an extra expense of regulation in the wall wart leads to lower
    profits to the maker.

    The "worst case" scenarios are:
    1) Unregulated (extremely high probability) wall wart, 16 to 18V to
    A 12V battery replacement cannot possibly damage the camera and will
    work well down to the indicated 10.2V as represented in the datasheet i
    2) Regulated wall wart, 12V to camera.
    Again a 12V battery replacement will do well.
  17. Robert,

    I am going to place the camera & transmitter on top of a Lego Mindstorms
    Roverbot (small autonomous robot). I'll be using it intermittently, at most
    a few hours a day. I want it as light as possible to reduce the load on the
    *Roverbot* batteries. The heavier the overall item of course, the faster
    the batteries on the *Roverbot*, not the camera, will burn out. If it's too
    heavy obviously, the Roverbot just won't move, but I don't think that's a
    problem. I'd like the battery "brick" to be about 3 inches by 4 inches.
    That should sit on top of the Roverbot quite nicely, since the Roverbot
    (Lego Mindstorms RCX brick computer), uses AA batteries itself.

    I definitely want rechargeables because it will get daily use for a long
    time to come.

    Big thanks for the detailed reply.
  18. Many of the battery chemistries discussed have initial actual voltages above
    12V for a nominal 12V configuration. If the camera expects regulated 12V
    these could damage it. I agree it's unlikely, but in light of the scope of
    the other data and advice being offerred, it is surprising that no one is
    even concerned about this.

  19. Robert Baer

    Robert Baer Guest

    For the lightest battery in a given volume, the Lithium
    non-rechargeables wins.
    Next comes the Eveready Alkaline non-rechargeables, AA size.
    Rechargeables would not have anywhere as long running time per charge
    compared to using those alkalines.
    Perhaps you could plan for the AA size and try the Eveready Alkaline
    first, and if not satisfied, then try the Panasonic NiCd P-100AASJ
    (Digikey #P252, Pg 1354) which is rated at 1000mAH, or the Sanyo
    equivalent (Mouser #639-KR1000AAU, Pg 1322).
    I think that the Alkalines will do well.
    Remember, the Roverbot uses them - so that makes for easier shopping.
  20. Good points. I wonder why rechargeables dissipate faster than

    I think for now I will go with the Eveready Alkaline AA's. I found an 8
    pack AA battery holder for cheap at Radio shack and a transformer connector
    too that fits the camera, so now I get to find out if the camera works off
    batteries. I'll be hooking it all up tomorrow.

    This is going to be part of my DID project (Duck Irritation Device). We've
    got these ducks in our neighborhood that I feed bread occassionally. My
    roverbot is going to follow them around and send back video images. I'm
    betting if I wait long enough I can catch one of them saying "AFLAC" on
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