Battery power to replace wall power supply?

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)?
Thanks,
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Robert Oschler
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Robert Oschler wrote:

*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!
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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?:
http://data.energizer.com/PDFs/a23.pdf
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?
Thanks, Robert Oschler
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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.
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minute
1300mah
Earl,
Ok, thanks. Do I need to put a smoothing or regulating circuit between the batteries output and the output jack?
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Robert Oschler wrote:

I'm not Earl, but probably not.
One caveat is that a 12 volt battery or pack frequently provides a little more than 12 volts when fully charged, fading down to around 80% of rated voltage (in this case around 10v) before it is considered discharged. It is unlikely that your camera will have any problems with the slight overvoltage of a fully charged pack or the undervoltage as the battery discharges, but it's worth mentioning.
Smoothing should absolutely not be necessary, assuming the only thing on the power bus is the camera/transmitter.
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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.

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

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SteveB wrote:

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).
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Robert Oschler wrote:

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: http://www.power-sonic.com/ps-1208.pdf 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 equipment). 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.
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Thanks for the detailed reply Robert. Are those PowerSonic batteries rechargeable?
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Robert Oschler wrote:

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 rating). 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.
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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.
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Robert Oschler wrote:

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.
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Good points. I wonder why rechargeables dissipate faster than non-rechargeables?
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 video.
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Robert Oschler wrote:

That would be a gas! As a point of reference, please measure the current drain from that 12V wall wart when the camera is in use and just sitting and waiting. Also, as has been mentioned, also measure the voltage to the camera (both modes). The second part is to satisfy the "worry wart" that implied that a 12V battery could damage the camera. Please post the values for all to see.
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Robert Baer wrote:

"worry wart", very punny, lol. Hey I've never heard of these power adapters being called a wall wart before.
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Matt,
Other Robert here (OP).
They're called wall warts in the music industry. If you are a musician with a lot of effects boxes (distortion pedal, phaser box, etc.), those wall warts are a pain to attach to a power strip because they don't sit next to each other well (think of the geometry). In fact there are companies that make "wart removers". A wart remove is just a very short power cord, usually about 8" at most, with a flat plastic panel at the end that has a single AC plug outlet. This moves the wall wart away from the power strip and allows you to easily plug multiple effects boxes into a power strip.
Thanks
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Robert Oschler wrote:

Yeh, sweet. They are a pain in the ass, even at home. So many things these days are low voltage so they come with one of these things. You'd think someone would design one that can be plugged in next to other ones or other plugs. I also didn't know you could get the wart removers but usually have to resort to 1m or 2m extension cords. You can get powerboards that allow for a couple of them but I still think it would be easier if the bloody things weren't so wide.
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Altronics in Australia sell slim line plugpacks (wall warts), that allow you to have two of them side by side in a double powerpoint.
http://www.altronics.com.au/index.asp?area=item&id=M9208
Regards
Andrew
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