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)?
*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!
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.
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.
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.
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):
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).
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.
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 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.
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).
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.
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?
While there are some pretty elaborate systems available, the last ROV I built used a 12V single board camera from Supercircuts. My power supply was a battery holder for 8 AA's and a set of rechargable alcaline. I didn't bother with a fuse or regulator. Just wired it frome the battery pack, through a switch, and to the camera. I get about 6 to 8 hours from the pack before I remove the batteries, drain them in a flashlight, and recharge them.
I've been using this system for about 3 years without ruining the camera. Most of the small cameras out now seem to be able to handle a pretty wide range of voltage. My current ROV project uses a small color cmos camera. It's specs say 9vdc, but it seems to run on anything from 7.2v up to 12v. I use a 9v battery for convenience.
You should be able to get a battery holder and some rechargable AA's with a charger at any radio shack.
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.
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 camera. 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 mentiond.
2) Regulated wall wart, 12V to camera. Again a 12V battery replacement will do well.
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.
Got any pictures of your ROV or video? I'd like to post them on my robot's site; especially if you have a write-up on it. If nothing else, stop by the forum and register, and I'll add you to the Member Sites list with a link to your ROV web page (if you have one).