I am looking to buy a wireless bullet camera (This One). It is powered by either an AC adapter or one 9-volt battery. I am trying to figure out how long it willl last on a battery. It claims that the power consumption is 200mW (milli-watts?). I read on wikipedia that a 9-volt has about 500mAh (milli-amp hours??), but I have no idea how to convert from watts to amp hours. TIA!
–FCOD
OK, power is voltage times current. If the camera uses 200 milliWatts, and takes a 9 volt battery, that means that the current it uses is .2 Watts / 9 Volts, or .022 Amps (22 milliAmps). The battery holds a total charge of 500 mAh, so draining it at 22 mA, it’ll last (500 mAh / 22 mA), or 22.5 hours.
amps = watts/volts
0.2 watts/9 volts = 0.0222 amps, or 22.2 milliamps.
Amps X hours = amp hours
0.5 amp hours/ 22.2 milliamps = 22.5 hours.
Well, I guess it’s unanimous. Thanks guys!
–FCOD
Bear in mind that the battery capacity (the amp-hour rating[sup]*[/sup]) only gives you a rough estimate of the charge life of the battery. The problem with the simple calculation is that the discharge curve varies with the load.
For one, the battery might last much shorter or longer than expected if you put a large load on it. Run several Watts off of that 9V, and it might only last a couple minutes instead of ~15 expected. The capacity ratings are always measured with a particular fixed load. It may or not be indicated, and is often expressed as a fraction of the capacity (e.g. 500 mAh @ C/10 means a 50 mA load was used).
The other thing is that the shape of the discharge curve (and by this I mean voltage over time) also varies. This is primarily a function of the type of cells used. Some cells begin to drop off almost immediately but drop gently, others last for a while at high levels then drop sharply. Again, this also can vary with the load you put on it.
The last concern would be variations in the power consumption and the tolerance of the device to low voltage. Given the nature of the camera + transmitter the load should be pretty constant. My guess is that the thing should mostly be able to tolerate a drop in voltage okay as well.
In your case, your load is probably in the middle range of the capacity rated current, so this simple calculation will probably be okay. In other words, don’t mind most of what I just said; but if your observed results end up very different, these considerations would be the reason.
[sup]*[/sup] Sometimes called ampacity, but this is confusing since the term usually refers to current capacity of a conductor.
Well, the reason is ask is to determine if I need to run a power cord to where I’m mounting the camera. The answer is most definitely. I’m going to construct some birdhouses with cameras in them for my backyard. The only way I could use the cameras on battery power was if it would last for months, and it’s not even close to that, so I’m going to have to run power wires.
–FCOD
Have you thought about using a solar panel with a rechargeable battery? You may be able to find an already packaged outdoor panel designed to charge the battery. Even if you’re not in full sun all the time, you shouldn’t need too big of a panel to get a continuously working battery.