My question, for all those lucky cell phone users out there, is how long does it take for a cell phone's battery to run down while on standby? How long during use? How often do you have to charge your cell phone and for how long?
Seriously, though, it does depend on the battery. There are cheap ones that have to be charged practically all the time and really expensive ones that'll last you a month on standby, at least. Then there are batteries that used to be good and have gone bad.
The truth is for story purposes you can choose just about anything you want because of the wide range of answers to that question. Anything reasonable, I'd say. Most batteries can last on standby for 24 hours, but if they're on their last leg you can have them requiring constant charging. Then you'd want to have your character complain about needing a new battery or new phone or something tho...
[This message has been edited by xarius (edited February 22, 2005).]
It also depends on how much I use some of the other functions. I have a camera-phone which takes pictures and 15-second video bits. I have a couple of games. And I can connect to the internet. Making a call and connecting to the internet wear down the battery the fastest, while taking pictures and playing games only moderately affect the battery.
I don't make or receive very many calls so I recharge every 4-5 days and it takes about 90 minutes.
Our phones will go about 12 to 20 hours on standby. But, use the phone for 20 or 30 minutes and standby gets cut drastically, sometimes we can't go more than a couple of hours. That's what car chargers were made for.
Another thing that may drain the battery is if I put the phone into a pocket and the buttons are pressed continually without my knowledge (until I take the phone out of my pocket and realized I just called the police a few times -- it's 999 in the UK, not 911).
Oh, and I hate mobile phones and only have it because my wife makes me have it. 
For a while I had a cell-phone that used sorta-normal looking rechargable batteries (they weren't normal, but they looked that way). I went ahead and always had a fully charged back-up set with me all the time. So, as you can see, it isn't just about the battery itself. Depending on the user, any system will vary substantially in practical reliability.
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My cellphone has a good battery, but when I'm underground, like the subway, or in a building with thick walls I have to turn it off otherwise it will drain itself looking for a signal.
Would a rural location have the same effect?
When I was cell phone shopping, I found that one company had nice cheap service, but if I left the city, I wouldn't have service. The company I went with has the second best coverage in Alberta. I can go just about anywhere in the province and still have service (unless it's way up past the arctic circle).
Advice: don't ask one of those phone reps. they will tell you it lasts more than a week. They lie... like salesmen. I should know, I am one, but not the lying type, at least I hope not!
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My mobile phone battery is crap! It will last about 2 days max on standby. Use of the phone for anything longer than the time it takes to text my wife "I love you" will drain the battery substantially.
Gee, HSO, I thought you lived in London. Doesn't eveybody send all their messages by Owl over there?
Therefore, we prefer modern technology as a means of protecting our beloved pets.
There are two big factors no one has mentioned. The old cell phones had NiCad batteries that were only good for a few hours of standby and less than an hour of talk. The NiCads were relatively fragile and their capacity could be greatly reduced by overcharging. The new phones use Lithium Ion batteries which hold a charge many times longer and are almost immune to overchaging.
The other factor is temperature. Put your character in a cold environment and any chemical battery will produce a lot less power. Make it winter outside and a partially charged phone will have its broadcast range greatly reduced by low voltage. If the character can warm up the battery the phone will work again, but the character might not know this or might not have time to do it.
[This message has been edited by Doc Brown (edited February 23, 2005).]
I have been through 2 dozen or more phones over the last 15 years. Best ever are Nokia. 7110 and 6310i when new will last a month on standby, and even with heavy use I was managing 7-10 days. Eriksons are less good, and flimsy too. Motorola suck.
But worst of all I recently upgraded to a Sharp GX25 flip phone. Sucks like a cuban Transsexual hooker on speed. I charged it 3 times in a week before I threw it away and changed to another Nokia. Battery now lasts a week, whish ain't so good as the older ones, but I guess thats the trade-off for built-in camera, radio, mp3 player etc.
http://www.write-across-europe.com
It fits with the tone of the story.Once again, I really appreciate all the answers I've gotten to this question.