Are you having problems with your mobile phone? You might want to replace the battery.
My Nokia N73 played up a few months ago. Even after charging it overnight, I found that it would switch off by itself in the middle of the day. At first I thought I had forgotten to switch it on, but in time I realized that something was wrong. I was inclined to believe that the phone was faulty and that I needed a new one. Anyway, I found that I could get a new generic battery for ~AUD$10, so I gave that a try. This time, the phone didn't shut down by itself, but the battery would suddenly run down in the middle of a conversation. I thought this further confirmed the phone was useless. But since the symptom was different from the first battery, I felt it was worth trying out with another new battery. This time I bought it from eBay for AUD$11. I am happy to say the phone has been working like new ever since I got this third battery.
My son's phone (a different model) also showed the same symptom as the second battery I had. He also thought he needed a new phone. Instead, I got him a new battery from eBay for AUD$9. It solved the problem rightaway.
I believe that the electronics in a phone far outlasts the lifespan of a battery. I believe that many people unwittingly throw away their phones after a few years when they could have just replaced the battery. The marketing strategy of service providers makes it very tempting to go out and get a "free" phone and a new plan every two years. The phones are by no means free. Those plans that look cheap, e.g. $19 per month, have very low cap values and most users end up paying much more than the $19 per month. Save yourself some money; buy a new battery the next time your phone plays up.