I have one of the original DS models, so it's around 5 years old, if older. A couple of years ago, I received a new battery for it for Christmas, since the old one wouldn't hold a charge anymore. Now I'm experiencing the same thing. And no, I haven't played it that often (I don't play it for months at a time), and I didn't leave it plugged in constantly. When I try charging it, the orange light stays on for about a minute, if even that, and the battery life is just under 15 minutes. Is it possible that the problem isn't with the battery, but the fact that the device is old?
Edit: I just found that if I plug in the charger while the DS is both on and open, the orange charge light stays lit, but the moment I shut off the DS or close it, the light shuts off. Simply opening the DS or turning it back on won't turn the light back on, so I have to turn it on and then replug it in.