Spindles

Hi All, Hopefully someone can help because this has been driving me nuts. I have a 1998 SR125 and ever since I bought it second hand a couple of months ago I've had intermittent problems where it won't start. I've bought a trickle charger and charged the battery both in and out of the bike. What seems to be happening is that the battery is not getting charged when I'm riding so eventually the battery just gets drained from using the lights and the ignition and then won't start and I have to charge the battery again. Any ideas?