Nobilis @ instructables.com has build a lead acid float charger out of an old phone wall charger:
If you do not use your motorbike on the winter or your car stands for weeks — you better have a float charger connected to the battery. Otherwise you probably encounter engine starting problems, and your battery’s lifetime is getting shorter. In a depleted battery the emerging sulfation on the negative plates is avoid the battery to take adequate charge again, and a vicious circle happens: the battery is unable to get fully charged, which increase sulfation, which avoids proper charging, and so on, until your rather new battery is ready to be recycled.
Lead Acid Battery float charger – [Link]