If you only have new, modern motorcycles I suppose you could stop reading now. Especially if you live in San Diego where the weather's warm and you can ride all year. For those of us who like vintage bikes life is filled with carburetors to clean and batteries to charge. I told you about Yamaha Carb Cleaner some time ago.
Battery Tender is the only way to go in the charging department as far as I'm concerned. They make a variety of models to fit all needs and budgets but the Junior has always worked good enough for me. The benefit of a BT is that they charge the battery without excess current which will quickly kill a battery. I don't know why or how but a car charger will knock off a good motorcycle battery faster than a cruiser pilot can down a 6-pack.
I used to recommend getting the cheapest, lamest battery charger you could find at the Dollar Store because it would only charge at .5 amps or less. Battery Tender is better because once it attains full charge it somehow throttles back to maintain that charge and no more. You can leave the charger on the battery all winter and it won't boil off the electrolyte.
Cycle Gear had them for $19.95 Thanksgiving weekend but they're $29.95 now. Amazon has both the 6v and 12v Juniors for $24.99 and $21.68. Free shipping on anything over $25 so you save if you order one of each (or order one of Kevin Cameron's books). Every moto outlet has them so you may be able to find even better deals. Post a reply if you find one.
Tell them I sent you, their response will be shocking!