The Yuasa part # for the stock battery is YB10L-B2.

When should you place a battery on charge? Whenever the open circuit voltage drops to the low 12V range. I usually hook up at 12.3V and charge till the voltage across the battery terminals reaches the mid 14V range and then disconnect the charger. One day later, your open circuit voltage will be in the high 12V range (12.9V).

Don't buy a car trickle-charger, they put out way too much current. Excess current causes physical damage to the battery from which it will not recover. If you use a charger that you would use to quick start your car the amperage will warp the plates in the battery. If you have a dead battery and try to jump start the bike with a car or a gas station charger you should pull off the regulator plug or one day you will fry it. Get a charger made for bike batteries.

In general, charging current on an ordinary lead acid battery should not exceed 10% of the AH capacity. The GS has a 12AH battery, so don't exceed 1.2 Amps and not for longer than 10 hours. It's worth investing in a good quality motorcycle battery charger. It will easily pay for itself over the life of a couple of batteries saved from an early grave!

Another note for people who would just say, Well, I don't need to keep the voltage up, I'll just recharge it next spring." The battery carries a specific gravity. When the battery discharges that specific gravity changes and the lead plates inside the battery sulfate, swell up, and destroy the battery. Remember, your voltage may come from the electrolyte, but your cranking power comes from the plates. A battery replaces the specific gravity by releasing a chemical from the plates. Once that chemical is depleted, the only cranking power you have left is in the electrolyte, and that doesn't last very long. You can take almost any brand new battery, completely discharge that battery 8 times, and the battery becomes useless. Your battery isn't a cornucopia of energy; it does have a certain designated lifespan.

A charger is necessary for any motorcycle. The kind you get depends on the battery. If you have a maintenance-free battery you have to use a battery-tender since they are a constant-current charger and do not trickle-down. You can use a big car charger on these kinds of batteries also but you have to pay attention the amps X hours formula and do no allow the current to drop more then one amp from the initial amperage.

You can use either a motorcycle type trickle charge or a battery-tender on the older style refillable batteries. If you use the trickle type you will want to monitor the charging progress with a hydrometer. These types of charges can overcharge a battery resulting in permanent damage. If you use the more expensive Battery Tender you don't have to worry about overcharge since they have an electronic sensor that shuts down the charging when the battery is up.  A battery tender is set at a certain voltage sensitivity. When the battery drops below that it comes on, charges, and then stops at the set voltage.

Buy an ampmeter gauge at Radio Shack and wire it into your charger. Store your batt. indoors and once a month put it on the charger. It should pull between several to 5 or 6 amp.s initally, but quickly start dropping. You don't have to stand there and watch it, but keep and eye on it. When it gets down to 1 or 2 amp.s  (probably within 1/2 to 1 hr.), it's done and you can park it for awhile. There is no need to do this more than once a month.

Want more battery info? Then check out the Yuasa battery site at http://www.yuasabatteries.com/  and click on "Literature".  Yuasa also makes some nifty chargers that you should look at. Check them out and look at the features.