Motorcycle batteries have one thing in common, they are all too small. Even the biggest ones are too small for what they have to do. These days, it seems as though nobody believes in kickstarters anymore, so a well taken care of battery is doubly important.
A battery will loose 1% of its charge per day just sitting there doing nothing. So you can see that you just can't park it, walk away, and come back in 4 or 5 months and expect it to work. Amazingly some of them do work after that kind of abuse, but a greater number of them don't. The cure? Trickle charge the battery at least once a month.

If you don't, it's OK, I don't care. It's not my Battery!!!
I have found that it is best to use a 1 Amp or smaller trickle charger. Most motorcycle batteries are small and just can't take the high rate of charge that a 6 or 10 Amp charger gives. A general rule of thumb is that a battery should not be charged at more than 1/20 its rated capacity. As you can see, even a big motorcycle battery is maxed out by a 2 Amp charger. The battery should be charged till it is gassing (bubbling) freely, but then stop, don't boil all your water out! The temperature of a battery must not go over 112 degrees. If it does, the battery may be ruined. If the electrolite is below the plates add distilled water till the plates are covered and/or to the top of the fill line. Always unplug the charger before removing the leads from the battery and keep the battery away from flames and sparks. Remember all that gassing in the battery is producing Oxyhydrogen Gas. This can and will EXPLODE the battery, covering you with acid which will burn you like you will not believe. Baking soda will neutralize battery acid and water will rinse it off and dilute it. Mixing baking soda and water together works great. You can use it anywhere on the bike or yourself to remove battery acid. Keep lots of both on hand.

If You Don't, It's OK, I Don't Care. It's Not My BODY!!!
If a battery is totally discharged, and you goof and connect the plus and minus leads backwards you can change the polarity of the battery. Plus is minus, minus is plus. I had a customer who did that. That was a fun one till I figured it out. I just drained the battery to zip, recharged it the right way... worked just fine... go figure.

Here is a trick that works maybe 60-70 % of the time...You have a battery that is almost junk, will not hold a charge. Put some Epsom salt, maybe 1/4 teaspoon, in each cell. Sometimes, not always, it will keep it going another 2-3 months. Won't hurt anything... the batterys junk anyway, but it might get you through till winter!

It's a funny thing, you can have a battery that reads 12.5 volts on a volt meter, but still will not light a small light bulb. Volts only tell you part of the story. The acid in the battery must be good. The Specific Gravity must be around 1.26 to 1.28 (in a fully charged battery) and you measure this with a hydrometer. The only trouble is this... there just isn't enough fluid in most motorcycle battery's to test with a big auto hydrometer. You must use one of the cheapy, small, 5 ball, pencil types. Oh well, at least they work and between the two you will have the whole story.