BATTERIES
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.
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. You might also make sure the water you use IS NOT salt water. It seems salt water and battery acid combine to make Chlorine Gas which will KILL you. Something to remember !
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.
I once worked in a shop that was responsible for a fleet of about 120 trucks and cars. Part of my job was making sure the batteries were up to snuff on all those vehicles. I did a lot of battery charging, battery changing, checking fluid levels, checking voltages, and the like. I found that a battery could have a good volt reading and be bad. It could also have the right hydrometer reading and be bad. You needed to check both. Even with BOTH volts and hydrometer readings good, I had one or two that just plain would not crank. Put a new battery in and problem solved. Perhaps they were sulfated badly. I could not tell as the cases were all black. This was quite rare. One or two out of hundreds and hundreds.
On motorcycle batteries you can usually see through the cases and see the white gook, sulfate, on the plates of the battery. Once you see the sulfate, that batteries on it's way out.
Remember too. On old British bikes, like Triumph, BSA, Norton, etc. They used a POSITIVE GROUND electrical system. It would behoove you to remember this if you have to work on one !
For a PDF file to print out a State of charge chart, Click 
For size and applications for Yuasa batteries, Click For more information, theory, lots of good stuff from Yuasa, Click For a number of handy PDF files on various battery related procedures, right from the horses mouth so to speak, Check out this handy page from Exide Batteries, Click Battery Council International, Click Here is a Zip file with a lot of Info on all types of Vehicle batteries, Click
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