When you are using your bike (or sled or riding mower), check the fluid level every month or so and add WATER ONLY (not acid) (I use tap water - I used to work in a battery plant and that's what they used - maybe if I used distilled water my batteries might last longer, but I usually get 5 or 6 years as it is) to bring it up to the correct level. Do this with the bike on the centre stand on level ground or remove the battery and service it on a level surface. You can't be sure of the level if the bike is on the side stand.
During the off season, TRICKLE charge the battery overnight every 6 weeks religiously. This is imperative, as batteries discharge if left to their own devices, and a battery that is at less than 90% or so charge can freeze, causing the housing to crack and leak acid when it thaws or warping the plates and causing possible internal shorts. If it doesn't freeze (eg. the battery in a winter machine stored forthe summer) the capacity can be reduced by leaving a battery in a discharged state for an extended period. In other words, if you leave it for 2 or 3 months, even in warm weather, you can reduce the battery's ability to produce current. I mark ahead every 6 weeks on my calendar when I put a machine (bike, lawn mower, anything with a battery) away and when that date comes up I start rotating the charger onto every battery in the shed for one night each.
When I worked at a battery factory we were told that if a battery was allowed to sit without a charge for 6 months in ideal conditions it would be "fully discharged", and that if a battery is left "fully discharged" for even a couple of days, the battery will not be able to be re-charged to it's full capacity again.
For a 12v battery, "fully discharged" means somewhere in the 9-10v range. If it is discharged below this point it will never be able to be fully re-charged again even if charged immediately. This is because the lead oxide in the plates changes to lead sulphate when the battery is discharged, and if the battery is discharged too far or is left discharged the lead sulphate crystals grow bigger and then can't readily turn back into lead oxide when current is passed through the battery by a charger.
Note: "Ideal conditions means absolutely no current drain. Since current drain can be caused by dirt on the battery, moisture in the air, and a number of other conditions, batteries stored in the plant were always re-charged every 2 months. Left in a vehicle, there are even more possibilities - perhaps a few microamps because of some dirt on a switch somewhere?
Here is how a lead/acid battery works:
When a lead acid battery is first put into service, it is filled with a specific amount of a specific type of electrolyte. The electrolyte is usually sulphuric acid dilluted 50/50 with water (H2O+SO2=H2SO4).
When you charge the battery, it creates a chemical reaction inside the battery which causes some of the sulphur atoms from the electrolyte to move to the plates and in so doing liberates some of the hydrogen atoms (because of the molecular bonds, I think), which vent to the atmosphere. Hydrogen is highly combustable (remember the Hindenberg), and you should never smoke or operate anything that could cause a spark near a charging battery.
When current is drawn from the battery, the chemical reaction causes sulphur atoms to move back to the electrolyte, but since there is no longer enough hydrogen to make H2SO4 with all of the sulphur & oxygen, it bonds with what it can and in the process liberates some of the oxygen atoms which don't have hydrogen to bond with them, which vent to the atmosphere.
Since hydrogen and oxygen (the components of water) are constantly being vented during charge/discharge cycles, but the sulphur only moves from the electrolyte to the plates and back, you only need to add water to top up the battery. If you add acid you will increase the ammount of sulphur in the cells, which will shorten the life of the battery.
The only reason for ever adding anything other than water to a battery is if some of the electrolyte has been spilled (eg. the bike falls over and isn't picked up quickly enough), and even then the specific gravity of the remaining electrolyte should be measured and the replacement electrolyte should be diluted with water to match in order to maintain the correct ammount of sulphur in the cells.
I only use tap water because I'm cheap & lazy. In my opinion, since I feel it is more work to obtain distilled water and use it than it is to trickle charge my off season batteries every 6 weeks, and I am getting more life out of a battery than many who use distilled water but aren't so fanatical about off season maintenance, I guess it's OK.
Note: I wouldn't use tap water with a high iron content in a battery. Even trace ammounts of iron in a cell can destroy it. Apparently it has to do with the iron oxide and chemical bonds.
In the battery factory they used municipal treated water (tap water) in all the new batteries. The main reason for using the purest water you can get (usually distilled - unless you have an electric vehicle with Ballard fuel cells, then you can collect really pure, newly made water from it's outlet) is that since pure hydrogen and pure oxygen are vented from the battery, any impurities in the water you add will stay in the electrolyte for the life of the battery. I decided long ago that it was better (IMO) to top a battery up a few times during it's life with impure water than to use it with low electrolyte levels. Maybe it's just me, but it seems that batteries don't need to be topped up as often as they used to. I remember doing monthly battery maintenance, but now I only need to add a bit of water 1 or 2 times a year. It's only a matter of time before someone figures out that bikes should have maintenance free batteries, and we won't need to add any.