Checking your battery before winter or before your next vacation trip can save you lots of grief. Checking the battery requires a bit more than peeking into the filler holes to see they have sufficient water.
First, a few cautions. Batteries give off two very explosive gases - hydrogen and oxygen. NEVER smoke or work with open flames around a battery. Also, the electrolyte is highly corrosive (it's a strong acid) so keep away from skin or clothing, and wear goggles to protect your eyes. If you get battery acid on you, wash it off immediately with large quantities of water or neutralize with a baking soda solution (yes, baking powder will do, too).
Make sure that the battery is securely mounted in the vehicle. The plates inside a battery can be severely damaged by bouncing, impacting its ability to provide power to your car.
If the battery and cable connections are corroded or loose, the car will be harder to start. During winter's cold weather more unburned gasoline dirtying the crankcase oil, which affects engine lubrication and performance (and the cycle continues…). All it takes is a light film of corrosion/oxidation, though very hard to see, to affect the ability of the battery to accept a charge. You should wire-brush clean the cable and battery terminals at least once a year. You should also clean other end of the cables, and the ground connection(s) to the body and/or engine, then tighten all the cable connections. You should also replace any corroded or frayed cables.
Clean the plastic surface of the battery by rinsing the surface with warm water after cleaning the terminals, and then give it a wipe-down with mild detergent to cut the grease, and rinse the battery top again. This will prevent an oily film from collecting dirt, creating a weak conductor that will slowly discharge the battery, over time. Commercial battery cleaners typically include a baking soda solution or ammonia, and household detergents are typically alkaline. Do not let any cleaning fluids get into the battery itself since even a few drops can substantially decrease the battery's efficiency and product life. Lastly, coat all the exposed metal parts of the terminals and cables with grease (not oil) to prevent further corrosion.
Fill each cell to just over the top of the plates inside the battery. Do not overfill, or you can lose some of the electrolyte, and risk that any spilled corrosive water/battery acid mixture will quickly corrode painted and unpainted metal surfaces. Refill using distilled water if possible, or at least clean water (if your tap water leaves chalky residue in your bathtub, you should use bottled water rather than tap water). If you find that you have to add water frequently, the cells may be cracked and you should get a charging system checkup.
Watch for other charging system problems like acid buildup on the outside battery surfaces or terminals, and watch for (much rarer) bulging of the battery, which indicates the positive plates have swelled.
Both over-charging and under-charging can be fatal to batteries. If the battery requires a recharge, use a slow charge rather than a "quick charge," t get battery power to the desired level. A badly sulfated battery can even explode during a quick charge. Normally, a battery should be charged at a rate that is no more than 10-20 percent of its rated capacity. For example, a battery rated at 55 ampere-hours should use a 5.5 to 11.0 ampere charging rate. By contrast, a quick charger will pump in current at rates of 35 to 50 amperes. The lower rates are typically provided by inexpensive home chargers, take a bit more time, but are much more "battery friendly."