Battery terminal corrosion can be as a result of several things: overcharging, poor electrical connections, leakage or simply not storing your battery correctly contribute to corrosion. The earlier you see signs of corrosion on your battery terminals, the more likely you are to halt the corrosion, before it gets to an irreparable stage.
Lead-acid battery terminals, the type of battery fitted in your car, probably suffer from corrosion more than most other types. They can be exposed to extremes in temperature and weather, contain sulphuric acid, which is highly corrosive and produce substantial amperes to start your car and, if electrical arcing occurs due to a poor connection, corrosion sets in. Common signs of corrosion on lead-acid battery terminals include: yellow-green sulphur deposits on and around the terminals, white deposits that are like powder, brown rust forming and pitting on the terminal poles. These are generally caused by leakage, overcharging, exposure to water and poor electrical connections respectively.
Non-rechargeable batteries seldom corrode; due to improvements in battery design and the fact that batteries produced by reputable companies are hermetically sealed. However, corrosion can still occur, particularly in non-alkaline batteries. Alkaline batteries, as the name suggests, don't contain acids, so corrosion is far less likely to occur. Signs of corrosion include brown rusting around the seal between the terminal and the outer casing, a whitish crust forming over the terminal and a thick liquid oozing from the seals. These conditions mean you need to dispose of the battery immediately; don't attempt to clean off the deposits.
Rechargeable Battery Pack
Rechargeable battery packs contain battery cells that are wired in series to increase output voltage. There are many internal connections, which can corrode. However, because the cells are inside a plastic casing, you only get to notice corrosion when it's probably too late to remedy it, as the only visible area are the two terminal connectors on the battery pack. As with signs of corrosion on non-rechargeable batteries, look out first for rust forming. It usually means water has somehow entered the battery pack. If you see chemical deposits, usually white through yellow to green, the chemicals inside the cells have leaked and are finding their way out of the battery pack. If you see rusting, you can try to clean it off using wire wool and then check again in a few days to see if it has reappeared, but if you see chemical deposits, it's sensible to dispose of the battery.
The way you store batteries is important. Storing them correctly prevents corrosion starting and once it's started, it can be difficult to stop, often resulting in you having to dispose of your batteries. Keep them in a cool, dark, dry environment. Somewhere that doesn't have excessive temperature fluctuations is also helpful. Regularly check the batteries for signs of early corrosion. If the batteries are stored in damp or wet conditions, the terminals will rust and turn brown. If the temperature is too hot, the chemicals inside the battery may expand causing the casing to rupture. You will see a greenish substance on the terminals that's sticky like glue, but don't touch it with your bare hands, as it can burn. Look out for white powder on the terminals; it usually means your battery is discharged and dead.