Many people take their car battery for granted and then curse it when it fails. This is not really fair though since there are tremendous demands on your car battery every day if you think about it. The main load comes when first starting your car, as it has to supply sufficient current to overcome the ineretia in the starter motor and get it to turn the engine over. The first sign of a failing battery is often a reluctant start up with the starter motor working more slowly than usual. This tends to occur in cold weather as lower temperatures reduce the battery's efficiency.
A car battery is thus a vital part of your car, as without it nothing will work at all. New car batteries are not so expensive when you think about it that way, and investing in a new one in time to prevent a sudden and complete failure is a wise move. More breakdown callouts are for a flat battery than almost anything else according to the roadside help organisations.
These days the car battery is fitted when the vehicle is new and there is virtually no maintenance required, but the average useful lifespan is about 3-5 years so if your car is approaching that age it is as well to bear that in mind and be prepared to replace the battery at the first signs of trouble. Sometimes of course the battery will be flat in the morning when you go to start the car because the lights have been left on, or because some electrical fault has developed that has drained current away. More often than not though, it is a sign that the battery is not holding an adequate charge any more and needs to be replaced.
When buying a new battery you need to replace like with like. The make of battery need not be the same but the specification needs to be. You should make sure the new battery is the right size to fit into the space available, and is of sufficient capacity, usually 12 volts. Some Japanese cars may have smaller batteries (even 6 volts), and a suitable replacement is sometimes not easy to source in the UK. Some even have different size terminals making it difficult to fit a diffferent but equivalent battery. This is worth checking before the time comes when you actually need a new battery.
A final point to mention is that new batteries generally come fully charged, but if not they need charging up before use, so this is also worth checking.