First test the alternator, get a digital multimeter from Dick Smith or somewhere similar, ($20), set it on 20V DC and test at the battery terminals(+ to + and - to -) when the car is warmed up and running.
If the alternator is OK, the reading should be around 14 volts. If it is less than 13, your alternator needs fixing, and if you leave it too long, so will the battery.
The alternator output varies with temperature, so make sure things are at normal operating temperature before testing.
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