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If your phone can turn on but the car takes longer than usual to start, that's when you should swap it out. The thing is, a lot of people only do something when it dies completely on the road, which is an unnecessary hassle - better to get ahead of it when it starts showing signs. Depending on the model and how old the battery is (they usually last between 3 to 5 years), it's worth getting a test done at a mechanic if you're unsure, because sometimes it's not the battery but an alternator failing or a corroded connection that gets fixed quick.
Is the battery having trouble starting the car, or is this more of a preventive thing? Usually you replace it when the car starts taking longer to turn over (takes more time to start) or when it's been around 4-5 years, whichever comes first.
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