Why does my car battery keep dying?

There are three distinct causes, and they need different fixes, so identifying which one you have matters more than replacing the battery again.

The battery may simply be worn out. Most last three to five years, and heat shortens that considerably more than cold does, despite cold being when failures become obvious. A battery that starts the car fine in summer and fails in the first frost was already weak; the cold merely exposed it. Most parts shops test batteries free, and the test measures capacity rather than just voltage.

The charging system may not be replacing what starting uses. If the alternator is failing, the car runs on battery power until it is exhausted, which produces repeated flat batteries and often a warning light. A voltage reading at the battery with the engine running distinguishes this immediately: a healthy system reads noticeably higher than a resting battery.

The third cause is a parasitic drain, meaning something continues drawing current after the car is switched off. Every modern car draws a small amount for its computers and alarm, but a fault, an aftermarket accessory such as a dashcam wired incorrectly, or a light staying on in a boot or glovebox will flatten a battery over days. This is the cause when a new battery also dies and the charging system tests fine.

Driving pattern is a fourth factor that is not a fault at all. Starting the engine uses a lot of energy, and a series of very short journeys never puts back what each start takes. A car used only for brief trips will slowly discharge, and an occasional longer run or a maintenance charger resolves it.