Gen4/gen5 Altimas all have a "smart" charging system that's entirely dependent on the Battery Current Sensor (located on the main ground cable next to the battery). If the sensor goes bad or the ground cable has high resistance, it can cause the smart system to get stupid and chronically over- or under-charge the battery. That results in the car "eating batteries" one after another, either from boiled plates if it's overcharging or repeated deep cycles if it's undercharging. Unfortunately, the diagnostics Nissan gives us techs for the current sensor are pitiful, and the BCM (which monitors the sensor) doesn't have the tools to detect a lying sensor, only a flatlined one. So the best diagnostic approach is usually to disconnect the sensor, replace the battery if it's weak, and see if that solves the problem. I wouldn't trust Nissan's "standard" battery testers either, or the ones typically used in a parts store. I've seen all of them call batteries "good" which were dead 20 minutes later from a simple ignition load. Nissan does have a new tester called the DCA-8000 which takes a bit more time and setup, but actually works right and gives trustworthy results. Ask the dealer to test your battery with that and not the "quickie" testers.