You generally can. The problem is that the batteries are generally so specific to the car, you can't just swap them out with any random battery and expect the same performance. Additionally, given the immense power most battery packs put out, replacing individual cells (or banks) of batteries can cause an imbalance that will damage or degrade the pack over time. The best solution most of the time is to replace the entire pack at once if there's damage to a bank, and have the installer take the rest of your battery as a core rebate.