2004 BMW 745Li Battery Replacement Cost

While the engine is running, the alternator recharges the battery and provides electrical power to the vehicle. The battery stores this electrical power and, when the driver turns the key, uses the stored power to start the engine. 

The battery is considered dead when there isn't enough charge to start the engine or run electrical components. Normally, the first sign is dim lighting in the interior of the vehicle, and when you try to start it, the engine may crank slowly, or not at all. A very rapid clicking noise is normally heard if there is any charge left in the battery, but in many cases the vehicle will be completely unresponsive. If the battery is losing charge while driving the vehicle, the battery warning light will illuminate, and the engine may stall.

Only with some help. A jump start will be necessary most of the time. Cars with manual transmissions can be push-started, but this is tricky and can be dangerous, so battery problems should be addressed right away.

There's no specific mileage interval or age when batteries fail. But the older a battery is, the more likely it is to die. Other factors that can affect battery life include the climate you drive in, how often you start the car (or let it sit idle), and proper maintenance of the starting and charging system.

If the battery dies, there may be other factors in its demise. Cars that have the symptoms of a dead battery should be properly tested using a machine. This device will test every part of the starting and charging system. If the battery's charge is too low, it will be recharged, retested and possibly replaced. Other parts, including the alternator, battery cables and wiring, may need repair or replacement, as well.

For most vehicles, battery replacement is typically fast and easy. First, apply the emergency brake, remove the keys from the ignition and turn off all electrical components.  Remove the negative battery cable first, followed by the positive cable. After removing both cables, loosen and remove the holding brackets. The battery will then be free for removal. When installing the new battery, always connect the positive cable first, followed by the negative cable. This order is important to avoid damaging the computers or software in the vehicle. Note that some vehicles place the battery in the trunk, the front or rear wheel well, or under the seats. In these instances, you may need to dismantle other components first.

We recommend using a battery rated identically to the original one. Using too small or too large of a battery will result in overcharging or undercharging, and will lead to repeated failure.

Replacing the battery is pretty straightforward for most cars. However, there is a risk of electrical shock or causing electrical damage to the car. That's why it's important to follow the right steps when replacing a battery. If the battery's location is hard to get to, you may be better off have a trained technician handle the repair.