2004 BMW 745i Battery Cable Replacement Cost

The positive battery cable transmits power to the vehicle's computer, and starting and charging system, while the negative battery cable is connected to the chassis to provide a universal electrical ground for the entire vehicle. This creates a closed-loop system where power can continuously flow through the battery, and power all vehicle functions.

The battery is considered “dead” when there is insufficient charge to start the engine, or run electrical components. When battery cables fail, this condition will occur. Normally, the first sign is dim lighting in the interior of the vehicle, and when attempting to start, the engine may crank extremely 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 to any input. If the battery cables fail to transmit power while driving, the battery warning light will illuminate, and the engine may stall from lack of electrical power needed to run. Note, the battery may have plenty of charge, but the same symptoms are possible as that charge has no route out of the battery. In many vehicles manufactured from 1996 until present, the check engine light may illuminate, alerting the driver that an on-board diagnostics trouble code has been stored.

Vehicles with manual transmissions can be manually started by pushing the vehicle to considerable speed, and releasing the clutch quickly if the battery is dead. This method is commonly called push-starting, and is an effective method of manually starting the engine using the vehicle's transmission to turn the flywheel. This will allow the vehicle to create electrical power using the alternator, but if the battery cables are faulty, the electrical power from the alternator may never reach the vehicle's computer, and will prevent the engine from running. The vehicle may be able to start using jumper-cables, but if the battery cables are completely faulty, there may be no change in symptoms. Lastly, if the battery cables show corrosion at the battery terminal, the symptoms may return as soon as the jumper-cables are disconnected from the battery terminals.

Battery cables fail at no specific mileage interval or age. The older a battery cable, the more brittle the sheathing will become, and exposure of the bare wire to the elements will accelerate failure. It is most typical to experience battery cable failure after several years of use, or in the case of improper installation. Also, battery cables are known to fail due to unintended friction and vibration when they rest unsecured on another component.

We have all heard of the term “dead battery”, or experienced this issue ourselves, but it is important to note that a “dead battery” does not necessarily mean the battery is at fault. Vehicles exhibiting the symptoms noted above will be connected to a starting and charging system test machine. This is an advanced device that allows automatic testing of every part of the starting and charging system by following onscreen instructions. The machine will display results, and indicate the possibility of battery cable failure, but may also require the servicing technician to complete visual inspections while testing is in progress. If the battery is found to be discharged below the required voltage, it will be recharged, retested, and possibly replaced before diagnostic work continues.

Battery replacement for most vehicles is typically fast and easy. The battery cables should be removed, any battery insulation material will be removed, and the battery retaining bracket will be loosened or removed. The battery will then be free to removed. The new battery will be installed in the reverse order of operations. Note that some vehicles place the battery in the trunk, the front or rear wheel well, or under the seats. In these instances, accessibility of the battery may dictate dismantling several components before replacement can being.

We recommend using battery cables rated identically, or larger than the original battery cables. Using too small of a battery cable will result in undercharging the battery, extremely high temperatures in the battery cables, fire hazard, and repeated failure.

A dead battery can be caused by several components. Most commonly, the issues causing battery power depletion are battery cables, failed connections due to corrosion, an electrical system malfunction causing battery drain while the vehicle is off, and long periods without using a vehicle. The battery must be tested prior to replacement, as the battery is very often replaced needlessly. Most commonly if the battery cables are at fault, corrosion is present at, or around the battery terminals, or starter motor, and replacement of the entire cable may not be necessary.

Replacing the battery cables is a great DIY project that can be completed in short order for most vehicles. However, there is a risk of electrical shock and other electrical component failure if arcing occurs at the battery terminals, or if the battery is not disconnected first. Prior to performing this service, the vehicle's battery location should be noted, as well as which components the battery cables power directly. If the location requires much disassembly, this job will be best trusted to a trained technician.