If you work carefully when removing the battery, without touching any foreign objects with the tool, there is no difference in the order of removing the terminals — the electrical system doesn't care. However, under the hood, conditions are far from sterile: the space is limited, there are pipes and metal parts around, and the tool can easily slip.
The key to understanding is in the design of the on-board network. The negative terminal of the battery is connected to the body, forming a "ground". While the battery is connected, any unpainted metal part of the car effectively becomes a negative contact: the engine, frame, mounting bolts, and even the battery clamping plate.
If in such a situation the positive is unscrewed first, and the negative remains in place, there is a risk of a short circuit. It is enough for the wrench to slip and touch the nearest metal part — and the positive will be shorted to ground through the tool. A spark occurs, a bang, and the smell of burning insulation appears. The result depends on the duration of the contact: sometimes everything is limited to a blown fuse, but with a longer short circuit, melted connectors and damage to the electronics are possible.
A rule that always works
To avoid such situations, a universal sequence is used:
- the negative terminal is removed first
- then the positive terminal is disconnected
- when installing, the order is reversed: first positive, then negative
After disconnecting the negative, the body no longer participates in the circuit. This means that even accidental contact of the positive terminal with a metal tool will not lead to a short circuit. Therefore, the negative wire is always removed first and returned last.
It is important to prepare before starting work. The engine must be turned off, the ignition off. Removing the battery with the engine running can cause a voltage surge from the generator: if on older cars this often went without consequences, then in modern cars the electronics can be damaged — from the radio to the engine control unit.
It is better to immediately remove the ignition key and put it in your pocket. If left in the cabin or on the body, it can cause an unpleasant situation: when de-energized, the alarm sometimes blocks the doors. As a result, the battery has already been removed, and access to the cabin is closed. As an additional measure, it is wise to leave one of the doors open. It is also worth considering that on some BMW models, disconnecting the battery can be perceived by the system as an attempted theft, which leads to a complete lock.
A minimal set of tools is usually enough for the job: an open-end wrench (most often 10, less often 12 or 13) and a socket wrench of 13 for attaching the battery. Gloves are desirable, since in case of electrolyte leakage you may encounter sulfuric acid.
Sometimes the terminal is difficult to remove: the tip literally "sticks" to the output due to oxides. Trying to knock it down with a blow is a risky solution, since the output is made of a soft lead alloy and is easily deformed. In such cases, it is better to treat the connection with penetrating fluid or a solution of soda, wait a few minutes and then carefully swing the terminal. Cautious prying with a screwdriver is allowed, but without sudden efforts.
After removal, it is useful to inspect the contacts. White or greenish coating indicates oxidation. It is removed with fine sandpaper or a special brush. Contamination increases resistance, which leads to undercharging of the battery and difficult engine starting.
The use of protective grease requires special attention. Its task is not to improve contact, but to protect it from moisture and acid fumes. An error occurs when grease is applied before installing the terminal: in this case, a dielectric layer appears between the contacts, which impairs conductivity. The correct order is different: the terminal is put on, tightened, and only then the connection is covered from the outside. Then the contact remains clean, and the outer surface is protected.
Working with a battery is associated with another risk that is often forgotten. Hydrogen and oxygen are released inside a lead-acid battery during charging and discharging. This mixture is easily ignited, and a small spark is enough — for example, one that occurs when connecting a terminal. Situations after prolonged attempts to start the engine or immediately after charging, when gas evolution increases, are especially dangerous. If the ventilation holes are clogged, gases accumulate inside the housing, and a spark can lead to its destruction with electrolyte splashing. Therefore, any sources of open flame near the battery are excluded.
After disconnecting the battery, the behavior of the car depends on its age and level of equipment. On older models, the consequences are minimal — most often only the clock is reset. In modern cars, everything is more complicated: the throttle settings may be lost, the automatic transmission "forgets" the driving style, the radio requires a code, the seat and mirrors will forget the driver and need to be recalibrated. To avoid this, an external power source connected via the cigarette lighter is sometimes used. It maintains the voltage at a level sufficient to save the settings.
After installing the battery, it is not recommended to immediately load the engine. It is better to turn on the ignition, wait for the self-diagnosis to complete, and then drive a few kilometers in a calm mode — during this time the systems will restore the operating parameters.
Features of "jump-starting"
When connecting to another car, the risk of error increases, especially in the dark or in the cold. Incorrect polarity connection leads to serious consequences: two batteries are included in the circuit, and when the engine is running, also a generator, which creates currents of hundreds of amperes and quickly overheats the wires.
To reduce the risk, several rules are followed:
- the negative wire is connected not to the terminal of the discharged battery, but to the metal part of the engine
- this allows you to divert the spark from the battery, where there may be gases
- the positive terminal is marked in red and with a "+" sign, often covered with a cap
- negative — black, with a "–" sign
- the positive terminal usually has a larger diameter, which eliminates incorrect installation