Disconnecting the car battery
Disconnecting the car battery is one of the simpler jobs on a vehicle – and yet mistakes can have expensive consequences. The wrong order, improper handling or forgetting a few precautions can quickly lead to a short circuit, control unit damage or the loss of important settings. In this guide, we show you step by step how to safely disconnect your car's battery – and later reconnect it, what you need to pay attention to in modern vehicles with start-stop system and on-board electronics and when it is time for a new battery.
The detailed guide with all steps: Disconnecting the car battery.
Why and when should you disconnect the car battery?
There are many occasions when disconnecting the car battery is necessary or sensible. Most commonly it is the battery change: a weak or defective battery must come out, the new one go in. Also for cleaning work on the terminal clamps or on the battery console, it is advisable to disconnect the battery from the on-board electrical system. If you are screwing on the alternator or the starter, disconnecting is mandatory to avoid short circuits. Likewise if you are taking the car out of service for several months: a connected battery discharges through the quiescent current of the control units – disconnecting prevents deep discharge, which can permanently damage the battery.
Safety precautions before disconnecting
Before you apply the spanner, you should take some basic safety measures:
- Switch off the engine and remove the key: The on-board electrical system must be de-energised. With keyless systems, press the start button until the ignition is completely off.
- Switch off electrical consumers: lights, radio, seat heating, blower – all off to minimise voltage spikes when disconnecting.
- Prevent short circuits: Do not wear metallic jewellery on your hands and arms. A ring that comes between the positive terminal and the bodywork can trigger an arc and cause severe burns.
- Have tools ready: A 10 mm or 13 mm open-end spanner (or a ratchet with socket) is usually sufficient for the terminal clamps, plus possibly a screwdriver for the battery holder. Keep the tool insulated or wear rubber gloves.
- Power supply for control units: In many modern vehicles, the control units are reset when disconnecting; this can put the engine control unit into a learning mode. If you want to avoid this, use an external OBD memory saver or a memory-save cable with a 9V block battery that is fed via the cigarette lighter. However, check beforehand whether your vehicle tolerates this.
The correct order when disconnecting the car battery
The golden rule is: Always disconnect the negative terminal first. This is the only way to prevent a short circuit that can occur when loosening the positive terminal if the spanner accidentally touches the bodywork – because the earth connection still exists. The detailed procedure step by step:
Step 1: Identify and loosen the negative terminal
The negative clamp is almost always marked with a black cable and the minus sign (-). Loosen the nut on the clamp with the appropriate spanner by one to two turns. Wiggle the clamp carefully and pull it off the terminal. Make sure the detached clamp is placed safely to one side with insulation – it must not come into contact with a terminal again.
Step 2: Loosen the positive terminal
Now loosen the positive clamp (red, +) in the same way. After disconnecting the negative cable, there is no longer any danger here, as the circuit is broken. Put this clamp aside as well.
Step 3: Remove the battery holder (optional)
If you want to remove the battery, also loosen the retaining plate or bracket at the battery base. Depending on the model, this is a screw (often 13 mm) or a plastic clip. Lift the battery straight out – 60 Ah models weigh around 15 kg, 100 Ah models around 25 kg. Best to use the carrying handles and keep your back straight.
Quick overview as a checklist
- Engine off, key removed, consumers off.
- Loosen and pull off the negative terminal clamp.
- Loosen and pull off the positive terminal clamp.
- (Optional) Loosen the holder and remove the battery.
Effects on the vehicle electronics
When the battery is disconnected, the entire electrical system is briefly de-energised. This causes volatile memories to be erased: the clock resets, radio stations are lost, seat and mirror memories disappear, and the window anti-trap automatic function loses its end positions. Modern vehicles often also have to relearn the throttle valve adaptation; this initially manifests as an uneven idle.
Tyre pressure monitoring systems (TPMS) deserve special attention. Many indirect systems that detect pressure via wheel speeds must be recalibrated manually via the on-board menu or after a longer drive following a battery disconnection. Direct systems with sensors in the tyres generally continue to transmit after disconnection, but may require a relearn procedure. If one of your sensors is defective or the sensor battery is exhausted, you will find suitable TPMS sensors for almost all makes in our range. For more technical details on tyre pressure sensors, see our TPMS overview page.
Reconnecting the car battery – the correct order
When reconnecting, reverse the order: First the positive terminal, then the negative terminal. Place the positive clamp on the terminal and tighten the nut. Then fit and secure the negative clamp. After the battery is connected, a small spark may occur because capacitors in the electrical system are charging – this is normal. Ensure the clamps are secure; loose connections can lead to voltage drops and increased contact resistance, which weakens the starter and subjects the on-board electronics to voltage dips (typically: below 9 V when starting).
On vehicles with a voltage-maintaining device, the OBD plug should be removed or the external battery disconnected before reconnecting. Start the engine and let it idle for a few minutes so that the control units can carry out their adaptation.
Battery types and their special features when disconnecting
Not every starter battery is the same. Depending on the vehicle type and equipment, different technologies are used, which differ in capacity, cold cranking current, cycle stability and price. Disconnecting is identical for all types, but choosing the right replacement battery requires some specialist knowledge:
AGM batteries
Absorbent Glass Mat (AGM) is the standard in vehicles with start-stop automatic and brake energy recuperation. Thanks to the bound electrolyte, the battery is leak-proof and can absorb high charge and discharge currents (up to 5 × I₁₀). Typical values: voltage 12.8 V, capacities from 70 to 105 Ah, cold cranking current according to EN 760–950 A, cycle stability at 50 % depth of discharge around 400–600 cycles.
EFB batteries
The Enhanced Flooded Battery (EFB) is a further development of the conventional flooded battery and is installed in simple start-stop vehicles without recuperation. It offers around 300–400 cycles at 50 % DoD, a rest voltage of 12.6–12.8 V and capacities in the range 60–80 Ah.
Gel batteries
Thanks to the fixed electrolyte, gel batteries are extremely cycle-resistant (500–800 cycles) and more deep-discharge-proof than AGM types. They are often used in commercial vehicles, motorhomes and for long periods of standing.
LiFePO4 batteries
Lithium iron phosphate batteries (LiFePO₄) are the lightweights among starter batteries. They often weigh only a third of a lead battery and offer enormous cycle numbers of 2000–5000 at 80 % depth of discharge. The voltage is constantly around 13.2–13.6 V, the internal resistance is extremely low, which enables cold cranking currents of 800–1500 A – even with small capacities (40–100 Ah). For classic cars or special applications they are an interesting alternative, but require a compatible charge controller.
| Type | Capacity (Ah) | Cold cranking current (CCA, EN) | Cycles (50 % DoD) |
|---|---|---|---|
| AGM | 70–105 | 760–950 A | 400–600 |
| EFB | 60–80 | 600–800 A | 300–400 |
| Gel | 60–110 | 700–900 A | 500–800 |
| LiFePO₄ | 40–100 | 800–1500 A | 2000–5000 |
When is it worth buying a new car battery?
If your current battery shows a rest voltage of below 12.4 V after charging with an external charger (recommended with IUoU characteristic, 10 % of the nominal capacity as charging current, i.e. 7 A at 70 Ah) or falls below the manufacturer's specification under load with a battery tester, you should consider a replacement. A typical capacity test with a 100 A load tester should still deliver at least 9.6 V after 15 seconds. Modern AGM and EFB batteries last on average 4 to 7 years, depending on cycle load and outside temperature.
In our range you will find starter batteries from all leading manufacturers. We deliver your new battery free of shipping costs within Germany by DHL. You can reach us by telephone on +49 7135 7194106 or by e-mail at [email protected]. Our team will be happy to advise you on the right battery choice and the optimum charger.
FAQ – Frequently asked questions about disconnecting the car battery
In what order do I disconnect the car battery?
Always the negative terminal first, then the positive terminal. This avoids a short circuit between the spanner and the bodywork, which can occur if the earth negative cable is still connected.
Why must I disconnect the negative terminal first?
If the negative cable is still connected, the bodywork is connected to the negative pole of the battery and carries earth. If you touch a metallic object (e.g. the bodywork) with the spanner at the positive terminal, a very high short-circuit current flows. After disconnecting the negative terminal, this circuit is interrupted.
Can I disconnect the battery to protect the car from theft?
Yes, that is a simple immobiliser. However, you may then lose stored settings and the alarm system will no longer be supplied with voltage. In vehicles with an electronic immobiliser, disconnecting does not provide additional protection, because the immobiliser remains stored in the control unit.
Will I lose my radio settings after disconnecting?
Yes, as a rule stored stations, sound presets and the code are lost. In newer vehicles, however, an anti-theft device may also be activated that requires a code after switching on again. Note the radio code beforehand or look in the vehicle folder. Some radios store the code in the control unit so that it is automatically recognised again after reconnection.
Do I need to relearn TPMS sensors after disconnecting?
With many direct systems with wheel sensors, relearning is not necessary because the receiving antenna has permanently stored the IDs. Indirect systems that work via wheel speeds usually need to be initialised via the on-board menu after a voltage interruption (e.g. "Save pressure"). If a sensor is defective, you will find suitable TPMS sensors for replacement in our shop.
What tools do I need to disconnect the car battery?
Usually a 10 mm or 13 mm open-end or ring spanner for the terminal clamps is sufficient. If the battery holder is bolted, you may need another spanner or a screwdriver. Insulated tools or rubber gloves increase safety. A small helper: a terminal clamp puller prevents you from slipping with the spanner.
Can I disconnect the battery if the car is standing for a long time?
Yes, this is even recommended in order to prevent discharge through the quiescent current (30–50 mA are typical). Before storage, charge the battery fully with a trickle charger (12.8–13.2 V depending on type). A disconnected, full AGM battery loses about 1–2 % of its charge per month.
How long can a car stand without a battery?
Indefinitely – however, the battery itself should not be deeply discharged. A disconnected battery can survive several months under good conditions (cool, dry). After about 6 months, you should check the voltage and recharge if necessary before it falls below 12.2 V (lead-acid) or 13.0 V (LiFePO₄).
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Germany
Tel: +49 7135 7194106
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