Disconnect a car battery’s negative terminal first and reconnect it last, because in nearly all vehicles the metal body is connected to the negative post, so a wrench resting on the body while it touches the positive post would short the battery. Keep sparks and flames away because charging and discharging release explosive hydrogen, wear eye protection and gloves because the electrolyte is sulphuric acid, and leave orange high-voltage cables on hybrid and electric vehicles untouched.
A lead-acid battery combines three hazards in one box: a large short-circuit current, an explosive gas and a corrosive acid. A hybrid or electric vehicle adds a fourth, a high-voltage pack.
The Order In Which You Disconnect Decides Whether A Slip Is Harmless
If the negative terminal is removed first, the circuit through the body is already broken, and a wrench that touches the body and the positive post afterwards does nothing. If the positive is removed first, the body is still connected to the other post, and the same wrench completes the circuit and shorts the battery. Reconnecting in the reverse order, positive first and negative last, keeps the same protection.
The parent article describes the other two hazards. Lead-acid batteries produce hydrogen gas during charging and discharging, and hydrogen is explosive; never create sparks near a battery and never smoke near one. Battery acid, sulphuric acid, causes chemical burns to skin and eyes; wear gloves and eye protection. See also Sulfuric Acid.
The parent article also notes that hybrid and electric vehicle batteries are high-voltage, 200-800 V DC, and can deliver lethal shocks; orange cables indicate high-voltage circuits. Do not cut, puncture or disassemble those packs without specialised knowledge. The ordinary 12 V battery in such a vehicle is handled as described here.
What You Need
- A wrench with its shaft taped, or an insulated one.
- Gloves (rubber or nitrile over leather) and eye protection.
- A battery carrier strap, or two people for a large battery.
- Baking soda, water and a brush, and a bucket of clean water.
- A well-ventilated, open work area.
Method: Negative First, Upright Always
Step 1: Remove rings, watches and metal jewellery.
Step 2: Loosen the negative clamp first and lift it clear of the post. Tuck it away or tape it so it cannot swing back.
Step 3: Loosen the positive clamp with the same care and keep the wrench from touching any metal other than the clamp.
Step 4: Lift the battery out upright, by the strap or carrier, and set it on a firm surface away from sparks.
Step 5: Clean terminals with a baking-soda and water solution and a brush, keeping the solution out of the cells (parent article).
Step 6: Store upright, cool and dry. Cover the posts so nothing metal bridges them.
Step 7: Reconnect positive first, negative last, in a ventilated spot with no flame nearby.
If Acid Gets On You
Flush skin with large amounts of water for 15 minutes (parent article). Eyes need continuous flushing with clean water and medical attention if available; see Chemical Burns. Neutralise acid spilled on surfaces with a baking-soda paste (parent article); do not put it on skin or in eyes.
Common Mistakes
- Removing the positive first. A tool touching the body shorts the battery.
- Leaving a wrench lying across the terminals. It welds itself there.
- Tilting or dropping a battery. Acid pours out.
- Smoking or using a flame while handling. Hydrogen collects around the battery.
- Charging a cracked, swollen or frozen battery. It can fail violently.
- Touching orange cables on a hybrid or EV. They carry lethal voltage.
Safety
Short circuit, hydrogen explosion, acid and high voltage
A bridged battery can drive a very large current through a tool, welding or burning it, and the starter motor alone draws 100-400 A (parent article). The gas released from the cells explodes when ignited; ventilate, and make and break connections away from the battery where possible. Sulphuric acid burns skin and blinds. Treat any orange cable as lethal. If a battery is hissing, hot, swollen or leaking, step back, keep flames away and let it cool before moving it.
What’s Next
- Learn next: Mechanical Hazards — springs, airbags and pressurised systems
- Combine with: Batteries, Alternators & Wiring — using the battery once it is out