What Is Jump-Starting?
Jump-starting (also called boosting) is the process of using a second vehicle's charged battery — or a portable jump-starter pack — to provide enough electrical current to crank an engine with a flat battery. It's one of the most common roadside tasks NZ drivers face, especially in winter when cold temperatures reduce battery capacity.
Done correctly, it's quick and safe. Done incorrectly, it can damage sensitive electronics, spark a battery fire, or — on rare occasions — cause a battery to rupture.
Before You Start: Important Warnings
- Modern vehicles with stop-start systems (many cars sold after 2015) often use AGM or EFB batteries and have sensitive battery management systems. Always check your owner's manual before jumpstarting — some manufacturers (e.g., BMW, Mercedes-Benz, Audi) require the jump leads to be connected to specific terminals in the engine bay, not directly to the battery.
- Never jumpstart a cracked or leaking battery — hydrogen gas can ignite.
- Don't let the clamps touch each other once connected to a live battery.
- Turn off all accessories (radio, lights, A/C, heated seats) in both vehicles before connecting leads.
- Use quality jump leads rated at least 400 A and with good insulation.
What You'll Need
- A set of jumper cables (booster leads), at least 2 metres long
- A second vehicle with a charged battery, or a portable lithium jump-starter pack
- Safety glasses (optional but recommended)
Step-by-Step: How to Jumpstart a Car
Step 1: Position the vehicles Park the good vehicle close to — but not touching — the flat vehicle, so the jump leads can reach both batteries comfortably. Turn off both engines. Apply handbrakes on both cars.
Step 2: Identify the terminals Locate the positive (+) and negative (−) terminals on both batteries. The positive terminal usually has a red cover or a (+) symbol; negative is black or marked (−).
Step 3: Connect red to the dead battery positive Attach one red clamp to the positive (+) terminal on the flat (dead) battery.
Step 4: Connect red to the good battery positive Attach the other red clamp to the positive (+) terminal on the good (charged) battery.
Step 5: Connect black to the good battery negative Attach one black clamp to the negative (−) terminal on the good battery.
Step 6: Connect black to an earth point on the dead car Attach the remaining black clamp to an unpainted metal surface on the engine block or chassis of the dead car — away from the battery and any fuel lines. This is the earth (ground) point. Connecting it away from the battery reduces the risk of igniting any hydrogen gas venting from the battery.
Step 7: Start the good car Start the engine of the vehicle with the charged battery and let it run for 2–3 minutes. This allows some charge to transfer into the flat battery.
Step 8: Start the dead car Attempt to start the flat vehicle. If it doesn't start after 3–4 seconds, wait another minute before trying again. If it doesn't start after several attempts, the battery may be too far gone — or there's another fault.
Step 9: Disconnect in reverse order Once the flat car is running, remove the leads in reverse order:
- Black clamp from the earth point of the formerly dead car
- Black clamp from the negative of the good car
- Red clamp from the good car positive
- Red clamp from the previously dead car positive
Step 10: Drive to recharge Drive the revived car for at least 30–45 minutes at highway speed (or keep the revs above 2,000 RPM). This lets the alternator recharge the battery. Avoid turning the engine off during this time, as the battery may not have enough charge to restart.
After Jump-Starting: What Next?
A battery that has fully discharged may never hold a proper charge again. Take the car to a mechanic or auto parts store for a battery load test as soon as possible. If the battery is more than 4–5 years old and has gone completely flat, replacement is likely the most reliable path.
Also have the alternator checked — a failing alternator is a common cause of batteries going flat.
NZ Costs
- Battery load test: Often free at Supercheap Auto, Repco, or a workshop.
- Replacement battery: $120–$300 depending on size and type (AGM batteries are more expensive).
- Roadside assistance jumpstart (AA or similar): Included with NZ AA membership or available on a call-out basis.
When to Book a Mechanic
- Battery goes flat frequently — alternator or charging fault likely.
- Car cranks slowly even after a jumpstart — the battery needs replacing.
- Warning lights stay on after the car restarts — modern vehicles may need a scan to reset battery management parameters.