How to jump start a car
Got a flat battery? This is how to jump start a car with jump leads or bump start it without them
Jump-starting and bump-starting cars are such familiar concepts that it’s strange to think of them disappearing as the world switches from petrol and diesel engines to electric power.
You may think bump-starting won’t be missed, but over the last century or so a push down the road from a couple of willing passers-by must have saved millions of drivers from being stuck with a flat battery. The same goes for jump-starting with leads from another car. An electric car with a flat battery will simply stay stuck until it’s charged again, but that’s progress for you.
Below you'll find our guide to the whole jump-starting and bump-starting process...
About jump-starting and bump-starting a car
The difference of course is that on current cars – hybrids excepted – the battery’s biggest drain comes from turning the engine over on start-up. In petrol cars the battery also provides the spark for igniting the fuel, while in diesels it preheats the glow-plugs which helps the fuel ignite under compression. Incidentally, it is possible to bump- or roll-start a diesel engine, but it’s harder because you’ll typically need to be moving faster to overcome the higher compression in the engine. If you need to bump start a big diesel car – especially in cold weather – towing or rolling downhill can help.
Bump-starting an automatic is not possible, as the gearbox will take exception to being driven from the wrong end – i.e. via the wheels, not the engine, and damage can ensue. You can jump start an automatic though, so it’s always worth keeping a set of jump leads in the boot for emergencies.
Whether it’s because of a battery that is past its best or one that is flat because the lights have been left on, most motorists will find themselves in need of a jump start at some time. Knowing how to put the spark back in your battery is a useful life skill which could save you a long wait for a recovery service and potentially help you avoid a bill too.
Slowly over time, your battery will lose its ability to hold a good state of charge. Meaning one morning (when you are probably running late) your car will refuse to start. This is more likely to happen in cold weather as the temperatures slow down the reactions inside the car battery cells and reduce the voltage. In addition, the demands on the car’s electrical system are increased as we use wipers, lights and heaters more in winter.
When this does happen, being able to jump start your car is critical. However, there are some common mistakes people make which could potentially fry your car’s delicate electronics, give you a shock or even cause a fire. If you are in any doubt at all, it’s something best left to the professionals, but by reading the guide below you’ll will get a good idea of how to jump start a car correctly and safely.
Jump start or bump start?
There are two common methods for starting a car which has a flat battery. One of these requires jump leads and another (running) car or a battery booster pack, while for the other you’ll need to be able to get the car rolling to ‘bump’ start the engine.
Whichever way you do it, you should get your car battery checked as soon as possible after jump starting the car. If the battery was flat just because you left a light on then it should recover, but if it failed suddenly there could be an issue with the car’s alternator or electrical system. It could be as simple as a dirty or corroded connection, though, so it’s worth getting it checked out.
How to jump-start a car with jump leads:
1. Position the two cars:
Move another car close to the car with the flat battery (Car 1) so that the batteries are close together. Some models have them mounted in the boot or even under a seat. Move all metal objects out of the way of both batteries and remove loose clothing and jewellery which could get caught in pulleys or cause a spark. Check that both batteries and your jump cables aren't damaged.
2. Connect the red jump lead:
Connect the red jump lead's crocodile clip to the positive (+) terminal on Car 2's battery, then the other end to the positive (+) terminal on Car 1's battery. Be careful not to touch the end of the clip to any other part of the car and make sure you are getting the terminal right. The positive terminal will be clearly marked and usually has a red cap.
3. Connect the black jump lead:
Connect the black jump lead's crocodile clip to the negative (-) terminal of Car 2's battery, then the other end to an earthing point on Car 1 – this is usually found at the end of the negative lead and is bolted to a solid part of the chassis.
4. Start the engine:
Start Car 2's engine. Wait about 5 minutes and then start Car 1. If the battery isn’t totally dead and the car turns over slowly, you may be able to do this straight away. When the car is started, remove the clips in reverse order to the above.
5. Run or drive the car:
Run or, preferably, drive Car 1 for at least 15 minutes to let the battery recharge so that it can start on its own. Don’t keep trying to start it though, or you will flatten the battery again.
Some models have inaccessible batteries and will usually have special terminals somewhere under the bonnet for jump starting – check the owner’s manual if this is the case.
Without cables – the bump start (only works for manual cars)
- If you are parked facing downhill, you won’t need a push. But otherwise you’ll need to gather some volunteers to help push your car.
- Push in the clutch (do not release until step 4) and put the car into second gear. Turn on the ignition.
- Have your friends start pushing the car, or let off the brakes if you're on a hill.
- Once you're moving at a fast walking pace, bring up the clutch quickly and the engine will start. Once it does, push the clutch pedal again and let the engine idle. Let the car run while it recharges.
Jump starting a car with jump leads: explained in detail
1. Safety always comes first when dealing with electricity. Make sure that there are no metal objects nearby, like tools or even jewellery, that could come into contact with the batteries or cables - and take off any clothing that could get caught inside the engine bay.
You should also make sure that the batteries of both cars aren't damaged. If a battery's plastic casing is broken or it looks like it is leaking, don't try anything - either remove it yourself, if you know what you're doing, or take it to a professional to be replaced. Likewise, if the cables are damaged it may be best to borrow someone else's or buy some new ones - they're not too expensive.
2. After you've moved two cars close together so that the leads will reach between the batteries, it's time to connect the leads. Make sure both cars are completely off with the keys out and place a clip on the red jump cable onto the positive terminal - marked with a plus (+) symbol - on the battery of the car that works (we'll call this Car 2).
Clip the other end of the red cable to the positive (+) terminal on the non-starting car (Car 1). Make sure both clips are on securely and will not snap off. You should also make sure the cables don't fall into the engine bay and obstruct any moving parts.
3. Connect one end of the black jump lead to the negative terminal - marked with a minus (-) symbol - on Car 2's battery.
The other end of the black cable should be attached to an earthing point on Car 1 - some cars have a dedicated point, but otherwise use any bit of solid metal attached to the engine block or chassis, like an unpainted bolt. This should be away from the battery and any fuel-related parts. A small spark as you connect it is nothing to worry about.
4. Now start Car 2 and let the engine run for a little while. After about 5 minutes you should be able to start Car 1. If it doesn't start, then turn Car 2 off, gently adjust the clamps to ensure a good connection and then try again. If this doesn't work you may need a new battery or other essential parts to be repaired.
Once Car 1 is running , the clips can be removed. Make sure you don't touch any electrical components and only handle the insulated parts of the jump leads. Remove the leads in the opposite order to before: Earthed end of black lead, then the black clip on Car 2, then the red clip on Car 1, and finally the red clip on Car 2.
5. You will need to drive Car 1 or keep it running for at least 15 minutes to charge the battery up, but once it is running normally it can be driven as normal - just don't turn it off before the 15 minutes are up or you risk having to repeat the whole process!
How to bump-start a car without jump leads: detailed push start guide
1. Before you try push starting a car, you need to make sure it's a manual car with a clutch. Automatics or cars with no clutch can’t be bump started.
You'll also need a group of friends to help you get the car moving. If you can't find any nearby, try asking some passers-by to help - they may even become your friends in the future, which would be a nice side-effect of having a flat battery. You could also be towed by another vehicle or, if you're on a hill and you can get the car moving without any help, you'll be able to start the car on your own.
2. Push in the clutch, and keep it depressed fully until step 4 (below). Put the car into second gear and turn the key so that the ignition light comes on.
3. Get your friends to start pushing the car, making sure that there is no traffic coming as you would when pulling off normally. If you're on a hill, let the brakes off so that you start moving.
4. Once you're on the move, at about 5mph, release the clutch quickly so that the engine and gearbox connect. The wheels will turn the gears and then the engine, starting the combustion cycle is the same way as your starter motor.
Come to a stop, leaving the engine running, and thank your companions for doing the legwork. Make sure you run the engine for at least 15 minutes to charge the battery before turning it off. You can use this time to make your friends a cup of tea, or simply drive away.
How to jump start a car with a portable power pack
Portable power packs are a way of starting a car without the extra effort of finding another car or a group of volunteers. These devices carry enough charge to jump start your car simply by attaching the built-in crocodile clips to your battery's positive terminal and to an earthing point - exactly like the steps for jump starting using another car (above). All you have to do then is turn the car on and remove the power pack, then leave the engine running to recharge the battery.
They are a wise investment if you don’t use your car very often or have a habit of leaving the lights on! They may also save you the bother of connecting up your car if another driver needs a jump start.
Check out our best mini jump starter packs here...