How to jump start a car safely, and what to do after
The correct lead order, jump packs versus leads, hybrids and automatics, and why a car that keeps going flat has a bigger problem than the battery.
Most people have a rough idea of how to jump start a car, and most of the damage we see comes from that rough idea. Leads on the wrong terminals, the black clamp on the flat battery's negative post, two cars touching, a donor engine revved to the limit. Modern cars are full of control units that do not like voltage spikes, so the order matters more than it did on a 1998 Fiesta.
Here is the method we use on the truck, the difference between leads and a jump pack, how to handle automatics and hybrids, and the part everyone skips, which is what to do after the car starts.
Before you connect anything
Make sure the problem really is a flat battery. Fast clicking and dimming lights, yes. Engine cranking at normal speed but not firing, no, and a jump will do nothing. Look at the battery itself: if the case is cracked, bulging or leaking, or it smells of rotten eggs, do not jump it. A frozen battery (possible after a hard frost on an already flat battery) can split when current goes in. Call for recovery instead.
Check the leads. Cheap thin leads with crumbling insulation are a fire risk and often cannot carry enough current to start a diesel anyway. Both cars should be switched off, in Park or Neutral with handbrakes on, close enough for the leads to reach but not touching each other.
How to jump start a car with leads: the correct order
The order below keeps the final spark away from the flat battery, which can give off hydrogen. Connect in this order and disconnect in exactly the reverse order.
- Red lead to the positive (+) terminal of the flat battery.
- Other end of the red lead to the positive (+) terminal of the donor battery.
- Black lead to the negative (-) terminal of the donor battery.
- Other end of the black lead to a bare metal earth point on the flat car's engine or chassis, away from the battery. Many cars have a marked earth bolt for this.
- Start the donor car and let it run for two or three minutes at idle. Do not rev it hard.
- Start the flat car. If it does not catch within ten seconds, stop, wait a minute and try again. Do not keep cranking.
- Once running, leave both cars connected for a minute, then remove the leads in reverse order: black from the earth point, black from the donor, red from the donor, red from the flat car.
- Do not switch the jumped car off. Drive it, as explained below.
Jump start a car with a jump pack
A lithium jump pack is what we reach for first on the truck, and it is what we would tell any driver to keep in the boot. There is no second car to position, the good ones have reverse polarity protection so a wrong connection just beeps at you instead of blowing something, and they are small enough to live under a seat.
Red clamp to positive, black clamp to an earth point or the negative terminal (follow the pack's instructions, they vary), wait for the ready light, start the car. If it does not start, disconnect, let the pack recover for a minute or so, and try again. Most packs allow a few attempts before they need a recharge. The weak point is that a pack left in a cold boot for a year will be flat when you need it, so charge it every few months.
Jump start an automatic car, a hybrid or a start-stop car
An automatic is jump started exactly the same way as a manual. The only difference is that you cannot bump start it by pushing, so if the jump fails, it is a recovery. Put it in Park, handbrake on, and follow the lead order above.
A hybrid has a high voltage battery that drives the wheels and a small 12 volt battery that boots the car's electronics. When a hybrid will not wake up and shows nothing on the dash, it is nearly always that 12 volt battery, which is often in the boot. Most hybrids have dedicated jump points under the bonnet, usually a positive terminal under a red flap and an earth bolt. Jump those, never the high voltage system, and never use a hybrid as the donor car for someone else, because its 12 volt system is not built to supply starting current to another engine. Check the handbook if in doubt, and if the handbook says no, ring us.
Start-stop cars run AGM or EFB batteries and a battery management system that monitors every amp in and out. They can be jump started normally, but they are the cars where a cheap replacement battery of the wrong type, or a battery fitted without being registered, causes trouble later. More on that below.
What to do after the jump start
The car is running, so the temptation is to drive to the shop, switch off and carry on with the day. Then it will not start at the shop. A jump start puts no charge into the battery, it just borrows enough to turn the engine. The alternator has to do the charging, and it needs time.
Drive for at least thirty minutes at normal road speed, not idling in a car park and not three short trips. The ring road, the N10 out to Paulstown and back, or the N77 towards Ballyragget will do it. Keep the heated seats, rear screen and blower off for that run so the alternator's output goes into the battery. Then get the battery tested properly that week. A load test tells you whether the battery recovered or whether it is on borrowed time, and most garages and motor factors in Kilkenny will do one for nothing.
Why the battery keeps going flat
One flat battery after lights were left on is bad luck. A second one within a month is a fault, and jump starting it again every few days is just delaying the bill. There are three usual causes.
The battery is finished. Batteries in Ireland last roughly four to six years, less if the car does mostly short runs. A tired battery charges fine and then loses it overnight. The alternator is not charging. If the battery warning light glows while driving, the headlights dim at idle, or the car dies while you are driving it, the alternator or its belt is the problem, and a new battery will go flat too. Something is draining it. A boot light that never goes off, an aftermarket alarm, a dashcam hard-wired without a cut-off, or a control unit that does not go to sleep can pull enough current to flatten a good battery in a few days. Finding a drain is a garage job with a meter, not a roadside one.
When to replace the battery, and the BMS registration point
Replace it when it fails a load test, when it is more than five years old and has started cranking slowly, or when it has been fully flattened more than once or twice. A deep discharge does lasting damage to an ordinary car battery, so a battery that has been rescued twice is rarely worth keeping.
On newer cars with start-stop or a battery sensor on the negative terminal, fitting the battery is only half the job. The car's battery management system has to be told there is a new battery, using a diagnostic tool, so that it charges it as a new battery rather than as the old worn one. Skip the registration and the car will undercharge the new battery, start-stop will stop working, and the new battery can be dead within a year or two. Fit the same type the car came with, AGM for AGM, and have it registered. We fit and register batteries at the garage in Three Castles, and if we jump you at the roadside we can tell you straight away whether your car is one that needs it.
Questions people ask about this
Which lead goes on first when jump starting a car?
Red to the flat battery's positive terminal first, then red to the donor's positive, then black to the donor's negative, then black to an earth point on the flat car. Disconnect in exactly the reverse order.
Can I jump start a hybrid?
Yes, through the 12 volt jump points under the bonnet, which boot the car's electronics. Never connect anything to the high voltage system, and do not use a hybrid as the donor car for another vehicle.
How long should I drive after a jump start?
At least thirty minutes at normal road speed with the big electrical loads switched off. Short trips will not put the charge back, and the car will be flat again the next morning.
Do I need to register a new battery on my car?
If the car has start-stop or a battery sensor on the negative terminal, yes. The battery management system needs a diagnostic tool to tell it a new battery is fitted, or it will undercharge the battery and shorten its life.
Does Kilkenny Recovery Truck do jump starts?
Yes, day and night across Kilkenny city, the county and the M9. Ring 087 288 2280, and we test the battery and alternator while we are there so you know whether it will happen again.